Pressure drop

17 Jan 2004
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United Kingdom

I've got a combi boiler and its serviced every year. Everything was fine, but the past couple of days I've noticed the pressure dropping on the system when the heating is off. I've increased the pressure by using the tap the engineer showed me (new home owner and no idea before this), but the pressure drops again.

Help or suggestions please :)
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Generally, do not make pressure adjustments.

The pressure will fluctuate from cold to hot, without making any more changes what is the lowest pressure and the highest pressure that you see at the setting you last made?

If the pressure drops to zero, you must know why it is doing so, as this could be a symptom of component failure.

Post the pressures and we can sort out what to do next.

What model boiler is it?

I've had a similar problem before, It happened when I vented the radiators. This dropped the pressure down to 1/2 bar, No problem, I thought, Ill just top up the boiler.

Unfortunately our top up valve is in a hard to reach spot and by the time i'd glanced at the pressure gauge on the combi I'd over pressurised it and the safety valve opened to drain the boiler. I now had the same problem as you have. It was the pressure release valve that was causing the issue with my combi. basically when the valve opens you can often get 'dirt' in it from the water, and it stays slightly open (a bit like a tap that needs a new washer) and your pressure slowly drops back down to zero when the boiler is not in use.

An easy way to see if this is the probable cause is to check the overflow pipe and see if there is a steady trickle of water coming out when the heating is off. You should be able to find this pipe running from somewhere behind the boiler, through the wall and draining outside.

If this is your problem you can remove the cover from the boiler and attempt to tighten the safety valve yourself, or more likely get a plumber out to fix a new valve, it should be a simple job and a fairly cheap part.

Hope this Helps
Thanks for the quick replies. The pressure was around 1.7 yesterday and overnight dropped to 0.6 (the current boiler is a Glow worm XtraFast). I did notice this morning that the overflow pipe outside had been leaking a lot.

Any further help is much appreciated :)
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You may be able to clear the valve leak. Pressurise the system to about 1.5 bar then open the valve smartly and let it close. Do this a few times to see if it stops leaking. If it doesn't, it's a new valve.

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