Slow leak in CH/DHW system

[Chris Hogg]

Vaillant Thermocompact VU GB 226E condensing boiler 15 years old, propane; sealed S-plan CH and DHW system (i.e. no header tank). Single story bungalow, CH pipes in the roof space with droppers down the walls to the rads in the rooms below.

The other day I noticed the pressure in the system was rather low. Topped it up to 1 bar, but over the next few days it slowly fell back again. I assume a pinhole leak somewhere. No sign of an obvious leak on the CH system (no damp spots on the ceiling, for example), although much of it is inaccessible. I've checked the boiler pressure relief blow-off pipe outside and it's dry, so probably not a leaking relief valve. Any other suggestions?

I see that there are additives available for sealing such leaks, for example Fernox F4

but there are other makes. Are these generally OK to use, or do they cause problems over time? Is the Fernox one the best or can anyone recommend a better alternative? I would add it through a radiator bleed valve, so it needs to be in the right sort of applicator for that.

[Alan Deane]

Maybe the coil in the cylinder is perforated, so it's leaking primary water into the DHW?

[Andrew Gabriel]

The symptoms don't quite match a failed pressure vessel.

Slow leaks can dry as fast as they leak when heating is on. If the heating is off overnight, have a feel around all the radiator pipework, valve stems, radiator blanking plugs and bleed points, electric valves, etc when the system is cold before it comes on in the morning.

If that doesn't find any problem, with the system cold/off, fill the system up to the normal hot running pressure for an hour, and then trying feeling around again. Afterwards, let the pressure down to the normal cold fill pressure by bleeding from a radiator bleed valve,

*not* from the pressure relief blow-off valve (because they often fail to fully close after being operated, if any dirt gets trapped on the valve seat).

I am finding that the O-rings used on many radiator blanking plugs over last 20 years only have a life of about 10 years before they start seeping and you see dried crud starting to appear. I intend to remove all mine and replace with 1/2" BSP taper thread plugs, like radiators used to use, and seal forever.

[harryagain]

Probably your expension vessel is knackered. As water heats, there is nowhere or it to expand so it escapes out of the safety valve. Check by taping a plastic bag over the SV outlet. Or look for it dripping at the SV outlet as the system warms up from cold.

If so, ou will need to fit a new expenasion vessel. Existing one may be inside the boiler cabinet. New one doesn't neccessarily have to go there (General purpose one is cheaper)

[Roger Mills]

He says he's checked that, and it's dry!

Not necessarily. It might just need recharging with air - but that's almost certainly not the problem anyway.

I would put my money on a weeping radiator valve, whose leak immediately evaporates when the system is hot - so it never shows.

[Mr Pounder]

In the late 1960s - early 1970s I was installing central heating. I came across many carpets rotted by water leaks that had not been noticed. The days of Baxi Bermuda and heating systems that were easy to understand.

[Roger Mills]

Easy to understand, maybe - but not very efficient. And usually - though not always - with gravity HW systems, increasing the inefficiency even more. I've had two Baxi Bermudas in different properties. Great in their time, but somewhat overtaken by modern technology.

[Having said that, I think there's now a condensing version available].

As far as water leaks are concerned, the systems to which you refer would have been open vented - so leaks could go undetected for a long period. At least with a non-vented system you start to notice a pressure drop when a leak occurs.

[Chris Hogg]

Many thanks for the replies; very useful, especially AG's. Looks like a cold start one of these days, but as the situation isn't critical ATM and a weekly pressure top-up is all that's needed, it'll have to wait until the New Year.

But no one has commented on the Fernox F4 leak sealer or similar. If all my attempts at locating the leak fail, is that a sensible way to go before calling in a plumber or CH engineer, with all the uncertainties that would entail.

[Roger Mills]

It might work, but it may depend on the nature of the leak. If it's something which doesn't move such as a compression joint, it will probably seal it. But if it's a leak from the gland of a radiator valve or from the shaft of the pump, it probably won't.

I had a couple of compression joints which I just couldn't seal in the system which I installed in my previous house about 45 years ago. I used something called (I think) "Bars Leaks" in that - which was intended to be used in car cooling systems - and it worked a treat.

[Olav M]

OMG that was a long time ago! Yes it was called Bars(or Barrs) Leaks, sold at garages/filling stations at the time. As packets on tear off cardboard displays. I have used it, on cars. It worked within 30 seconds of tipping into radiator.

[Roger Mills]

It was indeed a long time ago - but a quick Google reveals that you can still get it!

Or

if that doesn't wrap properly.

[Kevin]

I used one of these a few years ago on a sealed system with inaccessible pipework and a slow loss of pressure. Can't remember which make, but it was probably Screwstation's cheapy. Problem solved, no adverse effects that I've found.

Then on Tuesday this week, I found a wet patch of carpet where one of my wretched isolating valves was leaking past its "spindle". Ordered a bottle of Corgi brand sealer from Toolstation mid afternoon, they delivered it at 7.30 am Christmas Eve, drained off a bucket of water from the most accessible rad with a drain c*ck, tipped the bottle into the f&e tank and the job's a good 'un. 24 hours later, not a drop leaking. I should theoretically probably have used two bottles, as my system has quite a lot of radiators.

Why are those rotating ball isolating valves so utterly useless? Are any brands better than the generic Toolfix/BES cheapies?

[Chris Hogg]

Thanks for that. Helpful and gives me hope for a simple solution.

[Andy Burns]

There's no need to use tinyurl with eBay URLs, just chop the description out of the middle (or edit it to be brief) and chop off all junk after the item number, e.g.

or