Combi - pressure relief valve

31 Dec 2003
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United Kingdom
This question is associated with my earlier one about a Grant Combi 90 that keeps losing pressure but I have posted it as a new subject as I hope it may provoke responses from others...

In an attempt to cure persistent loss of pressure in a Grant Combi 90 I replaced the pressure relief valve as it seemed there was a slight drip from the outlet. It seems that there is still a slight drip which I guess is an indication that the valve is opening. The boiler fires OK and the rads get hot and we get hot water, albeit at low pressure.

What could be causing the system to overpressurise so that the valve opens or leaks? If the heat exchanger was furred up could this cause a build up of pressure?

Hope I sound desparate 'cos I am!

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As far as I'm aware the pressure in the heating side is decided by the user by means of the filling loop which uses mains supply. If for example you stop the filling at 1 bar then when the water gets hot it will rise to say 2 bar due to the fact water expands when heated. The extra volume has to go somewhere and it flexes the rubber diaphram in the expansion vessel. Now expansion vessels have to be pressurised on the air side before being pressurised on the water side, if it's not you find the water pressure rises far more than the 1 bar.
So I would say increasing pressure can be due to
(a) filling loop left in position and may have leaking valve allowing pressure to build up.
(b) A leak at the interface of the heat exchanger where mains supply cold water is transformed to hot water.
(c) A faulty expansion vessel, ie lost the pressure on the air side more then likely due to split diaphram.
All theae will result in the pressure relief valve opening, pressure dropping and then building up again and repeating the problem.
Thats how I understand it but I could be wrong
:rolleyes: :rolleyes: :rolleyes:
All sounds right to me Mandate. Snowsride what does the pressure gauge read?

The red 'target' needle on the gauge is set to 1.1 bar and I re-pressurise to this level. The pressure rises to over 2 bar when the boiler is burning and then it falls to zero when it is off.

The original pressure relief valve had a 'relief' pressure of over 3 bar but the replacement supplied by Grant's is only 2.5 bar - the problem occurred with the old valve and persists with the new one!

Mandate :-

The filling loop (a flexible hose) has a valve at each end. I open both valves to re-pressurise and then close them both. I assume this is what is meant by disconnecting the loop?

Don't know how to check the interface from the cold supply to the heat exchanger. Do I just look for leaks when it's cold as I assume when it's hot any leaking water may quickly evaporate and not be visible.

The expansion vessel is new so I think it is unlikely to to be the problem.


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Hi! A leaking valve on the filling loop would allow mains pressure to slowly build up on the heating side, hence the reason why it should be disconnected. If you still have the problem after disconnecting the loop then you can't blame the loop valves.
I believe expansion vessels can be tested with a tyre pressure gauge but this has to be done when the water pressure is zero. I also believe that if water exits the valve on the expansion vessel when you momentarily press the plunger as on a car tyre, then its a clear indication the diaphram is split.
I don't know a method of testing the interface, but with the loop disconnected and boiler off, how else can the pressure rise on the heating side, it can only come from the higher pressure of the mains suppy and mains supply goes into the boiler cold and comes out hot.
It's hard to understand how the pressure falls to zero having gone from 1.1 to 2 bar, unless the pressure release valve has opened and released the pressure. I take it this fall in pressure only occurs when the boiler is on and the pressure is above 2.5 bar.
If it was pressurised to 1.1 bar then the pressure fell it would be due to a leak in pipe work/ joints on the heating side.
:rolleyes: :rolleyes: :rolleyes:
Mandate - thanks

Will try disconnecting the loop this coming weekend so no more questions until next week!


You could have a leak anywhere on the system that causes this. Things may be fine when the water is cold, and the leak starts when the system heats up, then continues until the pressure is zero.

How old is the boiler?
I know this is an old post, but I wondered if you sorted this problem out? I have exactly this problem with this boiler. I need to get it fixed asap!


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