Combi boiler losing pressure ??

Forever_Red

I have a Vaillant combi boiler which is losing pressure on a daily basis. I have checked all valves and radiators but no leaks anywhere. When I pressurise the boiler to 1.5 bar and turn on the heating, the pressure starts to go up to 3 bar. Turn the heating off and the pressure will gradually drop to zero overnight. The heating will come on in the morning (on a timer) but won't heat the radiators up as it seems there is an airlock in the pump, turn the boiler off and then on and the pump seems to clear the air and starts to heat up the radiators. Even when there is no pressure in the system we can still get hot water through the taps. Someone posted elsewhere about the expansion vessel being below pressure.

Would the pressure go from 1.5bar to 3bar, with the heating on, if the air pressure is low in the expansion vessel?

Can a general DIYer pressurise the expansion vessel or is it best to get a plumber in?

Any comments appreciated.

Alan

Canucklehead

Hi

See the installation manual . You might find it on the Vaillant website or ,if it's an older model, Google.
The flexi connection from the expansion vessel might also be blocked.

Corgi Guy.

wildmandy

The pressure should rise when the water heats up.
Your variations are too much though.
Normally, this is a sign of a water leak.
In you case, perhaps you have air in the system?

Mandy

djheath

Combis do have expansion vessels that need to be topped up with air to around 10 psi or so which allows for the heated water to expand when the heating is on.

If your pressure gauge has reached 3 bar then the pressure release valve (prv) could have been activated and discharged water out of the system. Check for the pipe where the water is discharged out too - its usual outside the building near the vent for the boiler. If the prv has activated they sometimes dont seal after use and may leak water and so will need replacing.

It sounds like you either have:
a. a leak in the system - could be in the pipes under the floors.
b. A blocked pipe which is causing the pressure to rise and then to be discharged out the prv
c. no air in the expansion vessel causing the prv to function.

I would look at the following site for more guidance.
http://www.diynot.com/forums/viewtopic.php?t=37170

Please not I am just a diy'er so have no corgi, plumbing, or heating engineer experience which should be taken as gospel and a qualified plumber should be consulted first!:D

ariba10

We had the same problem and it was as "djheath" said.The air pressure in the expansion vessel had dropped.

It was pumped back up with a car tyre pump about 18 months ago and no problems since.

economiser

I had the same problem with a Vaillant combi. Did the same as ariba10 and pumped it up again. Problem kept recurring and installer diagnosed faulty pressure vessel. That would be expensive so I tried changing the air valve on the pressure vessel, same thing as a car tyre valve, and problem solved for a few pence.

Forever_Red

Cheers for the replies. I will have a go at topping up the expansion vessel and let you know how I get on. I think the PRV has lifted as well, as I put an empty plastic bottle over the copper pipe outside and found it was filling up when the pressure got too high. I maybe need the PRV replacing? Any idea on the cost of them?

Cheers

Alan

djheath

Forever_Red wrote: »

Cheers for the replies. I will have a go at topping up the expansion vessel and let you know how I get on. I think the PRV has lifted as well, as I put an empty plastic bottle over the copper pipe outside and found it was filling up when the pressure got too high. I maybe need the PRV replacing? Any idea on the cost of them?

Cheers

Alan

If its stopped dripping then, it could be ok and have resealed so I would follow the instructions on here on how to pump up the pressure vessel and see how you get on.
http://www.diynot.com/forums/viewtopic.php?p=251090#251090

To see how much a prv would be have a look at:

https://www.keeptheheaton.co.uk