Case Study 22-2: The Risks You Can't See — Dave Kowalski's CO Safety Audit

Background

Dave Kowalski heats with propane. In rural Michigan, where natural gas lines don't run, propane is the practical fuel choice for heating, cooking, and his water heater. Dave is a practical man: he handles most of his own home maintenance, keeps a serviceable shop, and doesn't spend money unless the problem is real.

He reads Chapter 22 of this textbook — specifically Section 22.7 on carbon monoxide and combustion appliance safety — and pauses.

He has one CO detector. It's in the hallway outside his bedroom. He cannot remember when he bought it.

He looks at the back of the detector: no expiration date visible, but a manufacture date of 2016. Nine years ago. CO detector sensor lifespan: 5–7 years. He is living with a CO detector whose sensor is almost certainly degraded past reliable operation.

He looks at the rest of his combustion appliance inventory: - 2007 propane furnace in the basement - 2011 propane water heater in the basement - Propane range/oven in the kitchen (cooking) - Propane fireplace insert in the living room (used occasionally in winter) - Attached one-car garage

He has propane combustion on three levels of a relatively tight house, an attached garage, and a CO detector that may no longer work.

The CO Detector Upgrade

Dave's first action is straightforward. He buys three combination CO/smoke detectors: - One for the basement, near the furnace and water heater - One for the main floor, near the kitchen - One for the hallway outside his bedroom (replacing the expired unit)

He selects Kidde units with a 10-year sealed lithium battery — these don't require battery replacement and have a hardwired expiration date that will remind him to replace the entire unit in 2035. Total cost: $105 for three units.

He tests all three by pressing the test button: all alarm. He notes the alarm sound — he wants to know what it sounds like before it sounds at 2 AM.

The Furnace Inspection Discovery

With CO detection addressed, Dave schedules the annual furnace inspection he's been deferring. A local HVAC service company sends a technician who has been servicing residential propane systems for 22 years.

The technician runs through the standard checklist: combustion analysis (CO in flue gas at normal levels), burner inspection (burners clean, good flame pattern), heat exchanger inspection.

The heat exchanger inspection — done with a mirror and flashlight, looking for cracks or holes in the metal — finds a hairline crack in one of the secondary heat exchanger cells. The crack is small, visible only under good lighting. The technician demonstrates: he holds a smoke pencil near the crack with the furnace running. The smoke is drawn in, confirming the crack creates a pressure differential that could allow combustion gases to enter the supply air stream.

The furnace is 18 years old. This is not unexpected — thermal fatigue is the primary failure mode for heat exchangers, and 18 years of heating cycles is a long life. The technician shuts down the furnace: operating with a cracked heat exchanger is not an option.

Dave is without heat in November.

The Emergency Decision

The technician gives Dave options: 1. Replace the heat exchanger: $800–$1,200 parts and labor, if a replacement heat exchanger is still available for an 18-year-old unit. No guarantee of the rest of the furnace. 2. Replace the furnace: $3,500–$4,500 for a new 95% AFUE propane unit.

Dave calls one additional company for a second opinion. They confirm the crack assessment. Their quote for a new furnace: $3,800 for a 96% AFUE unit, installed, permitted, and inspected.

Dave uses the Rule of 5000 from Chapter 23: $1,000 repair cost estimate × 18 years = 18,000. Well above $5,000. Plus: parts availability is uncertain. He approves the replacement.

The new furnace is installed the next day (emergency scheduling, premium rate). Dave is back to heat within 36 hours.

The Retrospective: What Could Have Happened

Dave thinks through the scenario:

If he had not had the furnace inspected — which he'd been putting off because the furnace "seemed fine" — he would have continued operating a furnace with a cracked heat exchanger throughout the winter. With only one expired CO detector in the hallway (and one that likely no longer responded accurately), he might have had no effective warning if combustion gases infiltrated the supply air.

Propane combustion products include CO, CO2, and water vapor. With a cracked heat exchanger, even small amounts of CO entering the supply air and being distributed through the house represent a significant health risk. The insidious nature of CO exposure — headaches and fatigue that might be attributed to illness — means weeks could pass without recognizing the source.

Dave does not typically consider himself at risk for "carbon monoxide poisoning" — that happens to people who run generators indoors or make obvious mistakes. But a cracked heat exchanger is exactly the "maintenance neglect" scenario that accounts for a significant portion of residential CO poisoning cases. No obvious mistake. Just an aging appliance that crossed a threshold.

The Water Heater and Garage Assessment

With the furnace issue resolved, Dave continues the appliance audit.

Water heater (2011 propane, atmospheric vent): Dave inspects the flue connector — the metal duct from the water heater draft hood to the chimney liner. One of the three sheet metal screws securing a section has backed out; the section is slightly misaligned. He repositions it, re-secures with a new screw, and tapes the joint with UL-listed foil tape. The draft hood's draft is tested by holding a match near it when the burner is running — flame is drawn in (toward the appliance), indicating proper draft. Good.

Attached garage: Dave was aware that vehicle exhaust in an attached garage can infiltrate the house — but he hadn't mapped the pathways. He checks the door between the garage and the house: solid wood, reasonably weatherstripped. He checks the walls shared with the house at the garage ceiling: there are three penetrations where wires and a water line pass through, none sealed. He also finds a gap around the water line entry that he can feel air movement through.

He seals all three penetrations with fire-rated intumescent caulk (required for garage-to-house penetrations by code for fire safety as well as air quality) and adds door sweep to the interior garage door. This addresses both CO from vehicles and general air infiltration.

Fireplace insert: Dave looks up his propane fireplace model and finds it's a direct-vent unit — sealed combustion, draws combustion air from outside and vents combustion gases outside, with no connection to indoor air. This is the correct type for indoor use. No action needed, though the glass gasket seal should be inspected annually.

Annual Maintenance Protocol

From this audit, Dave establishes an annual protocol:

Each fall before heating season: - Schedule furnace inspection and tune-up (the heat exchanger crack reinforces this — he'll never skip it again) - Test all CO/smoke detectors, verify expiration dates - Inspect water heater flue connector for rust, separation, or misalignment - Check and clean propane range burner grates and burner ports - Verify direct-vent fireplace glass gasket and exterior termination cap is unobstructed

Each spring: - Check the exterior termination caps on all direct-vent appliances and the furnace exhaust pipe (for wasp or bird nests, which can block exhaust and cause back-drafting) - Inspect garage-to-house penetrations for any new gaps

Total annual cost of this protocol: approximately $150 for the furnace inspection/tune-up, 30 minutes of homeowner inspection time. This is not optional maintenance. For a homeowner who heats with propane in a tight rural house, it's life-safety work.

The CO Detector Test Scenario

Three months after the detector upgrades, Dave wakes at 3:30 AM to a CO alarm. Both the basement detector and the main floor detector are alarming. He follows the plan: gets up, wakes no one because he lives alone, grabs his phone from the nightstand, exits through the front door, and calls 911 from his driveway.

Firefighters arrive within 12 minutes with a CO meter. They find a reading of 35 ppm at the main floor (the EPA 8-hour limit is 9 ppm; OSHA 8-hour limit is 50 ppm). The source: the propane fireplace Dave had used the evening before. He'd noticed the flame seemed lower than usual but hadn't investigated. The exterior exhaust termination cap had become partially blocked by a bird's nest built since spring. Combustion was incomplete and back-drafting slightly.

Firefighters clear the house, remove the bird nest from the termination cap (not a difficult fix), and verify that CO levels drop to safe levels once the house is ventilated. No one is harmed.

Dave notes: 35 ppm over a sleeping period of several hours would have produced symptoms — headache, fatigue — that he might have attributed to a cold. His new detectors alarmed at 35 ppm, before levels reached a truly dangerous threshold. The old expired detector may not have alarmed until levels were significantly higher.

The $105 for three CO detectors was the most important purchase he'd made all year.