Heating Equipment and Systems (Furnaces, Boilers, Heat Pumps)
Efficiency, venting, and delivered heat
Gas furnaces are rated by AFUE, which accounts for cycling and jacket losses, not just steady-state combustion efficiency. Condensing units recover latent heat (category IV venting, plastic flue, acidic condensate with a trap and neutralizer); non-condensing units (category I) need a properly sized chimney with draft and cannot be sidewall vented without a listed kit. Verify the temperature rise at every start-up: high rise means low airflow; low rise means excess airflow or overfiring. Manifold gas pressure and combustion analysis confirm the fuel side.
Hot water boilers require an expansion tank, air elimination, a relief valve piped to a safe drain, a low-water cut-off where required, and correct circulator sizing. Outdoor reset and primary/secondary piping improve performance but must respect minimum return temperatures to prevent condensing damage in non-condensing boilers.
Heat pumps are rated at 47°F and 17°F; capacity falls with outdoor temperature while building load rises. The balance point is where the two curves cross. Below it, supplementary resistance heat carries the difference, with a lockout to prevent unnecessary auxiliary use and a defrost cycle that temporarily reverses the cycle.
Traps: comparing AFUE to steady-state efficiency; sizing a heat pump from the cooling load without checking heating capacity at design temperature; ignoring aux-heat lockout settings; forgetting the condensate drain freeze risk on condensing furnaces.
Worked example: 80,000 Btu/h output at 1,200 CFM → ΔT = 80,000 ÷ (1.08 × 1,200) ≈ 62°F rise.