Chapter 19: Air Conditioning and Cooling Systems: How They Work and What Fails

The central air conditioner sitting outside your house is doing something almost magical: it's picking up heat from inside your home and dumping it outside, even on a day when outside is already hotter than inside. It accomplishes this by exploiting the physics of refrigerants — substances that absorb enormous amounts of heat when they evaporate and release it when they condense. Understanding this mechanism, even at a conceptual level, transforms AC from a black box into a system you can diagnose, maintain, and make smart purchasing decisions about.

This chapter covers everything a homeowner needs to know about residential cooling: how the refrigerant cycle works without requiring a thermodynamics degree, the specific components that make up a central AC system, how to read SEER ratings and translate them into real dollars, the critical refrigerant transition that affects millions of older systems, and the growing world of mini-split systems that many homeowners don't even know exists. We'll also cover the most common failures — why your AC might be cooling poorly, what causes that dangerous ice formation on the coil, and what to do about refrigerant leaks.

The Rodriguez family is back in this chapter: their 1982 townhouse has a central AC system that's nearly as old as the furnace we discussed in Chapter 18 — and nearly as problematic. Priya Chen-Williams and Marcus Williams are installing an entirely new system as part of their gut renovation and face the choice between central AC and mini-splits. Dave Kowalski, out in the country, runs window ACs rather than central air — a choice that makes sense in his situation, and we'll explain why.

19.1 The Refrigeration Cycle: How Your AC Moves Heat

Your air conditioner does not create cold. This is the foundational insight that clears up more confusion about AC than anything else. Cold is not a substance that gets pumped into your house. What your AC does is move heat — from inside your house to outside. What remains after the heat is removed feels cool to you.

The tool that makes heat movement possible is a refrigerant: a chemical compound that can absorb large amounts of heat when it evaporates (changes from liquid to gas) and release that heat when it condenses (changes from gas back to liquid). Different refrigerants do this at different temperatures and pressures, which is why they're engineered specifically for the conditions in AC systems.

Here's the cycle, step by step:

Step 1: Evaporation inside (heat absorption). The refrigerant arrives at the indoor coil (called the evaporator coil) as a cold, low-pressure liquid. Your home's warm air is blown over this coil by the air handler's blower. The refrigerant is colder than the air, so heat flows from the air into the refrigerant. As the refrigerant absorbs this heat, it evaporates — it boils into a vapor, just as water boils when it absorbs heat on a stove, but at a much lower temperature. The air, having given up its heat to the refrigerant, is now cooler and is pushed back through your ducts into your living spaces.

Step 2: Compression (raising the pressure). The refrigerant vapor — now carrying the heat it absorbed from your house — travels to the outdoor unit. The compressor squeezes the vapor to high pressure. Compressing a gas raises its temperature. The refrigerant, already carrying heat from your house, gets even hotter as the compressor squeezes it. This is not a problem — it's the mechanism. We need the refrigerant to be hotter than the outdoor air so it can dump its heat outside.

Step 3: Condensation outside (heat rejection). The hot, high-pressure vapor moves to the outdoor coil (called the condenser coil). A large fan blows outdoor air over this coil. The refrigerant is now hotter than the outdoor air, so heat flows from the refrigerant to the outdoor air. As the refrigerant releases heat, it condenses back to a liquid. This is where all the heat from your house — plus the heat added by the compressor — gets dumped outside. On a hot day, if you stand near a running outdoor AC unit, you'll feel warm air blowing out: that's your house's heat being rejected.

Step 4: Expansion (cooling the refrigerant). The high-pressure liquid refrigerant passes through an expansion device — either a thermostatic expansion valve (TXV) or a simpler orifice tube. As the refrigerant expands from high to low pressure, it cools dramatically. It arrives back at the indoor evaporator coil cold and ready to absorb more heat from your house.

The cycle repeats continuously while the AC runs.

💡 The "Moving, Not Creating" Insight This is why a heat pump can heat your house more efficiently than an electric resistance heater. The heat pump uses the same refrigerant cycle as your AC, just running backward: the outdoor coil becomes the evaporator (absorbing heat from outdoor air) and the indoor coil becomes the condenser (releasing that heat inside). You're not generating heat with electricity — you're using electricity to move heat that already exists in outdoor air. At moderate temperatures, this is dramatically more efficient than generating heat directly. See Chapter 18, Section 18.5 for the full discussion.

Moisture removal. Air conditioning does double duty: it cools and dehumidifies. When warm, humid air flows over the cold evaporator coil, moisture condenses on the coil surface — exactly as water condenses on a cold glass. This condensate drips into a drain pan and flows through the condensate drain line to the outside or to a floor drain. On a humid summer day, a central AC system might remove 10–20 gallons of water from your house's air. This moisture removal is not a side effect — it's a major part of what makes your home comfortable. High humidity makes you feel hotter even at the same temperature.

19.2 Central AC: The Outdoor Condenser and Indoor Air Handler

Residential central AC splits the refrigerant cycle between two locations, which is why it's called a split system. The outdoor unit (called the condenser unit, though it contains more than just the condenser coil) houses the compressor, condenser coil, and condenser fan. The indoor unit (the air handler or furnace with evaporator coil) houses the evaporator coil and blower. Refrigerant lines — a pair of copper pipes wrapped in insulation — connect the two.

The outdoor unit. The compressor is the heart of the system and the most expensive component. Residential compressors are typically hermetically sealed — the motor and compressor are in the same sealed housing, cooled by the refrigerant itself. This design is reliable and eliminates the need for shaft seals, but it means the compressor cannot be serviced internally; if it fails, it's replaced as a unit. Compressors typically last 10–15 years under normal conditions; they fail earlier if the refrigerant charge is wrong (too little refrigerant causes the compressor to run hot), if the oil in the system becomes contaminated, or if the system is frequently run under extreme conditions.

The condenser coil wraps around the inside of the outdoor cabinet. It's made of copper tubing with aluminum fins — the fins dramatically increase surface area to improve heat transfer. These fins are fragile. A bent fin doesn't necessarily kill performance but it does reduce airflow. There are fin straightening combs available at HVAC supply stores; they're cheap and worth keeping on hand if the fins take a hit from debris.

The refrigerant lines. Two copper pipes connect the outdoor and indoor units. The larger pipe (the suction line) carries cold, low-pressure refrigerant vapor from the indoor evaporator to the outdoor compressor. It should be cool and possibly sweating condensation on a humid day — this is normal. The smaller pipe (the liquid line) carries high-pressure liquid refrigerant from the outdoor condenser to the indoor evaporator. It's typically warm to the touch. Both pipes should be insulated, particularly the suction line; insulation that's cracked, missing, or soaked is worth replacing to maintain efficiency.

The indoor air handler. In homes with forced-air heating, the evaporator coil typically sits on top of (or inside) the furnace air handler. The blower that moves house air is the same one used for heating. In homes without gas furnaces — including pure heat pump systems — the indoor unit is a standalone air handler: essentially a cabinet containing the coil, blower, and (for heat pumps) electric auxiliary heat strips.

The evaporator coil. Sitting above the furnace or inside the air handler, the evaporator coil is A-shaped or N-shaped to maximize surface area in a compact space. It's inside the ductwork, so you can't see it without removing access panels. Dirt accumulation on the evaporator coil is a major cause of reduced AC performance — dirt insulates the coil, reducing heat transfer. Keeping the air filter clean is the single most effective thing you can do to keep the evaporator coil clean. If the coil is already heavily fouled, professional cleaning (often done with a chemical coil cleaner) restores capacity.

📊 Central AC System Component Lifespans | Component | Typical Lifespan | Replacement Cost | |-----------|-----------------|-----------------| | Compressor | 10–15 years | $800–1,500 (labor included) | | Condenser fan motor | 10–20 years | $200–400 | | Evaporator coil | 15–20 years | $600–1,200 | | Capacitor (start/run) | 5–10 years | $100–200 | | Contactor | 5–10 years | $75–150 | | Refrigerant lines | 25+ years | Replace if corroded | | Air handler/furnace | 15–25 years | $1,000–2,500 |

Capacitors and contactors are the most commonly replaced components and the least expensive. If your AC stops working, a failed capacitor is the first thing a technician will check — it's a $15 part that might cost you $150–200 for the service call, which is still a cheap repair.

19.3 SEER Ratings and What They Actually Mean for Your Bills

Walk into any HVAC equipment showroom or browse online and you'll see SEER numbers attached to every AC unit: SEER 14, SEER 18, SEER 24. Contractors often use these numbers to justify higher-priced equipment. Understanding what they actually measure — and calculating their real-world dollar value — prevents you from overpaying or underpaying.

SEER stands for Seasonal Energy Efficiency Ratio. It measures the total cooling output (in BTUs) of a system over an entire cooling season divided by the total electrical energy input (in watt-hours) over the same period. A higher SEER means more cooling per unit of electricity consumed.

Since January 2023, the U.S. Department of Energy moved to a new metric called SEER2, which uses updated test conditions closer to real-world installation. SEER2 ratings run slightly lower than SEER for the same equipment. When comparing equipment or looking at older specs, be clear about which standard you're comparing.

Minimum standards: - Northern states: SEER2 13.4 (equivalent to old SEER ~14) - Southern states (DOE's Southwest/Southeast regions): SEER2 14.3 (equivalent to old SEER ~15) - Equipment is available up to SEER2 ~24 (old SEER ~26)

Calculating the real dollar difference. Here's a worked example so you can apply this yourself.

Suppose you're replacing a 1998 SEER 10 central AC (3-ton system, 36,000 BTU/hour capacity) and choosing between a SEER 14 replacement and a SEER 18 model. You live in Dallas and run the AC approximately 1,400 hours per season. Your electricity rate is $0.13/kWh.

Electricity consumption per season = (BTUs per hour × hours) ÷ (SEER × 1,000) - SEER 10: (36,000 × 1,400) ÷ (10 × 1,000) = 5,040 kWh/season - SEER 14: (36,000 × 1,400) ÷ (14 × 1,000) = 3,600 kWh/season - SEER 18: (36,000 × 1,400) ÷ (18 × 1,000) = 2,800 kWh/season

Annual operating cost at $0.13/kWh: - SEER 10: $655/season - SEER 14: $468/season (saves $187/year vs. SEER 10) - SEER 18: $364/season (saves $291/year vs. SEER 10; saves $104/year vs. SEER 14)

The price difference between a SEER 14 and SEER 18 system might be $800–1,500. At $104/year savings, the SEER 18 pays back its premium in 8–14 years. If the system lasts 15–20 years, that's a reasonable investment — but only barely. If the price difference were $3,000, it would never pay back.

💡 The SEER Math Rule of Thumb Each SEER point improvement reduces operating costs by roughly 7%. Going from SEER 14 to SEER 18 is a 29% improvement in efficiency — meaningful, but the premium price must be evaluated against your actual operating hours and electricity rate. In a mild climate with low AC usage, high-SEER equipment pays back very slowly.

The Rodriguez family's aging system was a SEER 8 unit (pre-1992 equipment was often below SEER 10). Replacing it with a minimum-code SEER 14 system would cut their cooling bill nearly in half — a calculation that made replacement a no-brainer regardless of the efficiency tier chosen for the replacement.

19.4 Refrigerants: R-22 Phase-Out, R-410A, and What's Coming Next

The refrigerant in your AC system is not just a technical detail — it has significant implications for repair costs, system longevity, and environmental impact. The refrigerant landscape has changed dramatically over the past two decades and will change again in the next few.

R-22 (Freon). For decades, R-22 was the standard refrigerant in residential air conditioning. If your system was installed before approximately 2010, it almost certainly uses R-22. R-22 is a hydrochlorofluorocarbon (HCFC) that contributes to ozone layer depletion. Under the Montreal Protocol and U.S. EPA regulations, production and import of R-22 was completely banned in the United States as of January 1, 2020.

Here's what this means if you have an R-22 system: your AC is not illegal. Existing R-22 systems can continue operating. Recycled and reclaimed R-22 is still legal to sell and use in existing systems. But there is no new production. The supply of R-22 is fixed and shrinking, while demand from aging R-22 systems remains. The result: R-22 now costs $50–150 per pound to recharge, compared to $5–10 per pound just fifteen years ago. A system that needs 3–4 pounds of refrigerant added costs $150–600 just for the refrigerant — before the service call.

🔴 The R-22 System Decision Point If you have an R-22 system and it develops a refrigerant leak, you face a genuine economic decision: - Repair the leak and recharge with expensive recycled R-22: Makes sense if the leak is small, accessible, and the system is otherwise in good condition with useful life remaining. - Replace the system: If the system is over 15 years old (which almost all R-22 systems now are), the economics of continued R-22 investment rarely pencil out. A major leak combined with an aging compressor means you're pouring money into a system that will need replacement within a few years anyway.

R-22 systems cannot simply be "converted" to R-410A by draining and recharging with the new refrigerant. The two refrigerants operate at different pressures; the compressor, copper lines, and coils designed for R-22 are not rated for R-410A pressures. Conversion is not an option — replacement is.

R-410A (Puron). R-410A replaced R-22 as the standard residential refrigerant from roughly 2010 through 2024. It contains no chlorine (so it doesn't deplete the ozone layer) and operates at higher pressure than R-22. Systems designed for R-410A have heavier copper lines and compressors rated for the higher pressures. R-410A itself has a high global warming potential (GWP of ~2,088 times CO₂) — it's not an ozone depleter, but it's a potent greenhouse gas if it leaks.

R-454B and R-32: The next transition. Under EPA regulations and global agreements, R-410A itself is being phased down due to its high GWP. New residential AC equipment manufactured after January 1, 2025 cannot use R-410A in most applications. The replacements are lower-GWP refrigerants, primarily R-454B (GWP ~466) and R-32 (GWP ~675). These refrigerants are mildly flammable (classified A2L) — a significant change from R-22 and R-410A, which were not flammable. This requires updated installation practices and equipment design, but is not a safety problem under normal residential conditions.

If you're buying a new AC system right now, you're likely buying R-454B or R-32 equipment. If you already have an R-410A system, it's not affected by the new rules — you can continue using it and recharge it with R-410A as long as it operates (R-410A itself hasn't been banned yet; only its use in new equipment is being phased out).

📊 Refrigerant Transition Timeline | Refrigerant | Era of New Installations | Status Today | |-------------|-------------------------|-------------| | R-22 (Freon) | Pre-2010 | No new production since 2020; reclaimed only; expensive | | R-410A (Puron) | 2010–2024 | Phasing out of new equipment; fine to recharge existing | | R-454B / R-32 | 2025–present | Current standard for new equipment |

19.5 Mini-Splits: Ductless Systems, Installation, and When They're the Right Choice

Walk through many homes in Japan, Korea, or Europe and you'll find a different type of AC: a sleek wall-mounted unit in each room, connected by a small bundle of pipes to a compact outdoor unit. These are mini-split systems — also called ductless mini-splits or ductless heat pumps. In North America, they've been gaining market share rapidly, and for good reason: they solve problems that central ducted systems cannot.

How mini-splits work. The refrigerant cycle is identical to central AC. The difference is in the distribution method. Instead of one large indoor unit that distributes cooled air through a duct system, a mini-split has one or more small indoor air handlers — mounted on a wall, ceiling, or floor — each connected to the outdoor unit through a small penetration in the wall. This penetration carries two refrigerant lines, a condensate drain line, and electrical wiring — bundled together in a conduit typically 3–4 inches in diameter. No ductwork required.

Each indoor unit has its own thermostat control, so each room or zone can be at a different temperature. One outdoor unit can serve 2, 3, 4, or even 8 indoor units in a multi-split configuration.

The indoor unit types: - High-wall unit: The most common type, mounted on the upper portion of a wall. Slim profile, quiet. Works well for bedrooms, living rooms. - Ceiling cassette: Recessed into the ceiling with a grille that distributes air in four directions. Almost invisible. More complex installation. - Ceiling-suspended: Hangs from the ceiling, not recessed. Good for rooms with drop ceilings or when ceiling penetration isn't possible. - Floor-mounted: Mounted near floor level. Useful when high wall or ceiling mounting isn't practical.

When mini-splits are the right choice:

Room additions and bonus rooms. Extending ductwork to a new addition is expensive and may not be technically feasible. A mini-split provides comfort immediately with minimal structural disruption. This is arguably the most clear-cut use case.

Homes without ductwork. If you have a hot water boiler system for heating and no existing ductwork, adding central AC means either installing an entire duct system (expensive, disruptive) or adding mini-splits for cooling. Mini-splits are almost always the better choice for unducted homes.

Spaces with poor duct coverage. If one room in your house is always too hot or too cold regardless of what the central system does, adding a mini-split for that room solves the problem without requiring duct redesign.

Energy-conscious new construction. A very tight, well-insulated house has small heating and cooling loads. A mini-split system can be sized precisely for each room, avoiding the over-sizing that plagues many central systems.

Climate extremes. Premium mini-splits (particularly from Mitsubishi, Daikin, and Fujitsu) operate effectively down to -13°F and at high outdoor humidity — performance that central systems often can't match.

What mini-splits don't do well:

Whole-house retrofits with multiple zones can get expensive quickly. Each indoor unit adds cost, and complex multi-split systems require careful design. A 4-zone multi-split installed by a professional might cost $12,000–20,000, comparable to (or more than) a whole-house ducted system replacement.

Filtration. Mini-split filters are simple mesh screens that capture lint and large particles. They don't integrate with high-MERV filter media the way ducted systems can. If air quality is a priority, a ducted system with quality filtration may be preferable.

Aesthetic preferences. Some homeowners dislike the visible wall units. Ceiling cassettes are more discreet but more expensive to install.

💡 Mini-Split Installation: What You Need to Know Installation requires drilling a 2.5–3.5-inch hole through an exterior wall for the line set. A licensed HVAC contractor connects the refrigerant lines and charges the system — refrigerant work requires EPA 608 certification. The DIY mini-split market has grown, with "pre-charged" line sets that eliminate the need for field brazing, but the refrigerant connection still requires certification. Some manufacturers now offer "DIY-certified" systems specifically designed for homeowner installation, with pre-charged quick-connect fittings. These are legitimate products, but read the warranty carefully — some require professional commissioning to maintain the warranty.

Priya and Marcus's decision. The Chen-Williams renovation presented a clean slate. Their 1963 house had no existing HVAC system worth keeping. They considered three options: a new gas furnace + central AC with all-new ductwork, a central heat pump with all-new ductwork, and a multi-split system with no ductwork. Ultimately they chose a hybrid approach: a ducted heat pump for the main floor (where ductwork could be run in a new soffit without major disruption) and two wall-mounted mini-splits for the upper floor rooms, where ceiling heights were low and duct routing would have been painful. The combination gave them whole-house coverage without compromise.

⚖️ DIY vs. Professional: Mini-Split Installation - DIY possible: Mounting the indoor unit bracket, drilling the wall penetration, routing and securing the line set conduit, making low-voltage control wire connections. - Professional required: Brazing refrigerant connections (unless using pre-charged fittings), evacuating the system (pulling vacuum), verifying refrigerant charge, making 240V electrical connections to outdoor unit. - Bottom line: Even with pre-charged DIY kits, having a licensed HVAC technician commission the system is worthwhile. They'll verify proper operation, check refrigerant charge, and leave you with documentation for the warranty.

19.6 Common AC Problems: Reduced Cooling, Ice on the Coil, Refrigerant Leaks

AC problems follow predictable patterns. Most failures fall into a handful of categories, and knowing what causes each one helps you describe the problem accurately to a technician — and helps you distinguish problems you can address yourself from ones that require a professional.

Reduced cooling capacity. The system runs but the house doesn't cool adequately. The first question to ask: is the outdoor unit running? (You should hear the compressor and see the fan turning.) If yes: - Dirty air filter: A clogged filter restricts airflow over the evaporator coil. Reduced airflow means less heat transfer, less cooling. This is the most common cause of poor AC performance and the one you can fix in 5 minutes. Check it first, always. - Dirty evaporator or condenser coil: Both reduce heat transfer efficiency. Condenser coil cleaning is a homeowner task (garden hose, gentle rinse, spray from inside out). Evaporator coil cleaning usually requires a professional. - Incorrect refrigerant charge: Too little refrigerant means the system can't transfer enough heat. Too much is also a problem. Refrigerant charge must be checked with gauges by a certified technician. - Undersized system for the load: If the system has always struggled on the hottest days, it may be undersized. If it's newly struggling, the problem is maintenance or equipment failure. - Duct leakage: Conditioned air leaking out of supply ducts in unconditioned space (attic, crawlspace) means less cold air reaches the living space. See Chapter 20.

Ice on the coil. Ice forming on the evaporator coil seems contradictory — shouldn't cold things work better? Actually, ice on the coil is a problem that progressively worsens. Ice insulates the coil from airflow, reducing heat transfer further, which causes more ice. The entire coil can eventually be encased in ice, stopping cooling entirely.

Causes of icing: - Low airflow (dirty filter, blocked return, slow blower): Not enough warm air flowing over the coil; the refrigerant gets too cold. - Low refrigerant charge: Reduces pressure in the evaporator, dropping the refrigerant temperature below normal, causing ice formation. - Running the AC when outdoor temperatures are below 60°F: Many AC systems are not designed to operate efficiently below 60°F outdoors. The refrigerant cycle needs a certain temperature differential to function properly.

If you find ice on your AC's indoor coil: shut the system off (or switch to "fan only" mode) and let the ice melt — this takes 1–4 hours. Place towels around the air handler to catch melt water. Once fully thawed, identify and fix the underlying cause before restarting.

🔴 Don't Run an Iced AC Running your AC with an iced coil can damage the compressor. If the ice extends down the suction line to the outdoor unit, liquid refrigerant can reach the compressor — compressors are designed to compress vapor, not liquid. Liquid refrigerant in a compressor causes immediate mechanical damage. If you find ice, shut the system off.

Refrigerant leaks. Refrigerant leaks are the most expensive common AC problem because (a) finding them requires specialized equipment, (b) fixing them requires a certified technician, and (c) recharging the system is expensive, particularly for R-22 systems.

Symptoms of a refrigerant leak: - Gradual decline in cooling capacity over a season or two - Ice on the evaporator coil (low refrigerant causes low evaporator pressure) - Warm air from supply registers when the system is running - Hissing or bubbling sound near the indoor unit or refrigerant lines - Oily residue near refrigerant line connections (refrigerant oil coats the inside of the lines; when refrigerant leaks, it carries oil with it)

Common leak locations: - Schrader valves (the service ports on the refrigerant lines): These degrade over time. A straightforward, inexpensive repair. - Evaporator coil hairline cracks: Particularly common in systems over 10 years old. The coil is made of thin copper; vibration and chemical corrosion (from household cleaners containing formaldehyde) cause cracks. This is a significant repair — evaporator coil replacement costs $600–1,200. - Line set fittings and joints: Where the copper lines connect to the coil or compressor. Brazed joints can develop pinholes. - Condenser coil: Less common but possible.

A technician will use an electronic refrigerant leak detector or UV dye (injected into the system; glows under UV light at leak points) to locate leaks. Once found, the leak is repaired, the system is evacuated (vacuum pump removes air and moisture), and it's recharged to the correct weight of refrigerant.

Compressor won't start. This is often a capacitor or contactor failure — cheap components, expensive labor, but nowhere near as bad as compressor replacement. - Capacitor: Provides the electrical kick needed to start the compressor and fan motors. Capacitors degrade over time, especially in hot climates. A swollen or leaking capacitor is visually obvious if you open the outdoor unit's access panel. Replacement cost: $100–200. - Contactor: An electrically-operated switch that connects 240V power to the compressor. The contact points pit and corrode with time. Replacement cost: $75–150.

If you're comfortable with electrical safety (see Chapter 12 on electrical systems), reading a capacitor's label and checking it with a multimeter is within DIY range. Replacing a capacitor requires discharge procedures — capacitors store charge even with power off. If in doubt, call a tech.

⚠️ The AC Doesn't Turn On At All First check: Is the outdoor disconnect (the gray box on the wall near the outdoor unit) closed? This breaker-style disconnect is sometimes accidentally tripped. Is the circuit breaker for the AC not tripped? Is the thermostat set to "Cool" mode and the setpoint below the current room temperature? Is the furnace/air handler power switch (looks like a light switch on the wall near the unit) on? These checks catch a surprising percentage of "the AC is broken" calls.

19.7 End of AC Season and Startup: What You Should Do Each Year

The seasonal transitions — starting up in spring and shutting down in fall — are the right times for homeowner maintenance that keeps your system running reliably and efficiently.

Spring startup (before the first hot day): 1. Check the air filter. Install a fresh filter. 2. Clear the outdoor unit. Remove any debris, leaves, or vegetation that accumulated over winter. Trim back plants — maintain 18–24 inches of clearance on all sides. 3. Check the refrigerant lines. Look for damage to the foam insulation wrapping the suction line. Replace any sections that are degraded; missing insulation causes condensation and reduces efficiency. 4. Clean the condenser coil. Turn off power at the disconnect, then gently rinse the condenser fins with a garden hose, spraying from inside out (through the top or the panel access). Do not use a pressure washer — the fins are delicate aluminum and will bend. 5. Clear the condensate drain. Pour a cup of diluted bleach (1 tablespoon per cup of water) or white vinegar into the condensate drain pan to prevent algae and mold growth in the drain line. A clogged condensate drain is a very common cause of water damage (the pan overflows) and an AC shutoff (many systems have a float switch that turns off the AC if the pan fills). 6. Test the system. On a day when outdoor temps are above 60°F, run the AC for a full cycle. Check that cold air comes from the registers. Check that the drain line is flowing.

Fall shutdown (after the last hot day): 1. Clean the outdoor unit. Remove leaves, dirt, and organic debris that can trap moisture and cause corrosion. 2. Cover the outdoor unit? This is genuinely optional and somewhat controversial. A properly designed AC outdoor unit can handle weather. However, a breathable cover protects the coil fins from ice damage in climates with heavy ice storms. Never use an airtight cover — it traps moisture and provides rodent housing. If you cover the unit, mark your calendar to remove it before spring startup. 3. Note any problems from the season for your spring maintenance call. Strange sounds, reduced cooling — document them while they're fresh.

✅ Annual Professional AC Service — What It Should Include A professional AC tune-up (typically $80–150) should cover: - Measure refrigerant charge (temperature split across the coil or electronic gauges) - Clean condenser coil (chemical wash if heavily fouled) - Check electrical components: capacitor, contactor, wiring connections - Measure motor amperage draw (high amps = motor nearing end of life) - Verify thermostat operation and calibration - Check condensate drain and pan - Inspect refrigerant lines and insulation - Inspect blower wheel and housing

Dave Kowalski's situation with window ACs deserves a full treatment of its own — and so do the other cooling alternatives that make sense for specific situations.

19.8 Window ACs and Portable Units: The Rental Situation and Room-by-Room Cooling

Not every house has central air, and not every homeowner wants it. Window air conditioners and portable AC units are legitimate cooling solutions for specific situations — understanding them helps you use them well and avoid common mistakes.

Window Air Conditioners

A window air conditioner is a self-contained version of the central split system, with the compressor, condenser coil, evaporator coil, and both fans all in a single cabinet. The unit straddles the window sill: the indoor section cools your room, the outdoor section rejects heat outside.

When window ACs make sense:

Rental situations. If you rent, you almost certainly can't modify the building's HVAC system. A window unit installed without permanent modification to the building is typically allowed under most leases (verify this, of course). It provides cooling without capital investment in a building you don't own.

Houses without ductwork. Homes with hot water boiler systems, radiant floor heating, or older homes with no forced-air system at all don't have the infrastructure for central AC. Mini-splits are the better long-term solution, but a window unit is a viable low-cost alternative for one or two rooms.

Additions and bonus rooms. A finished room over a garage, a sunroom, or an addition that isn't served by the central duct system can be cooled effectively with a window unit. This is often simpler and less expensive than extending ductwork.

Supplemental cooling for a hot spot. If one room consistently runs 5–8°F hotter than the rest of the house — typically a west-facing bedroom in summer afternoon — a window unit provides targeted relief without requiring the whole-house system to work harder.

Sizing window ACs correctly. Window ACs are rated in BTUs per hour. Undersizing means the unit runs constantly and never gets comfortable; oversizing means it cools the air too quickly without adequately removing humidity — leaving you in a cold, clammy room.

📊 Window AC Sizing Guide: | Room Size | BTU Recommendation | |-----------|-------------------| | Up to 150 sq ft | 5,000 BTU | | 150–250 sq ft | 6,000 BTU | | 250–350 sq ft | 8,000 BTU | | 350–550 sq ft | 10,000–12,000 BTU | | 550–700 sq ft | 14,000 BTU |

Adjust upward 10% for rooms with high ceilings, above 8 feet. Adjust upward if the room is in direct sun for most of the day. Adjust upward 600 BTU for each additional person regularly occupying the space beyond two people.

Installation considerations: - The unit must be level side-to-side; tilted slightly (1/4 inch) toward the outside so condensate drains out rather than into the room. - Use the foam side panels and window sashes provided; gaps around the unit allow hot air infiltration that defeats the purpose. - Window ACs require a dedicated 15-amp or 20-amp circuit for larger units (12,000 BTU and above). Running a 12,000-BTU window unit on a heavily loaded shared circuit will trip breakers. - Install a window latch or security bar when possible — a window held open only by the AC unit is a security vulnerability.

Annual window AC maintenance: 1. Clean or replace the air filter (usually removable and washable) at least once per season. 2. Vacuum or gently rinse the condenser fins on the outdoor section (accessible through the rear) if visibly dirty. 3. Check that the drain is clear and water is dripping from the outdoor section (if no condensate drips on a humid day, the drain may be clogged). 4. Inspect the foam seals around the unit and replace if cracked or compressed.

Dave Kowalski runs window ACs in his rural house rather than central air. His 1955 ranch has no existing ductwork, and the cost to add central AC with full ductwork doesn't pencil out for a house he inherited and plans to eventually sell. He has a 10,000-BTU unit in the primary bedroom, a 6,000-BTU unit in the office, and uses fans and natural ventilation for the rest of the house. On hot nights when the bedroom door is open, the bedroom unit provides some relief to the hallway. On truly brutal days — which happen maybe a dozen times per year in his northern climate — he closes himself into the bedroom-office wing. It's not perfect, but it costs him under $1,000 in equipment and produces comfort where he spends most of his time.

Portable Air Conditioners

Portable air conditioners are freestanding units on casters that you place inside the room, connect to a window via an exhaust hose, and plug in. They require no permanent installation and can be moved between rooms.

The honest truth about portable ACs: They are significantly less efficient and less effective than window units of the same BTU rating. This is physics, not a brand quality issue. Here's why:

A portable AC exhausts its hot condenser air through a single or dual hose to the outside — but the unit itself is inside the room. The exhaust hose is hot and radiates heat into the room. The unit draws condenser cooling air from inside the room; for a single-hose unit, this creates negative pressure that pulls unconditioned air into the house through cracks and gaps to replace the exhausted air. The net result: you're cooling the room with one process and simultaneously adding heat back through another. Effective cooling capacity of a portable unit is typically 40–60% of its rated BTU — a 10,000-BTU portable AC delivers roughly 5,000–6,000 BTU of actual net cooling.

Dual-hose portable units (one hose brings outside air to the condenser, the other exhausts hot air) are better — they don't depressurize the room. If you need a portable unit, a dual-hose model is worth the premium.

When portable ACs are the right choice: - Where window type or HOA rules prohibit window units - For short-term or temporary use where installation of a window unit isn't warranted - For rooms with casement windows or sliding doors where window units don't fit

For permanent use in a rental or home without ductwork, a window unit or mini-split is nearly always a better choice than a portable unit if installation is at all possible.

19.9 System Sizing: Why Bigger Is Not Better

One of the most persistent myths in residential HVAC is that a larger air conditioner cools better. If the house is too warm, the answer must be more cooling capacity. In reality, an oversized AC system creates its own set of serious problems that a correctly sized system avoids entirely.

The Short-Cycling Problem

An air conditioner reaches its peak efficiency after running for several minutes — the refrigerant circuit stabilizes, temperatures equalize, and the system hums along at its rated efficiency. When a system is oversized for the space it's cooling, it reaches the thermostat set point quickly, shuts off, and starts again minutes later. This is called short-cycling.

Short-cycling is bad for several reasons:

Equipment wear. Compressor startup is the most mechanically stressful moment in the refrigeration cycle — high current draw, rapid pressure changes. A compressor that short-cycles 15 times per hour instead of 5 times per hour experiences three times as many startup events and can fail years sooner than its rated lifespan.

Reduced efficiency. Each startup draws extra current. The system never reaches the steady-state efficiency it's rated for. SEER ratings are measured at steady-state operation, not at startup. Frequent startups mean real-world efficiency falls well short of the nameplate rating.

Humidity problems — the most important issue for comfort. This point is critical and often missed. Air conditioning removes humidity from the air by condensing moisture on the cold evaporator coil. Meaningful moisture removal requires sustained runtime — the coil needs to stay cold long enough for significant condensation to occur. A system that runs for 3 minutes and shuts off removes far less moisture per hour than a system that runs for 15 minutes. An oversized AC in a humid climate cools the air to the set point quickly but leaves it humid — you end up with a cold, clammy house that feels uncomfortable even at 72°F because the relative humidity is 65% instead of 50%.

💡 Comfort insight: In humid climates (Southeast U.S., Gulf Coast, Mid-Atlantic, Pacific Northwest), humidity removal is often more important to perceived comfort than temperature alone. A house at 76°F and 45% RH feels more comfortable than the same house at 72°F and 65% RH. Right-sizing your AC for sustained runtime is the mechanism that delivers real humidity control.

Manual J and Manual S: The Right Way to Size Equipment

Proper HVAC sizing uses a calculation called Manual J — the industry-standard procedure developed by the Air Conditioning Contractors of America (ACCA). Manual J calculates the heat gain (for cooling) or heat loss (for heating) of your home based on:

• Envelope characteristics: Insulation R-values, window sizes and types, orientation (which windows get afternoon sun), air infiltration rate
• Internal loads: Number of occupants, appliances, lighting
• Climate data: Design temperatures for your location (the 99th-percentile hot day and cold day for your area)
• Duct losses: Estimated efficiency losses if ductwork is in unconditioned space

The output is a design cooling load in BTUs per hour — the actual peak load your home places on the cooling system. Equipment is then selected to match that load, with Manual S providing guidance on how to interpret equipment performance data at actual operating conditions.

Getting a Manual J done is not expensive. Most HVAC contractors who do proper design work will perform one as part of a system replacement proposal (some charge $100–$300 as a stand-alone service). The calculation takes into account your specific house — not a rule-of-thumb based on square footage alone.

⚠️ The "rules of thumb" problem: Many contractors size equipment by square footage alone — "400 BTU per square foot" or "1 ton per 600 square feet." These rules of thumb are better than nothing but can easily produce oversized or undersized systems because they don't account for insulation quality, window area, orientation, infiltration, or climate. A well-insulated, shaded house needs significantly less cooling capacity than a poorly insulated, west-facing house of identical square footage. If a contractor quotes you a replacement system based solely on the size of the existing unit or a simple square footage estimate without asking about your house's insulation, windows, and conditions — that's a red flag.

The Existing-Unit Trap

When an AC system dies, there's enormous pressure to replace it quickly — in the middle of summer, you're not inclined to wait two weeks for a properly engineered proposal. Many homeowners tell contractors "just put in the same size we had." The problem: the previous unit may itself have been oversized, which is one reason it ran roughly and failed early.

When replacing AC equipment, treat the new installation as an opportunity to get sizing right. If you've added insulation, replaced windows, or sealed air leaks since the original installation, your cooling load may be substantially lower than it was. Right-sizing the replacement reduces both capital cost and operating cost.

19.10 Evaporative Coolers: An Alternative for Dry Climates

In many parts of the country — the American Southwest, the Mountain West, and parts of the Great Plains — the standard refrigerant-based air conditioner faces a meaningful competitor: the evaporative cooler, commonly called a swamp cooler. In the right climate, an evaporative cooler delivers excellent comfort at a fraction of the operating cost.

The Physics of Evaporative Cooling

Evaporative cooling exploits a simple physical process: when liquid water evaporates, it absorbs substantial heat from the surrounding air. The heat doesn't disappear — it's absorbed into the water vapor — but the air temperature drops measurably.

You've felt this: step out of a pool on a dry day and feel the evaporative cooling as the water on your skin evaporates. On a humid day, the same process feels much less dramatic because the air is already saturated with moisture and can't accept more — evaporation slows, cooling diminishes.

This is the critical point about evaporative coolers: they only work well in low-humidity climates. When outdoor relative humidity is below about 50–60%, evaporative cooling is effective. Above 70%, it's largely useless. The American Southwest — Phoenix, Albuquerque, Tucson, Denver, Salt Lake City — has summer relative humidity regularly in the 10–30% range, where evaporative coolers excel. The Southeast and Gulf Coast — Houston, New Orleans, Atlanta, Charlotte — have summer relative humidity regularly above 70%, where evaporative coolers are ineffective.

How Evaporative Coolers Work

A swamp cooler is mechanically simple. A large blower draws outdoor air through water-saturated pads. As the hot, dry air passes through the wet pads, water evaporates and the air cools — typically 15–30°F below outdoor temperature, depending on outdoor conditions. The cooled, now-moist air is blown directly into the living space.

This is a fundamentally different cooling mechanism than a refrigerant-based system. The evaporative cooler adds moisture to the interior air; a refrigerant AC removes it. An evaporative cooler requires that windows be partially open to allow the added air volume to escape — the system works as a continuous air exchange, not as a sealed-loop recirculation like a refrigerant AC.

Types of evaporative coolers:

Whole-house swamp coolers: Large units mounted on the roof or on the side of the house, connected to the home's ductwork. These serve the entire house and are very common in the Southwest — many Phoenix and Tucson homes have roof-mounted evaporative coolers that are the primary cooling system.

Window-mounted units: Similar in placement to a window AC, but operating on the evaporative principle. Good for individual room cooling.

Portable units: Small evaporative coolers for personal or single-room use. Effective as personal comfort devices in dry climates; limited whole-room cooling capacity.

Operating costs. This is where evaporative coolers shine: a whole-house swamp cooler uses 1/4 to 1/3 as much electricity as a comparable refrigerant AC system. The blower motor is the only significant electrical load; there's no compressor. A whole-house unit running 8 hours a day costs $2–$4 per day rather than the $6–$15 per day of refrigerant-based cooling in similar conditions.

Maintenance considerations: Swamp coolers require more routine maintenance than refrigerant AC systems. The water pads (media pads) must be replaced annually (cost: $20–$60 depending on unit size). The water distribution system must be cleaned of mineral deposits (scale buildup from hard water). The sump (water reservoir) must be drained, cleaned, and disinfected at the start and end of each season to prevent mold and Legionella growth. In cold climates, the unit is drained, the pads removed, and the inlet damper closed at the end of the cooling season.

📊 Evaporative vs. Refrigerant Comparison: | Factor | Evaporative | Refrigerant AC | |--------|-------------|---------------| | Upfront cost | $800–$3,500 | $3,500–$8,000 | | Operating cost/day (whole-house) | $2–$4 | $6–$15 | | Effective humidity range | Below 50–60% RH | Any humidity | | Adds or removes humidity | Adds | Removes | | Requires open windows | Yes | No | | Annual maintenance | Moderate | Low-moderate | | Climate suitability | Arid/semi-arid | Universal |

19.11 Geothermal (Ground-Source) Heat Pumps

The standard air-source heat pump discussed in Chapter 18 moves heat between your house and the outdoor air. A geothermal — more precisely, ground-source — heat pump does something subtly different: it moves heat between your house and the ground. That distinction has significant consequences for efficiency.

Why the Ground is a Better Heat Source

Outdoor air temperature in most climates swings wildly — 95°F in July, 15°F in January. The efficiency of an air-source heat pump drops significantly at extreme temperatures: as outdoor air gets colder, it becomes harder to extract heat from it for heating, and the system increasingly relies on backup resistance heat.

The ground, by contrast, maintains a remarkably stable temperature year-round. At a depth of 6–10 feet (below the frost line and the influence of seasonal surface temperature swings), soil temperature in most of the continental U.S. stays between 45°F and 60°F year-round — regardless of whether it's August or January at the surface. In the Midwest, this might be 52°F year-round. In the Southeast, 65°F.

A ground-source heat pump exploits this stable temperature reservoir. In winter, it extracts heat from 52°F ground (easier than extracting from 15°F air). In summer, it dumps heat into 52°F ground (far easier than dumping heat into 95°F outdoor air). This temperature stability translates directly into efficiency: ground-source heat pumps typically achieve COPs (Coefficient of Performance) of 3.0–5.0, meaning 3–5 units of heat energy moved per unit of electricity consumed. High-efficiency air-source heat pumps peak at around 2.5–3.0 COP under ideal conditions, and performance degrades in extreme temperatures.

System Types

Horizontal loop: The most common residential installation where land is available. Polyethylene pipes are buried in horizontal trenches at 4–6 feet depth, typically in loops that may total several hundred feet. A fluid (water or antifreeze solution) circulates through the buried pipes, exchanges heat with the ground, and returns to the heat pump inside the house. Installation requires significant ground disturbance — trenching several hundred to a thousand feet — and available yard space. Cost is moderate compared to vertical loop systems.

Vertical loop: Used where horizontal space is limited (urban lots, heavily landscaped properties). Bore holes are drilled 100–400 feet deep, and pipe loops are inserted in each bore hole. Connecting multiple bore holes provides sufficient loop length. Drilling is expensive — $10–$20 per foot — and a full residential system typically requires 2–6 bore holes. Total drilling cost can reach $10,000–$30,000 before considering the heat pump equipment itself.

Pond/lake loop: If you have a pond, lake, or other substantial body of water on or adjacent to your property, a submerged loop exchanges heat with the water. Water is an excellent heat exchange medium, and installation is often less expensive than buried loops.

Standing column well: Used primarily in New England on properties with deep bedrock. A single bored well serves as both the ground loop — water is drawn from the bottom, circulated through the heat pump, and returned to the top of the well. Simpler than multiple bore holes but requires specific hydrogeological conditions.

Economics and Incentives

Geothermal systems have high upfront costs — total installed cost of $15,000–$40,000 is typical for a residential system, depending on size and loop configuration. Operating costs are significantly lower than any other heating and cooling system type: the combination of high efficiency and stable performance year-round means annual savings of $500–$2,000 per year compared to conventional systems, depending on your climate, house size, and the fuel displaced.

At those savings, payback periods range from 8–20 years — which makes the economics challenging to justify on pure financial grounds, especially if you don't plan to stay in the house long-term. However, several factors improve the calculation:

Federal tax credits: The Inflation Reduction Act provides a 30% federal tax credit for residential geothermal heat pump installation through 2032, with no cap. On a $25,000 installation, that's $7,500 directly off your federal tax bill. This credit substantially changes the economics.

State and utility incentives: Many states and utilities layer additional rebates and incentives on top of federal credits. In some jurisdictions, the combined incentive package covers 40–50% of installed cost.

Long system life: Ground-source heat pumps have longer lifespans than most HVAC systems. The indoor equipment (heat pump unit, air handler) lasts 20–25 years. The buried ground loops, made of HDPE pipe, are rated for 50+ years and are warranted by most manufacturers for 25–50 years. The extended operating life compared to a 15-year central AC system or 20-year furnace improves the long-term economics.

Comfort quality: Geothermal systems, because they operate at moderate, stable conditions rather than cycling between extremes, tend to deliver exceptionally consistent comfort — slow, steady heating and cooling that maintains temperature without dramatic swings.

🔗 Cross-references: Chapter 18 (heating systems, including heat pumps that share the refrigerant cycle), Chapter 20 (ductwork that distributes cooled air), Chapter 12 (electrical systems and the 240V circuits that power AC equipment).

The refrigerant cycle that cools your house is elegant in its physics and robust in practice — these systems routinely run for 15–20 years with only modest attention. Your job as a homeowner is to maintain the airflow (clean filters), keep the outdoor unit clear, and schedule annual professional service. The most expensive AC problems are almost always ones that started small — a dirty coil, a slow refrigerant leak — and went unaddressed for years. Pay attention to how your system sounds and performs; early diagnosis is cheap. Late diagnosis is not.