Venting and Combustion Air
Venting and Combustion Air covers how flue gases leave the appliance and how replacement air enters. Questions test vent category, clearances, termination, slope, combustion-air sizing, and the draft and CO checks used on service calls.
Vent categories and materials
- Category I appliances are naturally drafted and operate under negative pressure; they use Type B vent or a listed chimney liner. Single-wall vent connectors need 6 in. clearance to combustibles; Type B vent needs 1 in.
- Category III and IV appliances are positive-pressure and condensing; they use listed stainless or PVC/CPVC venting per the manufacturer. Never substitute PVC for a Category I natural-draft appliance.
- Vent connectors slope upward toward the chimney at about 1/4 in. per foot, are supported, and are as short and direct as possible.
Termination
A vent must terminate outdoors, clear of windows, doors, intakes, and combustibles. A common rule is at least 3 ft above a flat roof and 2 ft above any structure within 10 ft; always confirm the code table and manufacturer's instructions for the specific appliance. Keep terminals clear of snow and decks and away from bathroom or dryer exhausts that can cause recirculation.
Combustion air
Confined spaces need two permanent openings, one near the top and one near the bottom. For outdoor air, a common sizing rule is 1 in² of free area per 4,000 Btu/h; for indoor air, 1 in² per 1,000 Btu/h, with the space providing 50 ft³ per 1,000 Btu/h. Louvres reduce free area, so size for the louvre's free-area percentage, not the rough opening.
Worked example
A 100,000 Btu/h furnace sits in a 20 ft × 15 ft × 8 ft room = 2,400 ft³. Required volume = 100,000 ÷ 1,000 × 50 = 5,000 ft³, so the room is confined. Using outdoor air, each opening needs 100,000 ÷ 4,000 = 25 in² free area. With a louvre rated 75% free area, the gross opening is 25 ÷ 0.75 = 33.3 in², so use at least 34 in².
Traps
- Treating gross louvre area as free area.
- Terminating under a deck or beside an intake.
- Reducing vent diameter to fit a chase.
- Ignoring depressurization from exhaust fans.
- Assuming a draft hood guarantees safe draft; always spill-test and CO-test.
Basic Gas Appliance Installation and Servicing
Basic Gas Appliance Installation and Servicing tests the practical sequence: set the appliance, connect gas and venting, verify pressures and input, check combustion and temperature rise, and service common components. The exam favours questions about rating plates, manifold pressure, clocking, and flame-safety devices.
Installation checks
- Read the rating plate first: gas type, input, manifold pressure, clearances, vent category, and temperature-rise range. Confirm the appliance is certified and the orifice is correct for the gas supplied.
- Provide a shutoff valve, union, sediment trap, and a drip leg ahead of the controls. Level the appliance, maintain clearances, and connect the vent and combustion air before firing.
- Check inlet pressure with the appliance running: typically 7 in. w.c. for natural gas and 11 in. w.c. for propane. Set manifold pressure with a manometer: commonly 3.5 in. w.c. for natural gas and 10 in. w.c. for propane, but always follow the plate.
Servicing sequence
- Visually inspect the heat exchanger, burners, vent, and air openings.
- Leak-test the gas train; check inlet and manifold pressure under load.
- Clock the meter to verify input.
- Measure temperature rise and compare it with the rating plate.
- Perform a combustion test for CO, CO2, and draft; spill-test natural-draft appliances.
- Clean burners and the flame sensor, check ignition and flame supervision, and replace thermocouples or thermopiles with correct types.
Worked example
A furnace rated 60,000 Btu/h at 80% output delivers about 48,000 Btu/h. Airflow is 1,100 cfm and the measured temperature rise is 45°F. Output = 1,100 × 1.08 × 45 = 53,460 Btu/h. That is well above the rated output, so the unit is overfiring or airflow is low; check manifold pressure, orifice size, and blower or filter before adjusting anything.
Traps
- Reading manifold pressure with the burner off.
- Adjusting a gas valve by flame appearance instead of a manometer.
- Fitting a propane appliance with natural gas orifices.
- Overtightening a thermocouple and cracking the insulator.
- Cleaning a flame sensor with coarse sandpaper or leaving fingerprints.
- Ignoring a CO reading because the flame 'looks blue'.