Vaillant system boiler losing pressure (but doesn't appear to be a leak)

[James Carmichael]

Hi,

I've got a Vaillant system boiler (ecoTec plus 637). It's around 12 years old and has been maintained. We've just been away for a couple of weeks so, keen not to burn gas unnecessarily, I set all the zone thermostats/timers (hot water, 2 x radiators, underfloor heating) to off/frost protection.

I came back and the boiler had lost pressure so I repressurised. However, it was immediately strange. I probably have to correct from 0.9 bar back to 1.2 bar once or twice a year, and that normally takes a fair amount of time with the filling loop, Today, it almost instantly shot up to 3bar - I drained about 10cm in the bottom of a bucket out of a radiator to get it back down to 2.5 bar. That seems very little.

Slowly, over the day, it has depressurised again and - again - it's taken very little to get the pressure up.

While it's running all radiators appear to heating up nicely, as does the hot water. I have checked every radiator and can see no sign of any leaks. I've not changed the system recently or done anything else that could prang a pipe. No sign of damp patches anywhere.

I am thinking there is some sort of boiler issue rather than a plumbing break. The rapid rise in pressure when I open the filling loop suggests something is blocked?

(obviosuly, if it is a boiler issue I'll be getting a GasSafe engineer in.)

Any ideas?

Thanks
James

 

[Johntheo5]

Looks like the expansion vessel needs attention. Does the pressure rise rapidly when system heats up, if so and it reaches +3bar then a (pressure relief valve), PRV, will release some water.

 

[James Carmichael]

Thanks.

No, it doesn't seem to. I was assuming expansion vessel (there are two on the system, I think - one for the unvented how water cylinder and one in the boiler; the latter would be suspect here?) but the fact that the pressure doesn't just shoot through the roof as soon as the heating goes on confuses me. I should go and check the safety discharge pipe ...

 

[James Carmichael]

Ahh, yes, that's dripping.

Ok, thanks everyone - time to call in the gas engineer, internal expansion vessel needs some love!

 

[Johntheo5]

Also the PRV will need renewing if its passing at any pressure < say 2.7/2.8 bar.