Heating Systems and Equipment
This domain covers furnaces, boilers, heat pumps and resistance heat: efficiency ratings, airflow verification, combustion air and venting, and how heat pumps balance capacity against building load.
Core rules
- AFUE is seasonal efficiency; input x AFUE = output. Furnace airflow: CFM = output / (1.08 x temperature rise).
- Condensing furnaces (90%+ AFUE) produce acidic condensate, need PVC or approved venting, and must be pitched back to the unit.
- Gas input derates roughly 4 percent per 1,000 ft above 2,000 ft, per the manufacturer.
- Heat pump balance point is where capacity equals load; below it, supplementary heat carries the difference. Defrost cycles reduce net capacity in cold, humid weather.
- Hydronic systems need an expansion tank, air elimination, correct circulator sizing and often outdoor reset to match supply water temperature to load.
Worked example
A furnace has 100,000 Btu/h input at 92 AFUE, so output = 92,000 Btu/h. At a 45F temperature rise, required airflow = 92,000 / (1.08 x 45) = 1,893 CFM. If a technician measures a 60F rise instead, airflow is only 92,000 / (1.08 x 60) = 1,420 CFM: the unit is under-aired, so check the filter, blower speed tap and duct restriction before touching gas pressure. A high rise also overheats the heat exchanger and trips the limit.
Traps that cost marks
- Confusing input with output, or AFUE with steady-state combustion efficiency.
- Setting manifold pressure to cure an airflow problem.
- Using single-wall metal vent for a condensing appliance, or PVC where the listing forbids it.
- Ignoring combustion air requirements in tight enclosures.
- Assuming heat pump aux heat should be locked out at the balance point rather than at a lower economic setpoint.