I had a new combi boiler added to my ch system. Worked fine for a while but now the pressure keeps dropping and needs topped up with increasing frequency.

I’ve not looked at things in detail – is it most likely to be a radiator leak? Nothing obvious like that. Could it be something else?

It could be – combi boilers run at mains pressure (higher than a closed system with a header tank) so can leak.

A plumber can whizz through some pipe sealant which should sort it.

You must be losing water somewhere. Do you have a drain valve? ours loses water thru its drain valve.

Does sound like a leak. IME the most likely is the compression joints where the pipework goes into the radiator valves, followed by the rads themselves. As the combi runs at much higher pressure than an open system when you add a combi to an old system weaknesses in joints can start to leak.

How often are you topping up? Daily? weekly? hourly?

Did you use plastic piping? Push-fits often need a bit firmer a push than you think

Leaks on rads or the valves can be hard to find if its only a drip as the heat from the rads evaporates the water – try looking for staining

Cheers all. The culprit was identified shortly after I posted the above but I forgot to tell you 🙁

Downstairs neighbour came in saying there was water coming down the wall…turns out a thermo rad valve was leaking. I just turned them all off and drained the rest (the rad in question is currently disconnected), seems OK.

Off to B&Q for a new valve then…

While we’re at it I need to re-install a rad into the system, would I get away with re-connecting the pipes (9mm) without draining the system entirely?

Once I’m done can I simply allow water badk into the rad system and bleed radiators, or do I need to do anything with the boiler given it’s at the highest point and there’s bound to be air in there?

The new valve should be compression fitting so should go on existing pipework without total draining. Compression fittings can go on wet pipes.

You might get away just with a refill and bleed of the rads – is there no bleed point on the boiler or nearby? I have a vertical pipe above the boiler with a bleed point on top just for this.

I’ve re-filled the system without draining the boiler and it makes a terrible noise if you don’t. I’m surprised I haven’t blown a valve somewhere along the line

OK I’ll speak to my plumber, thanks guys.