Pressure in boiler keeps dropping to nil, any help?

[Rumours]

We've just swapped the radiator in the bathroom for a heated towel rail. All radiators have been bled. Topped up the water so the pressure was at 1 bar. Heating comes on and all is fine until the heating goes off and the pressure then drops to nil. We then have to re-top it up to put the heating back on. Any ideas why? Its gas central heating, and a combi boiler.

Hmm def not a leak anywhere???

DH has checked and says not. He put a jug under the pipe outside where the overflow thing is and it keeps filling up when the heating goes off.

Sounds like a leak or your overflow valve is sticking and emptying the water out of the system. Either way, the water is going out of the system somewhere I'm afraid.

So it will be nothing to do with the new heated towel rail. The boiler was fine before this.

Thanks for your replies

Oh we had this. Twice in different houses . It got fixed. If there was no water coming out the pipe then it would be a tiny leak some where. I am trying desperately to remember why water came out ie which bit needed replacing. A valve maybe ?? phone a boiler person and they should know.

I suspect the towel rail is a coincidence . Annoyed I can't recall more

It's possible that when you topped up the pressure, you accidentally went too high, and it came out out through the pressure relieve valve that goes through the wall behind the boiler. Once the valve has released it seldom seals perfectly again. This can also happen if you have not fully turned off the topping-up valve.

However:
Look at the pressure gauge in the morning, when the system is fully cold. Take the reading and write it down, with the time. Turn it on and let all the radiators get hot. Take the pressure again.
Let it get fully cold again. Take it again.

What are these three pressure readings?
If it goes up a lot when hot, and goes down a lot when cold, your pressure vessel probably needs pumping up.

For either of these faults, you need a boiler repairman. Neither is a complex repair. Have the annual service done at the same time.

Agree, sounds like a leak.

If it's coming out outside it's either your pressure relief valve which is letting by. They are very difficult to reseat and very often need replacing. However unless the boiler was over pressurised when topping it up I would doubt it would be that from what you have said.

Is the pipe plastic? If so sounds like your condensate pipe which will dump water outside - perfectly normal. In which case you have a leak somewhere - very hard to spot when heating is on as it may evaporate before it could be seen. Try wrapping toilet paper around the new joints on the new rad, that should show if there's a leak there.

If nothing shows Get a gas safe registered engineer to check it over.