Heating Systems: Combustion, Furnaces, Boilers, and Hydronics
This domain covers combustion, furnace and boiler operation, hydronic piping, and efficiency. The exam tests excess air, draft, condensing versus non-condensing equipment, and hydronic components.
Combustion and efficiency
Complete combustion of natural gas needs about 10 ft³ of air per 1,000 Btu input; excess air ensures complete burning. Combustion analyzers report O₂, CO, CO₂, and flue temperature. AFUE is annual fuel utilization efficiency, not just steady-state combustion efficiency. Condensing appliances recover latent heat, reach 90%+ AFUE, and require a condensate trap and corrosion-resistant venting.
Hydronic basics
A closed loop needs a circulator, expansion tank, air elimination, relief valve, and properly sized piping. The relief valve protects against overpressure; its discharge must terminate safely. Primary/secondary piping decouples boiler flow from system flow.
Worked example
A furnace has 100,000 Btu/h input and 80% AFUE. At 1,000 full-load hours, output is 80,000 Btu/h × 1,000 h = 80 MMBtu. Input required = 80 ÷ 0.80 = 100 MMBtu, or 1,000 therms. At $1.20/therm, fuel cost is $1,200. A 95% AFUE unit would use about 842 therms.
What costs marks
- Confusing AFUE with combustion efficiency.
- Omitting the condensate trap or using wrong vent material on a condensing furnace.
- Returning water below about 130°F to a non-condensing boiler, causing flue-gas condensation and corrosion.
- Plugging a relief valve or piping discharge where it can scald.
- Ignoring draft and combustion air; a tight basement can backdraft appliances.