Heating Equipment and Combustion Systems
This domain tests furnace categories, venting, combustion air, gas pressure, and combustion analysis. Safety-related answers usually win when a question looks ambiguous.
Categories and venting
- Category I appliances are non-condensing and vent into listed B-vent or a lined masonry chimney.
- Category IV condensing appliances (90 percent AFUE and up) vent in PVC or CPVC sloped back toward the furnace, with a condensate trap and often a neutralizer.
- Follow listed clearances to combustibles; single-wall vent connectors need notably more clearance than B-vent.
Combustion air, IMC and NFPA 54
- Indoor air method: 50 cubic feet per 1,000 Btu/h of total input.
- Two permanent openings, each 1 square inch per 1,000 Btu/h when all air comes from outdoors; vertical ducts 1 per 4,000; horizontal ducts 1 per 2,000.
- Derate openings for louver free area, typically 75 percent for metal and 25 percent for wood.
Gas pressure and analysis
- Natural gas: supply 5 to 7 in. w.c., manifold commonly 3.5 in. w.c. LP: supply 11 to 13 in. w.c., manifold commonly 10 to 11 in. w.c.
- Clock the meter to verify input against the rating plate; check temperature rise against the nameplate range, often 35 to 65 F.
- A combustion analyzer reads O2, CO, CO2, stack temperature, and excess air; air-free CO should stay well under 100 ppm.
Worked example
A 1 cubic foot meter dial completes one revolution in 32 seconds. 3,600 / 32 = 112.5 cubic feet per hour. At 1,020 Btu per cubic foot, input is 112.5 x 1,020 = 114,750 Btu/h. Compare that to the rating plate before adjusting manifold pressure.
Traps that cost marks
- Setting manifold pressure with the burners off.
- Sizing combustion air openings without derating for louvers.
- Assuming high CO2 means clean combustion; excess air and CO tell the real story.
- Venting a condensing furnace into an unlined masonry chimney.