Heating Systems and System Performance Study Guide
This domain, Heating Systems and System Performance, tests combustion heating, electric heat, heat pumps, and the performance checks that prove safe, rated operation. Expect gas pressure, combustion air and venting, ignition and flame sensing, temperature rise, airflow, limits, AFUE, heat-pump balance point, defrost, and auxiliary heat.
Core rules and concepts
- Natural-gas manifold pressure is typically about 3.5 in. w.c. and LP about 10–11 in. w.c., but always use the rating plate. Measure with a manometer; clock the meter only with other appliances off.
- Temperature rise = supply-air temperature minus return-air temperature. It must fall within the rating-plate range. Low airflow raises the rise; high airflow lowers it.
- Furnace output ≈ 1.08 × CFM × ΔT for standard air. Compare measured output with rated input times efficiency.
- Flame rectification proves flame; a dirty flame sensor reduces microamps and causes lockout. Combustion air, draft, and CO testing are safety-critical.
- Heat pumps need defrost, a balance point, and correctly staged auxiliary heat. AFUE rates furnaces; HSPF and COP rate heat pumps.
Worked example
A furnace is rated 60,000 Btu/h input at 80% AFUE, so output ≈ 48,000 Btu/h. Measured airflow is 1,000 CFM and temperature rise is 45°F. Output ≈ 1.08 × 1,000 × 45 = 48,600 Btu/h, close to rating. If the rise were 60°F, calculated output would be near 64,800 Btu/h-impossible for this furnace-so suspect low airflow or a measurement error. Check filter, blower speed, and duct static before adjusting gas pressure.
Traps
Adjusting manifold pressure to fix temperature rise; clocking the gas meter with other appliances running; measuring return temperature too close to the furnace; ignoring CO and draft; using AFUE as a heat-pump efficiency rating; energizing auxiliary heat without checking capacity.