Bosch combi boiler

[joesdaddy]

My combi boiler keeps losing pressure I top it up to 1 bar and it takes roughly half a day to go down to 0.5 of a bar it is bleed properly so there is no air in the system . The first thing I thought of was leaks so I have checked everywhere rads piping and boiler cant find a drop anywhere. I have also kept a eye on the overflow from the pressure release valve and nothing is coming out of that,any help greatly appreciated cheers

[boxtops]

I would get a man in - I have a combi boiler and I never have to touch it.

[AlBags]

Possible pressure vessel failed.
Set pressure to 1 bar when rads hot .. check bleed again and monitor over time. Is there a bleed at top pipework in boiler? Bleed that too.

[themayor]

Hi joesdaddy. Had the same problem with my Bosch Combi (only 13 months old), it turned out to be a porous heat exchanger. It was still under warranty so they just changed it for a new one, no problems so far but its only been 9 months yet.
Engineers paid umpteen visits, trying all the different things that make combi's loose pressure, tried to convince me that there was a slow leak in the system and wanted all the floor boards up. Eventually admitted that it was a known fault with my boiler type.
Good luck in getting a quick diagnosis.

[heathfield]

As the water heats in your system it expands. Since it's a 'closed system' there has to be somewhere for the extra water to go. So your boiler has in it an 'expansion vessel'. This is a cylindrical tank divided in two internally by a rubber diaphragm, with water from the system on one side, and air under a bit of pressure on the other side. If you're really sure that your system has no leaks anywhere else, it's quite possible that there could be a leak in the rubber diaphragm, and that's allowing the water to escape. It's not an uncommon occurrence, and if this is what's happening, the only answer is to have the expansion vessel replaced.

[AlBags]

There's an ECHO in this room.

[AlBags]

I will say tho .. that the heat exchanger theory would work.
The water would slowly leak and be burned off as steam. You would be unlikely to see this in the exhaust gas/plume.

[themayor]

AlBags. Therein lies the clue, on a very cold morning my boiler would send so much "steam" out of the flue we thought that the "the Mallard had pulled up outside". The engineer who finally diagnosed the problem said that if the other engineers had listened to my explanation they would have known what the fault was much earlier.

[joesdaddy]

Cheers for all your answers still trying to find a external leak but i think i would have found one by now as its losing roughly 0.5 of a bar each day does anybody know how much water this would be , it seems to lose more when the heating as been on

[AlBags]

That would depend on the system volume ..
That is set by the heat exchanger size, radiator type and amount of pipework .. and the bore!