Central Heating System Losing Pressure

[trickydicky]

The domestic hot water system works fine but when the central heating system comes into play the pressure in the system gradually builds up until it reaches 3 bar which activates the pressure release valve. After this, there is no pressure in the system and water needs to be added to bring the pressure up to 1 bar. Then, the process repeats itself. Is this a major problem with the central heating system or one that I can easily repair? Any help or advice would be greatly appreciated.

[Christian McArdle]

It is a major problem. Water is clearly getting into the system. You need to identify where. Some possibilities.

(a) DHW heat exchanger in a combi boiler. (b) Leaking indirect coil in an unvented hot water cylinder. (c) Filling loop not disconnected after filling.

Basically, anywhere mains pressure water comes into contact with the heating primary circuit is suspect. Depending on what the final diagnosis is, fixing it could be cheap and easy, or very expensive.

Christian.

[Ron Lowe]

Sounds more like the expansion vessel has lost it's air volume.

It could be that the diaphragm has burst, and the thing is full of water, or it may just need re-charging via the schrader valve.

Read here:

down to the section: I suspect the expansion vessel is faulty, how do I check?

[Aidan]

Inadequate air charge in the expansion vessel? The water can't be compressed & can't absorb it's expansion when the system heats up. Pressure rises to 3 bar, excess water is discharged. System cools down, water contracts, pressure drops, boiler lock-out when next required.

See

[Christian McArdle]

Yes, I didn't read the OP's post thoroughly enough. I would agree that this is a more likely scenario.

Christian.

[trickydicky]

Thanks for the advice, I'll look at that. I've noticed another problem, which may or may not be related, which is that the air-vent cap, which should be left loose, leaks water when left in this position. On the matter of the expansion vessel, how much would it cost to replace? It's a Potterton boiler and the manual advises that the replacement should be external to the system. What do you think? Again, your help and advice is greatly appreciated.

[Christian McArdle]

If you do it yourself, about 20 or 30 quid.

Christian.