Heating Fundamentals and Equipment
What this domain covers
The Heating Fundamentals and Equipment domain tests heat production, combustion, efficiency ratings, and the airflow that moves heat into the conditioned space. In Hawaii most heating work is commercial or at higher elevation, but the exam still uses standard mainland fundamentals, so do not skip gas, electric, hydronic, or heat-pump basics.
Core rules
- Input vs. output. A furnace's input is the gas energy rate; output is what reaches the air. AFUE is the annual efficiency, not a steady-state combustion number.
- Condensing vs. non-condensing. An 80% furnace vents with a metal flue and dilution air; a 90%+ condensing furnace has a secondary heat exchanger, PVC/CPVC venting, and acidic condensate that must drain and neutralize where required.
- Combustion safety. Manifold pressure is roughly 3.5 in. w.c. for natural gas (LP is different); the flame sensor proves flame, the rollout switch proves no flame outside the heat exchanger, the pressure switch proves draft, and the high limit protects against low airflow.
- Heat delivery. Sensible heat equation: Btu/h = 1.08 × cfm × ΔT at sea level. The nameplate temperature rise is the field check.
- Heat pumps and electric heat. COP and HSPF2 rate heat-pump efficiency; balance point is where capacity equals load, below which auxiliary heat runs. Electric resistance delivers 3.41 Btu per watt.
Worked example
A furnace has 80,000 Btu/h input and 80% AFUE, so output is 64,000 Btu/h. Return air is 68°F and supply air 118°F, so ΔT = 50°F. CFM = 64,000 ÷ (1.08 × 50) = 1,185 cfm. The nameplate range is 35–65°F, so 50°F is acceptable; if the measured ΔT were 80°F, suspect low airflow (dirty filter, wrong blower tap, crushed duct) before condemning the heat exchanger.
Traps that cost marks
- Using input instead of output in the airflow equation.
- Replacing a high-limit switch when the real cause is restricted return air.
- Calling a furnace 90% efficient while the flue is still metal and not condensing.
- Confusing auxiliary heat (normal second stage) with emergency heat (manual lockout of the compressor).
- Quoting cfm/ton for heating; that rule is a cooling approximation, not a heating check.