How do we track down a mystery leak in our plumbing system?

[minipie]

Ever since we moved in 8ish years ago we’ve had a slow leak somewhere in the house. We can tell because it’s a pressurised system and over say 6 months the pressure drops from 1.6 bar to 0.5. Loads of air collects in the radiators and taps on the top floor. We bleed out the air, top up, 6 months later it’s down again. No signs of wet or stained patches anywhere (well apart from the leaky roof one... another story).

Any ideas how we could track down the leak? Ideally without taking all the floors up (and of course it could be in a wall anyway...)

PS it’s definitely not the overflow valve as we’ve had that replaced and it still happens.

We have had the same problem for about the same amount of time would be interested to hear the answer

It sounds exactly like faulty pressure relief valve in the boiler. Sometimes a faulty valve can have a permanent slow leak or sometimes just blow off occasionally when it heats up or cools down. Just noticed you say you have changed the overflow valve but I will leave it up for any other viewers.

The other reason I have come across is a damaged diaphragm in the expansion vessel, this fills water the air expansion gap causing the system to over pressure. You can check this by pressing in the Schrader valve (the car tyre type valve on the expansion vessel) If any water comes out the diaphragm is damaged. Either way, unless there is a leak, the only way water can leave the system is a leak that you would notice or some way through the pressure relief valve to a pipe outside.

The easiest way to check for any of these problems without taking the boiler cover off is just to tape a plastic bag over the end of the overflow. leave it until the end of the week and If there is any water in it then you have a problem with the relief valve or the expansion vessel or some other obscure problem with the boiler.

Just as an afterthought, some boilers today also have overflow pipe to take away any condensation. The one from the pressure relief valve normally empties into gully or other drain, sometimes and in most new installs it should be fitted with a tundish so you can check if any water is leaking and may go straight into a waste pipe. It should be somewhere near the boiler and looks something like this one. www.amazon.co.uk/McALPINE-Top-Inlet-Dish-TUN-4/dp/B01I54MHF8?tag=mumsnetforum-21

And that's a very small leak over six months. Check every single radiator valve. I have seen small leaks on these that have been leaking for years without the homeowner even noticing.

Thanks so much KitchenGuy! Yes we have a new pressure relief valve - it gave up completely last winter, we could hear the water flowing away as fast as we topped up and the boiler gave up due to lack of pressure. We had it replaced and thought that would solve the mystery leak, but no.

I will check out your other suggestions especially the expansion vessel.

I have a nasty feeling though that it may be a radiator valve. Would there be signs of the leak above the floor if it was, or could it all be under the floor?

Hubby just checked and water came out of the expansion vessel.
Why would that make the pressure drop though? Surely by now all of the air has been replaced by water

There is some chemical you can pour into your heating system (in via a radiator) which will fill any small leaks it finds. You need to run the heating to force it round the whole system. It only cost us about £50 to have someone come in and do it for us

Sounds like your expansion vessel has gone South which means every time the system heats up the water will leave via the blow off. Somewhere air is being pulled in as the system cools as the pressure turns negative in the system relative to outside.
Once that happens the air that's been pulled in will act as the expansion vessel and everything will work mostly as normal, albeit with cold radiators where they're dry and with air in that will be rusting the radiators.
Get the ev replaced, a new prv if needed and it should be ok

We had a loss of pressure that only happened when the heating was running. In the end we had a forensic plumber come in with a stethoscope to narrow down where the leak might be happening. We were all pretty sure that it was downstairs as we have wooden floors upstairs and there were no signs of damp on the downstair's ceilings. He narrowed it down to the most likely one, which was in the kitchen. We took up the flooring (Karndean tiles so easy enough) and sure enough there was a small damp patch. We had the builder who had laid the floor in the kitchen come in and dig under the radiator. There turned out to be a tiny, tiny pinhole in the piping, so small that it only lost steam under pressure, not water which is why the pressure only dropped when the heating was running.

Minipie. A small leak like that is hard to spot and sometimes they only leak and pull in air when they heat up or cool down. I usually just run my hand slowly around the underside of the valve, fitting and pipe, slightest bit of water on the hand and that's all it takes to cause problems. Sometimes to check I will run a piece of tissue paper around as well. It's easy to notice a tiny wet spot on that blue roll tissue. Also look also for any slight staining on the floor around the valve area, sometimes you can be unlucky and it just runs straight down the pipe and under the floor boards but should still feel wet. Good luck

Alexalee. As Johnd2 mentions. There is meant to be air in the expansion vessel but that air is meant to be isolated from the rest of the system. The air acts like a spring to absorb the expansion and contraction of the water as the temperature changes. With no place for expansion, when your system heats up it will just blow off any excess pressure through the pressure relief valve. By the same token, when it cools down it shrinks creating negative pressure in the system and suck in air through seals which are not designed to stop air under negative pressure. If there is water coming out of the valve then it's certain your diaphragm has gone and you need to replace the expansion vessel. You don't necessarily need a Gas Safe boiler guy as you don't usually have to remove any gas parts to replace it, so any plumber who does boilers will do.

Thanks guys. Any idea what sort of cost I am looking at? I am in South East London.