CENWAT Flue Requirements & Pressure Relief 3

Question 1

If a PRV repeatedly discharges on a sealed central heating system, what is the most likely cause?

Accepted Answer

Waterlogged or failed expansion vessel

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

To allow the system to be filled or topped up with mains water to the correct cold fill pressure

Answer

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?

Accepted Answer

To prevent backflow from the heating system contaminating the mains water supply

Answer

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)?

Accepted Answer

As specified by the boiler manufacturer, typically 1–1.5 bar

Answer

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.

ACS CENWAT Practice Test

ACS CENWAT Practice Test

CENWAT Flue Requirements & Pressure Relief 3