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 http://tinyurl.com/pum2e54 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.
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
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
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.
[email address is not usable -- followup in the newsgroup]
Probably your expension vessel is knackered.
As water heats, there is nowhere or it to expand so it escapes out of the
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
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.
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.
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.
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.
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.
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.
Overlooking the Clyde where it meets the Sea
It was indeed a long time ago - but a quick Google reveals that you can
still get it!
Or http://tinyurl.com/phunvjl if that doesn't wrap properly.
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.
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 cock, 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?
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