Chapter 18 Key Takeaways: How Heating Systems Work

The Physics Foundation

Heat moves by three mechanisms: conduction (through materials in contact), convection (carried by moving fluids — including air), and radiation (as electromagnetic waves). Every heating system exploits one or more of these. Knowing which one your system relies on helps you diagnose failures.

Gas Furnaces

  • The combustion sequence has five stages: inducer pre-purge, ignition, gas valve opens, heat exchanger warms, blower starts. Each step has a safety interlock.
  • Modern furnaces use hot surface igniters or intermittent pilots — no standing pilot light (which wasted $5–15/month continuously).
  • AFUE (Annual Fuel Utilization Efficiency) is the key efficiency metric. Standard furnaces: 80% AFUE. High-efficiency condensing units: 90–98% AFUE.
  • A condensing furnace vents through PVC pipe (not a masonry chimney) and produces condensate water — both of these are normal and correct.

The Critical Safety Issue: Heat Exchangers

  • The heat exchanger is the sealed metal barrier separating combustion gases from household air. It must not be cracked or perforated.
  • A cracked heat exchanger allows carbon monoxide (CO) into the household airstream. CO is odorless and cannot be detected without a monitor.
  • Warning signs: yellow/orange burner flame, visible soot outside the combustion area, flame rollout, CO detector activation.
  • A cracked heat exchanger cannot be repaired — the furnace must be replaced. This is non-negotiable.
  • Annual service must include opening access panels and visually inspecting the heat exchanger. If your technician doesn't do this, find one who will.

Hot Water Boilers

  • Boilers heat water and circulate it through pipes to radiators or baseboard units — no ductwork required.
  • Advantages: even, comfortable heat; easy to zone; very long equipment life (25–35+ years).
  • Key components: circulators, zone valves, expansion tank, pressure relief valve.
  • Expansion tank failure and inadequate system pressure are the most common maintenance issues.
  • Bleeding trapped air from radiators is a simple homeowner task.

Heat Pumps

  • A heat pump moves heat rather than generating it — it extracts heat from outdoor air and delivers it inside via a refrigerant cycle.
  • COP (coefficient of performance) can be 2.0–4.0+, meaning 2–4 BTUs of heat per BTU of electricity.
  • The balance point is the outdoor temperature below which aux heat is needed. Modern cold-climate units maintain heating capacity to -13°F.
  • Aggressive thermostat setbacks (large overnight drops) are counterproductive — recovery may trigger expensive aux heat.
  • Annual service by an HVAC technician familiar with heat pumps is worthwhile; refrigerant work requires EPA 608 certification.

Oil and Propane Systems

  • Oil furnaces and boilers work similarly to gas equipment but require annual nozzle and electrode replacement.
  • Underground oil tanks are a serious liability — corrosion can cause leaks with $20,000–100,000+ cleanup costs.
  • Propane is heavier than air and accumulates in low areas — never ignore a propane smell.
  • Low propane tank levels (below 20%) can cause pressure problems in cold weather.

Electric Heating

  • Electric resistance heat (baseboard, radiant) is 100% efficient but expensive to operate — electricity costs more per BTU than gas in most areas.
  • Best applications: supplemental heat in specific rooms, areas with low electricity rates, locations where gas supply isn't available.
  • A heat pump is dramatically cheaper to operate than electric resistance heat (3x less or more) — if you're heating with electricity, a heat pump is almost always the better investment.

Safety Summary

🔴 Immediate action required: - CO detector alarming with furnace running - Gas smell near furnace or meter (evacuate; call gas company from outside) - Yellow/orange burner flames - Flame rollout visible from burner area - Confirmed cracked heat exchanger

⚠️ Schedule professional service promptly: - Furnace that short-cycles (turns on and off frequently) - Delayed ignition banging sound - Furnace over 20 years old with no documented heat exchanger inspection - Propane smell (any amount — investigate)

The Annual Rhythm

Schedule professional heating system service in September/October — before heating season. Test CO detectors when clocks change in fall. The cost of annual maintenance is $80–150; the cost of deferred maintenance is measured in thousands of dollars and potentially lives.