Why does my boilers pressure keep changing?

[olyboy123]

hello,

I've recently moved and a new boiler was fitted before we moved in. It's a main heating Eco 25 combi.

We've only until now used it for hot water. The pressure sits at about 2.5 bar when not in use and when the boiler kicks in it drops to about 2 bar. We've just started using the central heating which makes the pressure gauge rise to about 3 bar (in the red). It's seems to stay at this level until I turn the heating off at which point the boiler hisses and the pressure drops to about 1 bar. It then gradually rises throughout the day until it gets to about 2/2.5 bar. Seems to repeat this process everytime the heating goes on. Any ideas? Cheers! Oly

 

[Nozzle]

You heat water, it expands. If the pipe system was solid, it would split, but there is a special vessel built in the system which has capacity for the extra volume of water. That vessel has a sac inside it which is pressurised with air while the system is cold. When you heat the system, the bag with air in gets squished as the pressure of the entire system goes up.

Nozzle

 

[olyboy123]

Thanks for the explanation nozzle. Is there any reason this only happens with the central heating then? And not the hot water. So the pressure change doesn't have have anything to do with filling loops or other? Just the natural pressure changes with heat?

 

[Madrab]

For a kick off, it's too high, the pressure should be set to about 1-1.5bar, if it's rising slowly through the day, then the filing loop may be passing. When the boiler runs the pressure should rise a little but your system should not be filled to 2.5bar.

When the heating goes off and you hear a hiss @ 3bar, that's the safety relief valve opening. Your boiler should not be doing this. You need to get the filling loop and the Expansion vessel checked

Get a GSR engineer in to give it a service ASAP!

 

[olyboy123]

thanks for the reply. the situation has updated a little. So the pressure now constantly rises, slowly, but seems to get to 3bar and then stays there, so i bleed the rads and the pressure builds again to 3bar. I've detached the rigid filling loop and the isolation valves are working (the only valve that doesn't seem to work is the heating return isolation valve on the boiler, but the filling loop valves bypass this for repressurising anyway i suppose?). Strangely with the loop detached the pressure still rises. Im wondering if it could be a faulty pressure gauge?

There is i've found an additional filling loop (old style with a white plastic knob to turn on and off) lower down the pipes, connecting the cold water inlet and the heating return, so I guess I should look at this next to see if it's passing. Beyond that I'm lost! Think i'm going to have to get someone in.

 

[Rampage77]

If that's not passing then you have a faulty DHW heat exchanger, you will probably need a gas engineer for that.