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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Combi Boiler system problem https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1666088601 Message started by mike4369 on Oct 18^th, 2022, 11:23am

Title: Combi Boiler system problem
Post by mike4369 on Oct 18^th, 2022, 11:23am

Following a change of towel rad in bathroom the system (15mm copper throughout) is losing pressure. The new rad has been bled but more air is blked out almost daily. Unfortunately the bathroom floor (house empty at time) was replaced by bathroom fitter and vinyl laid before I could confirm no leaks at new rad Boiler ha s had a pressure test and no problem there but when reconnected to system pressure drops. No obvious leaks, even on ceiling under new rad, Fernox leak sealer added 3 day ago but problem still there. My one concern is the amount of topping up done daily to restore pressure with no visible sign of water anywhere plus the amount of air bleeding out of new rad. Any advice would be appreciated

Title: Re: Combi Boiler system problem
Post by woodsmith on Oct 19^th, 2022, 8:52am

With no obvious signs of water anywhere my first guess would be the pressure release valve in the boiler. You could put a container underneath where it vents outside the house to see if it collects any water. It’s not that unusual for them to leak after having work done. Otherwise you could drain the whole system down, you need to get all the water out, then pressurise the system with air. You can normally then hear where the leak is coming from.

Title: Re: Combi Boiler system problem
Post by mike4369 on Oct 19^th, 2022, 11:26am

Thanks for the reply
As said the boiler flow and return taps were shut and pressure stayed at 1.5 bar overnight. Is the PRV on the system side so that pressuriing the boiler would not be affected by a leaky valve. I dont yhink the previuos houseowner had and protector in the water as there seemed to be a lot of black gunge in the rad I drained off to get the leak sealer into the system. So maybe when the new rad was fitted some gunge had been disturbed and found its way to the PRV.
Will go back and check some more possibilities

Title: Re: Combi Boiler system problem
Post by woodsmith on Oct 20^th, 2022, 9:11am

The PRV is in the boiler so if you have isolated the boiler it would seem that it can’t be the PRV. However I assume that you didn’t fire up the boiler with it isolated, which is not a good idea, so it could still be the PRV as it may be opening when hot. Another thing to look at is the expansion vessel, normally a red can inside the boiler. If this has failed it will be full of water and you can tell just from the weight if it’s full or not. Some sludge in radiators is normal even with inhibitors.

Title: Re: Combi Boiler system problem
Post by mike4369 on Nov 5^th, 2022, 11:36am

Does anybody know of a reliable auto vent valve for radiators. I have fitted an Aladdin type - it worked once but not again. I am trying to purge the air from the sytem as well as turning off 1 radiator at a time to try and isolate the fault.
I has been suggested that changing the bleed and blanking valves had cleared the problem of air getting into the system.

Title: Re: Combi Boiler system problem
Post by woodsmith on Nov 6^th, 2022, 10:13am

The Aladdin auto vent is the only one I know of. I’ve never used them because, if the system is working, you shouldn’t need to regularly bleed radiators but I’ve heard they work ok. If you have a leak in your system, unless the radiator itself is leaking, turning radiators off is not going to help you find the leak. If the bleed or blanking plugs are leaking you will see an obvious black stain. Can you not get the plumber who changed the towel rad to come back and find the leak?

Title: Re: Combi Boiler system problem
Post by mike4369 on Nov 6^th, 2022, 11:08am

Thanks for the reply. As said originally I have searched ( a lot of the pipework is surface mounted so can be seen) the system for leaks and cannot find one. There is no sign of water on ceilings or walls, the new rad has been checked - no leak there. The system loses pressure in about an hour so if there is a leak it would have produced a wet patch somewhere. The system is topped up daily and the amount of air that is vented is significant. I will keep trying various options and see what actuall fixes the problem.

Title: Re: Combi Boiler system problem
Post by woodsmith on Nov 6^th, 2022, 12:41pm

If you pressurise the system and then leave the boiler off does it still lose pressure? Or only lose pressure when the boiler fires up?

Title: Re: Combi Boiler system problem
Post by mike4369 on Nov 7^th, 2022, 5:26pm

Thanks everyone for the replies. Problem solved - there was a leak is the pipework - finally manifested itself last night when some water came through ceiling. Turned out to be a screw holding floorboad nicked a pipe causing ever so small drip. Plumber said it must have been there for some considerable time as screw was almost rusted away.