Water Loss From Central Heating?

[Anthony Bowles]

The combi boiler has (for quite a while now) been losing water

The pressure guage which is usually at 2 will eventually drop to 0 or so. This can happen over a matter of hours or days. It only loses water/pressure when the central heating is running. It's been fine over the summer period when all we ran was the hot water so I presume it's only losing water when the CH kicks in.

We assume that the water loss must be from downstairs as any water loss from upstairs would be showing now. I've been under the floorboards today and cannot find any part of the plumbing that is leaking (it's all pushfit). The only visible water is (presumably) condensation along a cold water pipe. Additionally most of the pipes are covered with dust and I can't see any "streaks" that might have been left by water.

The only part of the system I cannot see is the copper pipes that run from the boiler through a concrete floor and out to under the floorboards.

Anybody any ideas as to where the water loss could be from before I have to call a plumber out?

Cheers

Anth

[look]

Check for seepage from the radiator valves - tissue paper is a good indicator. Also around any automatic airbleed.

Slow seepage can be difficult to trace if it evaporates from hot pipes/valves - try with the pump working but the CH temperature turned right down..

Robert royall at which net

[Lee]

Although it sounds like it should affect both modes, try the trick of tying a plastic bag over the pressure relief valve outlet to check whether it's leaking. Might be a failing expansion vessel that can't cope with the volume of the entire system.

Lee

[Set Square]

You presumably have a pressure relief valve which spills water to the outside of the building in the event of the pressure rising too much? You need to locate this, and check whether anything is coming out - by tying a plastic bag round the open end of the discharge pipe. If water collects in the bag, this is your water loss.

If water *is* being discharged through this valve, the most likely cause is a problem with the pressure vessel - which is causing the pressure to rise too high, so that a discharge takes place. Then, when it cools down, the pressure is too low because of the water loss.

Check this first - and then come back, and we'll tell you what to do next.

[Anthony Bowles]

That's the problem. I tied a large freezer bag to it last night and there is a substantial amount of water in it this morning. Actually when I got up I noticed that the guage was nearly empty even though my other half had topped it back up about 20 minutes before I looked.

Is this normally a costly repair?

Cheers

Anth

[Set Square]

It may well be something you can do yourself. If the pressure relief valve is opening, it's due to one of two causes. Either the valve itself is faulty, and is opening at too low a pressure, or there is a problem with the pressure vessel which is causing the pressure to rise too much.

You cannot easily check the valve, unless you can watch the pressure gauge while it is discharging. But I would check the pressure vessel first.

The pressure vessel is a bomb-like device which contains both water and air under pressure - separated by a diaphragm. As your system heats up, the water expands - and the expansion is accommodated by the pressure vessel, whose air gets more compressed with only a modest increase in pressure. If the diaphragm fails, or if some of the air is lost, the water loses its "cushion" and excessive pressure occurs when the system gets hot.

The pressure vessel may well be inside the boiler casing, or nearby. It will have a Schrader valve (like a car tyre valve) which enables you to check and adjust the charge pressure. With the water system unpressurised, the air pressure should be around 0.7 bar (10 psi). If it is significantly less than this, blow it up with a car tyre pump. If water comes out of the Schrader valve, the diaphragm is shot, and you need a new pressure vessel.

If the system works ok after you have re-inflated the vessel, fine. If the problem recurs after a short time, there is probably a leak and the vessel will have to be replaced.

If the vessel *is* shot, it is often easier to leave it where it is and to fit a new one elsewhere in the system. That way you can use a generic vessel rather than an exact match, and the old one won't do any harm as long as it isn't leaking water to the outside world.

[Anthony Bowles]

Excellent. It took a few attempts to get it right but it seems to have sorted itself out. Watching the boiler with the case off, it looks like the pressure was building up pretty quickly (from 1 to 3) and then it would trickle out of the pipe.

However it seems to be fine now. The heating's been on 3 times since last night and so far no loss.

Cheers

Anth