More boiler woes

[Pete Verdon]

Hi,

Based on some advice here last week about sticky TRVs, I discovered that that was indeed the problem with my duff radiator. However, in the process I seem to have made the very minor leak I did have (top up every few months) significantly worse (top up every couple of days). I suspect this happened when I closed every other radiator to ensure water was flowing to the suspect one, but the stuck valve meant that the suspect one was closed too. The boiler made some rather worrying banging noises and the temperature "gauge" (row of LEDs) shot up fast until I turned off the power.

Unlike the former slight weep somewhere, the system is now losing enough water that I have to worry where it's going. I also need to keep an eye on the pressure to keep the boiler out of lockout when it's needed.

I've checked all the radiator valves, which were dry, and I've had a look up inside the boiler cabinet which doesn't seem to be dripping wet anywhere. I've kept an eye out for wet patches elsewhere in the house with no result.

Is it possible for a boiler heat exchanger to fail in such a way that it leaks into the DHW? (I don't really drink from that so I wouldn't necessarily notice a little boiler water mixed in). Or leak into the combustion chamber and evaporate out the flue?

I think I would have noticed by now a failed joint above the ground-floor ceiling, which leaves only the concrete ground floor. This is half tiled (hallway, kitchen, loo) and half laminate (living/dining room) so really not accessible. Is there any cunning way to locate a leak in a concrete floor short of waiting long enough for it to become saturated, stripping back to concrete, and looking for the damp patch?

This is a Chaffeateaux Brittony combi, in a 1990s three-bed semi.

Cheers,

Pete,

Worried :-)

[chudford]

It could be that the pressure relief valve opened when the boiler overheated. It might now be stuck open slightly. Check the pipe from the relief valve, which should feed outside somewhere, and see if there is any water dripping from it.

[Ed Sirett]

Water would go into the primary FROM the DHW. You probably have a very small leak somewhere. I take that there is at least one radiator that you normally never shut off, if not you could have the boiler overheating again sometime.

Internal leak sealer (e.g. Fernox LS-I) might help.

The rate of leakage is quite small and you will have to hunt around fairly hard for it.

[Roger Mills]

My guess is that in shutting off all the radiators, you disturbed at least one of the lockshield (or non TRV) valves so that it is leaking round the shaft. You may not notice this when the system is hot because any water leaking out will immediately evaporate. You need to have a close look at all of the valves when the system is cold - maybe temporarily raising the cold pressure via the filling loop. [But bleed some water out of a radiator to reduce it again before turning the boiler on]. If any valves *are* leaking, you'll need to tighten up the gland nuts, or even to re-pack the glands by completely undoing the nuts, winding some hemp or thin string round the shaft below the nuts, and then doing the nuts up again.

The alternative - as others have suggested - is that the pressure relief valve is leaking, so you need to check that. Hang a small container under the outlet pipe and see whether any water collects in it (when it isn't raining!). Are you sure that the expansion vessel is working correctly? What is the pressure gauge reading when the system is (a) cold, and (b) hot?

[Mathew Newton]

A sandwich bag held in place with a rubber band is a good alternative

- it helps identify even the smallest of leaks over time.

Mathew

[Pete Verdon]

This seems unlikely to me - OK, it might evaporate when hot, but it's still going to leak when cold so I'd expect to find a damp patch on the floor of water that leaked out and away from the valve while it was cold. The heating's only on for a couple of hours in the morning and a little more in the evening.

It would be good if it were that, especially if it can be knocked closed again rather than replacing. The boiler schematic shows a relief valve but the parts diagram doesn't, so I don't know where it's physically located. I'll have a look tonight.

The boiler only has a single (flexible, transparent) drain pipe coming out. I assumed this was the condensate drain - is it likely that the relief valve drains into here too?

How would I tell? Surely even a faulty one wouldn't cause the system to constantly lose pressure?

Depends how recently I've topped it up. It needs doing more or less daily at present to keep the boiler running.

Cheers,

Pete

[Pete Verdon]

Duh, of course :-)

Um, there isn't one without a TRV, but apart from when I deliberately turned them all off I've never had the boiler overheat.

Hmm, ok. If I manage to eliminate any other possibility I might give that a try. Presumably it doesn't create a problem of gumming things up?

It seems like a big leak to me :-). If this counts as "small" in your professional opinion then perhaps I can relax slightly :-).

Pete

[Tim Downie]

If there's no expansion capacity in your system then every heating cycle will expel water out of the pressure relief valve as it has nowhere else to go. Is it possible that you bled a lot of air out of a radiator recently? It may be that the pressure vessel hasn't been working for some time but by bleeding a large amount of air out of the system you're removed expansion volume that has been disguising the failure of the expansion vessel.

The point is that if the pressure rises quickly and too high as the system warms up, the expansion vessel isn't working.

Tim

[Heliotrope Smith]

The safety relief valve is on the far right of the boiler. You should see the head of the valve somewhere under the pump, it is most likely a plastic head coloured red or black. Don't touch it yet. Trace the 15mm pipe coming from it and dissappearing through the wall to the outside and check if water is or has been discharging. If this is the case then switch off the boiler and top up the pressure. If the discharge continues with the boiler off then the problem is most likely a faulty valve, you could try turning the head of the valve a few times to clear any crud that may have got trapped under the seating. When turning the knob continue turning in the same direction untll you feel a 'clunk'. Do this a few times and keeping topped up then test again to see if it still leaks. If it does then renew the valve.

If the discharge only happens when the boiler is running then the problem is very likely to be a fault with the pressure vassel.

[Roger Mills]

A lot depends on whether the expansion vessel is working. If it *is* it will probably absorb an expansion of 3 or 4 litres between cold and hot - so this would be the amount you are losing. If this is leaking *inside* the house, you should certainly notice it.

If the vessel *isn't* working, the expansion water will be dumped outside, through the pressure relief valve, and there may be none inside.

[Roger Mills]

Oh yes it would! If it's full of water without any air on the other side of the diaphragm, it will have no expansion capacity. So any expansion as the system heats up will cause a rapid rise in pressure and the relief valve will blow.

The air side of the expansion vessel will have a schrader valve (like a car tyre valve). You can check the pressure with a car tyre pressure gauge. With the heating system cold and the water side unpressurised, the air pressure should be about 0.7 bar (10 PSI). If it's less than this, re-charge it with a tyre pump. If water comes out of the valve, the vessel is shot (ruptured diaphragm) and needs to be replaced.

[Ed Sirett]

The leak is small interms of finding it, but too big to leave unattended.

[Pete Verdon]

Great - thanks very much!

I checked this morning and it was indeed a dripping pressure-relief valve. I jiggled it about a bit and still couldn't get it to seal, so I've now replaced it (fortunately it seems to be something most places keep in stock). So far everything seems fine - I'm away for the weekend so let's hope it's still OK when I get back.

Thanks everyone for your suggestions.

Pete

[Tim Downie]

What was the pressure while this was going on?

Just don't forget, if it was weeping when the pressure was high then it could still be your pressure vessel that's at fault, not the valve. If you don't know what the pressure was then you may have replaced a valve doing what it's supposed to do!

Tim

[Pete Verdon]

No - it was still dripping even with the boiler partly drained. The new one doesn't drip at all.

It appears to be all-plastic, so it's not surprising that it might have got a little worn.

Thanks,

Pete