boiler losing all water pressure every 4 hours.

[paulanderika]

We have a new combi boiler, installed in May, now losing all water pressure about every 4 hours and shutting down. We have been advised there must be a large leak under our concrete floors but no sign is really showing. Utility and kitchen floors appear damp but after digging up trench under utility room radiator, there was no leak there. Can anyone offer advise on how to make progress, or tips on detection, cant keep on resetting boiler. Have only been in this house 18 months and thought we were making progress up dating utilities.

[The Natural Philosopher]

It doesn't take a large leak to depressurise a CH system.

You could feed it a barium meal and go around with a geiger counter...

Is it possible to isolate sections of it at all?

[John Rumm]

Have you watched the pressure gauge as the CH heats up? You may find it it going up too high (i.e. 3 bar or more). If this is the case then the pressure relief valve will probably open and vent some water outside. Hence once it cools there is no longer enough pressure in the system.

You could tie a plastic bag under the blow off pipe to see if it collects any water.

See the sealed system FAQ for information on pressure vessels:

[Lobster]

Well - to depressurise it every four hours must mean there's a reasonable volume of water coming out in that time - ie measured in pints rather than egg cups I'd have thought? Certainly enough to be readily detected if it wasn't leaking some nefarious like under the floor. I'm sure it would be coming through the ceiling, for example, if there was a leak anywhere upstairs.

Where else is there pipework which you can't eyeball? When you say you've dug a trench in the utility room floor do you mean you've exposed all the buried pipework, or is there more?

If you can be convinced the leak is under there somehwere, one approach might be to bypass the under-floor bit by running new pipes above ground somewhere - less attractive but less invasive??

David

[Ed Sirett]

There is a FAQ for this. See below.

[Graham.]

Barium sulphate, as given by mouth or as an enema is radio-opaque to X-rays, not radio-active.

[Roger Mills]

There may not be a leak at all from the pipework. It is quite possible that the expansion vessel has lost its air/nitrogen charge so that the water in the system cannot expand when it gets hot without building up an excessive pressure which causes the pressure relief valve to open - and water to be expelled outside the house. Then, when the system cools again, the water contracts and all pressure is lost.

Find the discharge pipe, and hang a container under it and see whether it collects any water.