CENWAT Flue Requirements & Pressure Relief 3 — Questions and Answers
Question 1: If a PRV repeatedly discharges on a sealed central heating system, what is the most likely cause?
- Waterlogged or failed expansion vessel (Correct answer)
- Blocked PRV seat
- Insufficient system water
- Gas pressure too high
Correct answer: Waterlogged or failed expansion vessel
Repeated PRV discharge usually indicates a waterlogged or failed expansion vessel that can no longer absorb system expansion, causing pressure to rise and exceed the PRV set point on every heat cycle.
Question 2: How should the expansion vessel on a sealed system be checked?
- Turn off and drain the system, check and adjust the pre-charge pressure of the expansion vessel using a tyre pressure gauge to match the system cold fill pressure (Correct answer)
- Simply check for external corrosion
- Test it by raising system pressure to 3 bar
- Replace it every 5 years without testing
Correct answer: Turn off and drain the system, check and adjust the pre-charge pressure of the expansion vessel using a tyre pressure gauge to match the system cold fill pressure
The expansion vessel must be tested with the system drained (or isolated and water pressure released on the vessel side) by checking the nitrogen pre-charge pressure and recharging to the system cold fill pressure if low.
Question 3: What is the purpose of a filling loop on a sealed central heating system?
- To allow the system to be filled or topped up with mains water to the correct cold fill pressure (Correct answer)
- To drain the system
- To fill the system with inhibitor
- To connect a pressure gauge for commissioning
Correct answer: To allow the system to be filled or topped up with mains water to the correct cold fill pressure
A filling loop provides a temporary connection between the cold mains supply and the heating system, allowing water to be introduced to achieve the correct cold fill pressure. It must be disconnected after use.
Question 4: Why must a filling loop be disconnected after use?
- To prevent backflow from the heating system contaminating the mains water supply (Correct answer)
- Because it reduces system pressure
- Because it causes air ingress if left connected
- Because it is a legal requirement of Part L
Correct answer: To prevent backflow from the heating system contaminating the mains water supply
A connected filling loop could create a direct connection between the (contaminated) heating system water and the potable mains supply, risking backflow contamination — it must be disconnected to provide an air gap.
Question 5: What pressure should a sealed system be set to when cold (before heating)?
- As specified by the boiler manufacturer, typically 1–1.5 bar (Correct answer)
- 3 bar (same as PRV setting)
- 0.5 bar
- 0.1 bar
Correct answer: As specified by the boiler manufacturer, typically 1–1.5 bar
The cold fill pressure is set per the manufacturer's recommendation, typically 1–1.5 bar, which allows the pressure to rise safely as the system heats up without reaching the PRV set point.
Question 6: What is a safety valve test procedure for a sealed system PRV?
- Manually lift the test lever briefly to confirm the valve opens and reseats correctly, then check for signs of weeping (Correct answer)
- Raise system pressure to 3 bar to test opening
- Replace the PRV annually without testing
- Test only if the boiler fault code indicates a pressure issue
Correct answer: Manually lift the test lever briefly to confirm the valve opens and reseats correctly, then check for signs of weeping
The PRV is tested by briefly lifting the test lever to confirm it opens and reseats; if the valve weeps after testing it may need replacement as debris can prevent proper reseating.
CENWAT Flue Requirements & Pressure Relief 3
If a PRV repeatedly discharges on a sealed central heating system, what is the most likely cause?