Combi losing pressure when CH on

[tr12345]

Hi

My combi is losing pressure, but only when the CH is on. If the CH is not running, and the boiler is just providing DHW then the pressure remains fine.

I have checked the PRV and nothing is leaking there.

Any ideas what might cause this, or what test I should try next?

Thank you

 

[ianmcd]

check the pressure guage when the boiler is cold then turn the heating on and check the pressure again after half an hour or so and if it is near 3 Bar you have an expansion problem

 

[tr12345]

It went up to 2.4 bar, but when turned off below 1 again with F25 lockout. What should the pressure max out at when the boiler is running?

Does that sound like under-pressurised or failed expansion vessel?

What I don't understand is WHERE is the water going? No leaks and nothing out the PRV

Thank you

 

[ianmcd]

Your expansion vessel needs re-charging , the PRV might need replacing at the same time if this has been on going, your expansion vessel should be serviced every year same as your boiler but they never are

 

[tr12345]

Hi Ian, I re-pressurised the expansion vessel this afternoon, and all seems good now, fingers crossed. Many thanks for your help.

I would still like to know where the water was going though?!?!

Cheers

 

[ianmcd]

It would have been going out the expansion releif pipe, wouldnt always show just when heating sytem was hot

 

[tr12345]

That's what I would have thought, but when I sellotaped a sandwich bag over the end, there was nothing in it. If it happens again I'll use a balloon as that would have a tighter seal.

Also I've read that once activated, PRVs tend to leak, but this one is dry as a bone. Perhaps I've just been lucky.

Anyhow, thanks again for your help. Much appreciated.

 

[ianmcd]

Also I've read that once activated, PRVs tend to leak, but this one is dry as a bone. Perhaps I've just been lucky.

if they are constantly activating and poor water quality , then yes high failure rate , but some just re seat without any problems, post a pic of where you are attaching the bag

 

[old duffer]

check the pressure guage when the boiler is cold then turn the heating on and check the pressure again after half an hour or so and if it is near 3 Bar you have an expansion problem

would you be able to expand (pun intended) and explain this logic please

 

[old duffer]

encountered the exact same situation. Can you see why this was happening?

 

[ianmcd]

encountered the exact same situation. Can you see why this was happening?

because when the water expands, the EV is there to absorb this expansion, if that fails next line of defence is the PRV , it opens at 3 Bar to protect the boiler, once it has released the excessive pressure then when cold the boiler or system pressure is too low, so you add water and it simply cycles again

 

[old duffer]

because when the water expands, the EV is there to absorb this expansion, if that fails next line of defence is the PRV , it opens at 3 Bar to protect the boiler, once it has released the excessive pressure then when cold the boiler or system pressure is too low, so you add water and it simply cycles again

Volumetric (Cubic) Thermal Expansion

Volumetric temperature expansion calculator.

www.engineeringtoolbox.com

 

[RandomGrinch]

Volumetric (Cubic) Thermal Expansion

Volumetric temperature expansion calculator.

www.engineeringtoolbox.com

knowledge and understanding is what most strive for.....but cuting and pasting things is neither one nor the other

 

[tr12345]

if they are constantly activating and poor water quality , then yes high failure rate , but some just re seat without any problems, post a pic of where you are attaching the bag

I can't get to it easily at the moment due to some rubble, but it's 15mm copper bent back to the wall, so definitely the PRV outlet.

The pressure never got to three, but about 2.4 bar. So I don't understand it. But all is still good and added inhibitor today.

Thanks again Ian.

 

[ianmcd]

Even at 2.4 it might have been letting by, they say 3.0 Bar but in reality they are simply a spring loaded valve and dont actually open bang on 3.0 Bar most are just cheap crap but safer that they open earlier rather than too late , glad you got it sorted