pressure drop in thelia 23 combi boiler

Have a look at the overpressure blowoff pipe ( the overflow pipe ) again, but do so when the system fires up, as it may be that there is a problem with your expansion vessel ( regulates the system pressure when the water gets hot ). I would imagine that another way to tell if this was he problem would be to watch the pressure gauge when the system ( i.e. the central heating ) comes on. As the CH water is heated, if the pressure gauge creeps up to 2.4 or 3 bar ( depending on what type of overpressure valve you have ), and then zooms down to a low figure all of a sudden, that'll be the overpressure valve opening and venting water outside ( at great velocity ).

Otherwise it sounds like you have a leak somewhere, but it must be fairly big to lose system pressure so quickly. Beware the fact that if the CH water is very hot when it leaks, it may evaporate and not cause a puddle. have you examined all the CH piping yourself, even under the floor?

Andy.

overpressure

If the mains pipe is a low pressure < 1 bar, then the system water will work its way into the mains if the plate heat exchanger is holed. Sometimes the water out of the tap is brown or tastes bad.

thanks very much;

plumbers have checked this out- and refilled the expansion vessel. also i have observed pressure while turning on, off, and anything in between -i really don't think it hits anything high enough, or has dramatic enough drop to be the overpressure valve as described. at the moment, starting with the system at zero, when switched on, the pressure will not rise at all - until manually filled - and then falls gradually (but very quickly) from that moment on. we're filling it

no- have examined anything visible as best as possible, but trying to investigate everything about the boiler before taking up floorboards/concrete. if that really needs doing then we'll go down that route as cheerily as possible- my reluctance is based on the fact that previous engineer stopped the very same thing from happening by fiddling with the boiler only (very technical, i know..) now we are getting a little bit of wetness on the pipes near the 'tap' where we fill the pressure up. but we can't see if/where it is coming from, and it seems to appear at random. well, any more ideas very welcome! helenski x

Firstly read the FAQ below.

If you are filling up that often then you do have a leak somewhere and if it were anywhere visible you would know about it. If it were in the boiler itself it would only be a short while before the boiler was dripping wet.

I presume that there could be some places where there is pipework where a big leak could go undetected?

I presume the work that was done 2 years ago was to fix a leaking pressure releif valve or was it to fix some sort of leak inside the boler?

OK you have a big leak somewhere.

This could be condensation because the pipes will be at around 10C on account of the mains cold water coming in.

I presume you have something like a ground floor flat with a wood floor? So you could be putting a lot of water down there under the floor with any problems.

I've just had a similar problem on a 2 y.o system. It turned out to be a faulty/leaking heat exchange coil in the indirect water cylinder.

However this is a combi boiler so that can't happen here. If the DHW heat exchanger is holed then the pressure would rise to mains pressure which is at least 1 bar+ and likely 3 bar+

Pleeeese help meeee........

I have the same problem but,

I have a constant drip from the overflow pipe and this increases when turn on the boiler for heating the rads. I am having to fill the boiler and I have tried to drain air from al the rads but to avail. There is a small amount of what could be condensation under the boiler not that much. It seems that this happens at the onset of winter. Any help would be appreciated

-- mohed