Heating Equipment and Combustion Safety
Combustion fundamentals
Perfect combustion of natural gas (roughly 1,000–1,050 Btu/ft³) needs about 10 ft³ of air per ft³ of gas; real appliances run 25–50% excess air. Products of complete combustion are CO₂, water vapor, and heat; incomplete combustion yields CO, a deadly and odorless gas. A combustion analyzer reports O₂, CO, CO₂, stack temperature, and air-free CO - the number that matters, because dilution can mask a problem. Steady-state efficiency ≈ (gross stack loss from temperature and CO₂/O₂) subtracted from 100.
Safety devices and venting
Know the function of the flame sensor, thermocouple, hot-surface igniter, rollout switch, high limit, pressure switch (proves draft on induced-draft units), and draft hood/spill switch. Category I appliances vent into a chimney with draft; Category IV condensing units vent with PVC under positive pressure. A blocked flue, disconnected vent, or backdrafting water heater spills CO into the space - test draft with a smoke or draft gauge and check for spillage at the hood.
Traps
Confusing manifold pressure specs (natural gas typically 3.5 in. wc, LP about 10–11 in. wc) and forgetting LP appliances need different orifices and a regulator. Also: sizing combustion air openings (50 ft³ per 1,000 Btu indoor; two permanent openings for confined spaces), ignoring altitude derating, and reading CO in diluted flue gas instead of air-free CO.
Worked example
Clock the meter: 2 ft³ of gas in 60 seconds = 120 ft³/hr. At 1,050 Btu/ft³, input = 120 × 1,050 = 126,000 Btu/hr - compare to the rating plate to confirm the orifices and pressure.