Another combi pressure loss thread

2 Sep 2010
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United Kingdom
Hi All.
I will try and keep this short.

Started loosing pressure in Jan and over the weeks the pressure loss got worse, placed a plastic bag under the pressure relief valve outlet pipe and no leak.

Tried the no nonsense leak sealer and the leak stopped, 3 weeks later, the pressure was dropping from 1.5 to 0 everyday, so found 2 leaks on the grandee combi, 1st was the brass automatic air vent valve which I replaced and 2nd was the 3 way port valve which I also replaced but still loosing pressure, found that the expansion vessel was full of water so replaced that, the expansion vessel was supposed to be pretty loaded at 1 bar but was showing zero on a air gauge, so I put more air into the vessel to 1 bar..

Since fitting the new expansion vessel, I have found that I am only loosing .2 of a bar everyday and I need to top it up every 4 the day, but I would like to fix the problem.

What I want to know, can I isolate the central heating from the combi boiler by closing the 2 taps either side of the water pump and leave it for 24 hours to see if there's a drop in pressure. By doing this, if their is a pressure drop, then it's still the combi boiler or if no pressure then it's the central heating.

If it's the central heating, then I can replace the valves, if that doesn't work, then my plan is to replace the pipe work above floor level and replace the radiators. The original pipe work is under concrete and it's over 30 years old and to avoid chopping up the floor looking for leaks, it would be cheaper just to replace the pipework and rads.

I could try my luck and try using the expensive leak sealer but it might not solve my problem..

On another note, since fitting the expansion vessel and putting 1.5 bar of pressure into the system, I have noticed that the pressure only goes up to 2 bar when the central heating is on and not 2.5 bar like it did before hand. Any ideas?

Thank you for any help.

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