Pressure loss on Combi heating system

[Simmmy]

Hi all....i have a pressure loss on a combi heating system....the system is stable when the heating is switched off....stable for over a week at 1.5 bar...but after a hour or so of activity on central heating there is a loss ... a fairly big loss ...the trouble is is that i can't find the loss...i've checked all radiators and all valves on those radiators...there is no sign of any leakage to plaster work around the house....all radiators are full...i bled them last week ...so where can it be....i've looked around the boiler itself and have come up blank...no leaks to be found.

All help appreciated.

Simmmy

[Ed Sirett]

See FAQ.

[Simmmy]

Thanks for the reply....do you suspect a release valve or expansion tank leak?

It's to big a job for me but it will save me ripp>

[andrewpreece]

You could try pressurising your CH system with your filling loop, and see if the pressure release valve blows at 3 bars ( don't go much above that whatever happens ). However, I expect your expansion tank has lost its air. Look back at the various combi threads over the last 10 days or more.

Andy.

[Simmmy]

Thanks ... i have done what you said and looked back and came across this post... "Central Heating Boiler problem"

I have a Vokera 12-48 R.S Mynute Central Heating Boiler at the moment which is behaving a bit strangley! When the Heating comes on the pressure rises gradually up to and beyond 3 bars (which is when I panic and turn it off!) However when I switch it off the pressure drops back down again below 1 bar.

I have recently changed a broken fan but other than that nothing has changed since it used to work.

Any help would be much appreciated.

These are the very same problems i have had recently...this is what someone recons the problem is

[Simmmy]

Thanks ... i have done what you said and looked back and came across this post... "Central Heating Boiler problem"

I have a Vokera 12-48 R.S Mynute Central Heating Boiler at the moment which is behaving a bit strangley! When the Heating comes on the pressure rises gradually up to and beyond 3 bars (which is when I panic and turn it off!) However when I switch it off the pressure drops back down again below 1 bar.

I have recently changed a broken fan but other than that nothing has changed since it used to work.

Any help would be much appreciated.

These are the very same problems i have had recently...this is what someone recons the problem is

[John]

You could then try buying a new pressure relief valve as its very likley that doing what you suggest will result in system dirt lodging under the seat of the valve causing a slow drip which may not be curable:-(. Never deliberately over pressure a relief valve on a system whuch has been in use for any length of time.

I would suggest you fit a polythene freezer bag over the outlet pipe of the relief valve with an elastic band to secure it. Then see if it collects water in it when your boiler heats up. This will tell you if the water is going out of the relief valve. Watch the pressure gauge after you have charged the system while cold and switched it on to heat up the radiators. An excessive pressure rise will indicate a lack of somewhere for the water to expand into as the temperature rises. This means your expansion vessel may have lost its gas precharge, it may have a blocked connector pipe between vessel and system or it may never have been big enough in the first place (more or less in that order)

[andrewpreece]

I have no idea what it costs. You need to find your expansion vessel thingy, possibly painted red, about a foot or more across at the back of the combi, a bit doughnut shaped. Unscrew the cap on the bicycle valve attached to it ( gingerly ): if water comes out the diaphragm in the expansion vessel has leaked - time for a new expansion vessel. If air comes out then check the pressure on it and pump it back up to what it should be ( I confess I don't know ) with a bicycle pump, and dab a bit of soapy water across the inlet to the valve to see if it is leaking air. If the vessel has leaked and you want to repair it yourself you may be able to avoid the grief of dismantling the combi to get at the vessel by attaching a new expansion vessel on the CH pipework at an appropriate place elsewhere. You'll have to drain the CH down etc etc. Spare parts can be bought on a number of websites, Google for more info. NB I am not an expert! DYOR etc.

This question comes up every week without fail!

Andy.

[Simmmy]

Thanks...

Much appreciated.

[Ed Sirett]

Which is why we have a FAQ (with suggested answers).

[Simmmy]

I have looked outside at the outlet pipe and it suggests it has been leaking....water round the pipe and also on the ground underneath.

Whats the course of action now?

"I would suggest you fit a polythene freezer bag over the outlet pipe of the relief valve with an elastic band to secure it. Then see if it collects water in it when your boiler heats up. This will tell you if the water is going out of the relief valve".

I would'nt say there was a excessive rise in water pressure whilst the heating is running....just a loss of pressure after it cools back down...again i can find no leaks to any plaster work inside the house or at any radiators and boiler.

Thanks

[Ed Sirett]

Sounds like the pressure relief valve has gone weak. This will need replacing. The boiler manual will tell you how, if your are competent to do so.

[Simmmy]