Testing, Commissioning, Servicing and Troubleshooting
Domain 5 - Testing, commissioning, servicing and troubleshooting
This domain tests whether you can prove an appliance is working correctly and diagnose it methodically when it is not. Exam questions favour sequence-of-operation logic, measured values, and the difference between a symptom and a cause.
Commissioning checklist
- Confirm the installation, then leak-test at operating pressure with soap solution or an electronic detector.
- Purge air from new piping to a safe location before lighting.
- Set manifold pressure with a manometer at the pressure tap, with the appliance firing and the correct orifices installed.
- Clock the meter to verify input against the rating plate; if it is off, check orifice size and pressure before touching the gas valve.
- Measure temperature rise (supply minus return) at steady state and compare with the nameplate range.
- Perform combustion analysis: O2/CO2, stack temperature, draft and CO air-free. Rising CO or rising stack temperature means investigate, not adjust.
- Prove the safeties: flame sensing, pressure switch, high limit, rollout switch, spill switches and CO alarms where required.
Servicing and diagnosis
- Flame pattern should be steady blue with a defined inner cone. Lifting, flashback, yellow tips or soot point to primary air, dirty burners, wrong orifice or low pressure.
- Spillage or rollout means stop. Check flue, heat exchanger, combustion air and building pressure before replacing parts.
- Thermocouples read roughly 10–30 mV open circuit; a pilot that holds when jumpered but not otherwise is usually a dirty pilot or weak thermocouple, not the valve.
- Flame rectification normally produces 1–10 µA. Low microamps come from a dirty sensor, poor flame impingement or a bad ground.
Worked example
A furnace lights, runs about 6 s and locks out, with normal manifold pressure and a good flame. The flame sensor measures 0.6 µA where the manufacturer expects 2–6 µA. Cleaning the sensor with steel wool and cleaning the burner ground strap brings it to 4 µA and the furnace runs continuously. Replacing the gas valve would have been wasted money: the valve was closing because the control had lost proven flame.
Traps
Turning manifold pressure up to "cure" CO; clocking the meter while other appliances are running; taking temperature rise before the appliance stabilises; condemning a gas valve on a flame-sensing fault; and leaving an appliance running with spillage because it passed last year.