Installation, servicing and troubleshooting of gas-fired appliances
Installation
- Confirm the appliance is certified for the gas supplied and for the altitude; check the rating plate for input, manifold pressure, clearances and vent category.
- Maintain clearances to combustibles, service access, and protection from water, freezing, lint and corrosive vapours. Do not install a condensing appliance where the drain can freeze.
- Gas train: manual shutoff valve, union, sediment trap at the appliance, approved connector, and a regulator where required. Check that the appliance connector is not concealed or routed through a wall.
- Vent and combustion air must match the appliance category; verify vent connector slope, supports and termination before firing.
- Start-up: leak test, purge, set manifold pressure with a manometer while operating, clock the meter to confirm input, measure temperature rise and static pressure, and perform combustion analysis for O2, CO2, CO, flue temperature and draft.
Servicing and troubleshooting
- Work in a sequence: interview and verify the complaint, visual inspection, then electrical, gas and airflow checks. Confirm the sequence of operation before replacing parts.
- Combustion analysis: high CO with normal O2 suggests flame impingement, recirculation or a dirty burner; high O2 and low CO2 suggest excess air; low O2 with high CO2 suggests insufficient combustion air.
- Temperature rise above the rating plate range points to low airflow: filter, blower, evaporator coil, duct static pressure or blower speed. Correct airflow rather than lowering manifold pressure.
- Delayed ignition points to dirty or misaligned burners, wrong orifices, low pressure, or a weak igniter. Short cycling can be high limit, thermostat, flame sensor, pressure switch or venting.
Worked example: a furnace runs five minutes and trips the high limit. Manifold pressure is 3.4 in w.c. against a 3.5 in w.c. rating; temperature rise is 75°F against a 35–65°F range; supply static is 0.9 in w.c. against a 0.5 in w.c. maximum. The low gas pressure is not the cause of the limit trip. The high temperature rise and high static pressure show restricted airflow. Check filter, coil, blower wheel and duct restrictions, then recheck rise and CO.
Traps: adjusting manifold pressure to cure limit trips; measuring manifold pressure with the burner off; replacing safety devices instead of finding the cause; assuming a new gas valve fixes an ignition fault; not checking clocked input.