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.

Is this a major problem with the central heating system or one
that I can easily repair?

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.

"Christian McArdle" wrote in message
...
Is this a major problem with the central heating system or one
that I can easily repair?

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.




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 he
http://www.makewrite.demon.co.uk/SealedCH.html
Scroll down to the section:
I suspect the expansion vessel is faulty, how do I check?

--
Ron

trickydicky wrote:
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.

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
http://www.makewrite.demon.co.uk/SealedCH.html

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.

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

Christian.

Christian McArdle wrote:

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.

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

Christian.

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.

On the matter of the expansion vessel, how much would it
cost to replace?

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

Christian.