CENWAT Commissioning and Fault Finding 2

Question 1

A customer reports that radiators are hot at the top but cold at the bottom. What is the most likely cause?

Accepted Answer

Sludge (magnetite) accumulation at the bottom of the radiators

Answer

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?

Accepted Answer

21 mbar

Answer

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?

Accepted Answer

The system is oversized for the heat demand, or circulation is restricted

Answer

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?

Accepted Answer

Check the electrical connections, fuse spur, and whether the motorised valve end switch is making

Answer

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?

Accepted Answer

Zero pressure drop, no drop is permitted

Answer

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.

ACS CENWAT Practice Test

ACS CENWAT Practice Test

CENWAT Commissioning and Fault Finding 2