CENWAT Commissioning and Fault Finding 2 — Questions and Answers
Question 1: A customer reports that radiators are hot at the top but cold at the bottom. What is the most likely cause?
- Air trapped in the system
- Sludge (magnetite) accumulation at the bottom of the radiators (Correct answer)
- The pump is running too fast
- The thermostatic radiator valve is faulty
Correct answer: Sludge (magnetite) accumulation at the bottom of the radiators
Radiators that are hot at the top but cold at the bottom typically indicate a build-up of iron oxide sludge at the bottom of the radiator. The sludge settles due to gravity and restricts water flow through the lower sections. Treatment involves a system powerflush or removal and manual flushing of affected radiators.
Question 2: What is the correct gas operating pressure (working pressure) at the meter for natural gas in the UK?
- 10 mbar
- 21 mbar (Correct answer)
- 37 mbar
- 50 mbar
Correct answer: 21 mbar
The standard operating pressure for natural gas at the meter outlet in the UK is 21 mbar with a tolerance of plus or minus 2 mbar, so 19-23 mbar is acceptable. If the working pressure drops below 19 mbar with all appliances running, this indicates a problem with the gas supply.
Question 3: A condensing boiler is cycling on and off frequently (short cycling). What is the most likely cause?
- The flue is too long
- The system is oversized for the heat demand, or circulation is restricted (Correct answer)
- The gas pressure is too low
- The condensate pipe is blocked
Correct answer: The system is oversized for the heat demand, or circulation is restricted
Short cycling typically occurs when the boiler reaches its set temperature too quickly, either because the boiler is oversized for the property heat demand or because circulation is restricted with TRVs closed on most radiators without an adequate bypass. Solutions include checking the bypass valve and verifying boiler modulation range.
Question 4: What should you check first if a room thermostat is calling for heat but the boiler does not fire?
- Replace the boiler immediately
- Check the electrical connections, fuse spur, and whether the motorised valve end switch is making (Correct answer)
- Check if the gas meter is prepayment
- Replace the thermostat batteries only
Correct answer: Check the electrical connections, fuse spur, and whether the motorised valve end switch is making
If the thermostat is calling for heat but the boiler does not fire, the first step is to check the electrical circuit. Verify the fused spur is on, check wiring connections, and critically check if the motorised valve is opening and its end switch is making. If the valve does not fully open, the end switch will not close the circuit to energise the boiler.
Question 5: When conducting a gas tightness test on a domestic central heating installation, what is the acceptable pressure drop over the test period?
- 5 mbar in 2 minutes
- Zero pressure drop, no drop is permitted (Correct answer)
- 1 mbar in 2 minutes
- 4 mbar in 1 minute
Correct answer: Zero pressure drop, no drop is permitted
A gas tightness test on a domestic installation requires zero pressure drop over the prescribed test period. The system is pressurised to 20 mbar on the installation side of the meter and monitored. Any pressure drop indicates a leak that must be found and repaired.
Question 6: What does it indicate if a flue gas analyser shows oxygen readings significantly higher than expected, for example above 9%?
- The boiler is operating perfectly
- There is excess air entering the combustion chamber or a flue leak (Correct answer)
- The gas valve is stuck open
- The system water temperature is too low
Correct answer: There is excess air entering the combustion chamber or a flue leak
Significantly elevated O2 readings in the flue gas indicate excess air in the combustion process. This can be caused by a cracked heat exchanger, a poorly sealed flue connection, incorrect burner pressure, or damaged seals. Excess air reduces efficiency and can mask dangerous CO levels by diluting them.
CENWAT Commissioning and Fault Finding 2
A customer reports that radiators are hot at the top but cold at the bottom.
What is the most likely cause?