Heating Systems (Gas, Oil, Electric, and Heat Pumps)
Gas and oil fundamentals
Combustion requires fuel, oxygen, and ignition in the right ratio. Natural gas manifold pressure is typically 3.5 in. w.c.; propane is about 10–11 in. w.c. Know the ignition sequence: call for heat, inducer proves draft via pressure switch, ignition source energizes, gas valve opens, flame proves through flame sensor, blower delays on. Trap: a dirty flame sensor causes short cycling, not a no-heat lockout with no flame. Oil systems use a pump at roughly 100 psig, a nozzle rated in GPH, electrodes set to a specific gap, and a cad cell to prove flame.
Electric and heat pump heat
Electric resistance heat uses sequencers or contactors to stage elements. Heat pumps move heat rather than create it: COP and HSPF describe efficiency, balance point is where capacity equals building load, and auxiliary/emergency heat makes up the difference. Defrost cycles periodically reverse the valve or shut the outdoor fan to melt frost. Trap: emergency heat locks out the compressor - it is not the same as auxiliary heat.
Temperature rise
The single most useful furnace check is temperature rise: return-air temperature subtracted from supply-air temperature must fall inside the data-plate range. High rise means low airflow; low rise means excess airflow or overfiring.
Rule of thumb and worked example
CFM ≈ Btu output ÷ (1.08 × temperature rise). An 80,000 Btu/h input furnace at 80% AFUE delivers 64,000 Btu/h. If measured rise is 40°F, airflow ≈ 64,000 ÷ (1.08 × 40) ≈ 1,480 CFM. An out-of-range rise always sends you back to the blower chart and duct static pressure.