CH pressure dropping

I keep having the same problems with my CH system every year or two, but I can never remember exactly how it was fixed. The problem is this:
o The CH doesn't work, and that's because the water pressure is too low (0.4 bar, the cut-off point on my system).
o Repressurising to 1.0 bar fixes that, for day or so, then the same thing happens once the system cools down.
o There are no interior leaks, but today I noticed the pressure relief valve was letting out water. The water pressure was 3.0 bar at that point.
I don't think it's suffered from overpressurising in the past (so that the relief valve would open and get clogged). So what would cause the pressure to rise too high? (Presumably the pressure relief valve is operating correctly.) The expansion vessel? When I checked the air pressure a year or two ago, it was exactly the same as the water pressure; which sounds about right...)
Thanks,
--
Bartc


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On 02/02/2012 17:28, Heliotrope Smith wrote:

Got it in one!
Chances are that the expansion vessel just needs re-charging with air - with the water system de-pressurised, as Heliotrope says.
If there's no air in it, the water has nowhere to expand to, hence the pressure rise and triggering of the PRV. Then it cools down and contracts, and the pressure falls because there's no resilience in the system, and hey presto - vicious circle!
--
Cheers,
Roger
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Roger Mills explained :

On the other hand, if it is not found to be that....
These very slight leaks can be very difficult to find. An easy way to find a persistent problem is to drain the system and pressurise it with air. Air leaks out much faster, does no damage where it leaks and usually can be heard escaping.
--
Regards,
Harry (M1BYT) (L)
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Feel I must point out here to the OP and anyone else that may be tempted to pressurise an empty heating system with compressed air that this could be a very highly dangerous practice!
Without the correct equipment and experience at best there could be damage to boiler parts and pipework and or controles.
At worst an explosion could affect life or limb.
An air test would normaly only indicate that there is a leak somewhere, I would not want to put my head in the same room to listen for any sign of a leak for fear of it getting blown off!
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On 02/02/2012 22:06, Heliotrope Smith wrote:

Only if the 3 bar pressure relief valve was seized shut.
That said, I don't think I would do it myself.
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This is very different from running at 3 bar with water. A central heating system pumped up to 3 bar with air represents a large amount of stored energy, which if let-go suddenly by failing plumbing parts could certainly affect life or limb.
I did pressure test each part of my central heating system during installation, but only small sections with one, or at most two radiators. Even that was quite frightening, and I made sure there was no one else in the house.
--
Andrew Gabriel
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Newshound wrote:

The expansion vessel is designed to blow as a secondary safety measure. Sometimes these blow at 1.5 bar as they may be poorly made.
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On 02/02/2012 18:06, Harry Bloomfield wrote:

Yebbut, OP said that the pressure went up to 3 bar and triggered the PRV. This wouldn't happen with your type of leak.
--
Cheers,
Roger
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I pushed in the valve last night with a ball-point pen, and fortunately just air come out.
When I tested the pressure this morning (with water pressure at 0), it was only about 0.2/0.3 bar. Filled it to 1.0 bar or so (the valve is awkwardly placed very near the wall and angled towards it, so some pressure is lost fiddling to get the pump connector off). Hopefully that will be it!
Thanks,
--
Bartc



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On 03/02/2012 11:20, BartC wrote:

Good. Let us know what happens when the system gets hot and then cools again.
--
Cheers,
Roger
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