Checking radiators for a leak

[Prudence99]

The pressure on my 4 year old Worcester Bosch combi is on all over the place.

It is reading 0 when the boiler is switched off and varies between 1 and 3 when switched on. I think the 3 may be because I overpressurised it! I wasn't sure what to do so went outside and I noticed that the little copper pipe that goes out through the kitchen wall to the outside had started dripping, every 3 seconds. I'm not entirely sure how much water eventually came out of it because it was dark and raining.

After a short while I noticed the pressure starting to fall on the pressure gauge and it went down eventually to about 1 and a half. Today pressure is again at 0, I haven't switched it on yet.

I'm now wondering if I have a leak in the radiators. Is it best to have the central heating running so the radiators are hot or is it best to check the joints and valves (or whatever your call them) when the radiators and cold?

I can't think of any other reason why the pressure gauge reads 0 when switched off.

Many thanks.

Sounds like your expansion vessel has either lost pressure or just broken. You'll need a gas engineer to check it.
The expansion vessel is basically a pressurised balloon to let the water expand as it heats up. If there is no pressure then it will just be full of water and nowhere to expand, and if the balloon has burst there will be water on the wrong side.

Hi, thanks for your reply.

I switched the boiler on about 20 minutes ago to see what would happen. Pressure gauge is now registering 2 and all radiators working. Is it safe to have it on or is it likely to blow up?

Shouldn't blow up as the safety valve will let any over pressure out. However it's not good in the long term to run it like that.

That's reassuring anyway about it not blowing up. Have a service booked in but not until beginning of November as that was the first available date.

Thanks again.

You can top it up to get it to start up, and it will release the over pressure when it warms up through the pipe you saw outside.
To reduce the wear on the system I'd turn the radiator water temperature control on the boiler down as much as you can, turn off any radiators you don't need, and turn the heating on and off as little as possible. Then it's less likely to need topping up and releasing the pressure.

It's up and running at the moment, but I didn't repressurise it today and the gauge is on 2. Should I switch the boiler off and repressurise some more?

No if it's working then leave it as it is. Just get someone to look at it as soon as they have time.

Have you bled air out of the radiators? Air will expand greater than water which will mean a greater pressure in the system.
So blead the radiators before putting more water into it.

if the pressure goes substantially up when hot and substantially down when cold, it's your expansion vessel. If it has not been serviced in several years it might just need pumping up. Otherwise it will need a new one (not very expensive). You need a boiler mender.

If the maker's vessel inside the boiler is expensive or unobtainable, you can have an external one fitted anywhere on the pipework. It is a bit unsightly so if you can have it fitted in the boiler cupboard it will be neatest.

The longer you leave it, the colder the weather will be, and the busier the menders. No-one bothers calling them until it gets cold.

p.s.

don't bleed air out until you've had it mended. Air is compressible so will take up some of the expansion.

Actually that's a good point, if you're handy, you can make a temporary expansion vessel by draining a radiator and not bleeding it but leaving it on, then the air will act like an expansion cushion.