After being disturbed due to other work the pressure relief valve
appears not to be sealing and the pressure drops to zero. I can keep
the pressure up by cracking open the inlet tap just a tad, getting
about 2 bar, but of course water then drips out of the outlet pipe.
Whilst waiting for someone to come and fix it a few questions:
1) Am I causing any problem by doing what I'm doing?
2) What happens if I just let the pressure drop to zero?
3) Is the water coming out just tap water or will there be CH radiator
water as well? ie is the pressurising isolated or part of the
You're diluting any corrosion inhibitor and adding more limescale. You
may also over-pressurise things. Technically, i think you may be
breaking a water byelaw by connecting the radiator system to the mains
Once they have been lifted, these valves quite often don't seal again
IME. It is the pressurising system for the radiators (and the heating
coil in the hot water tank, if you have one).
I would not advise "drip feeding" it by cracking the filling valve
because you are losing inhibitor all the time the relief valve leaks,
but perhaps with one exception.
If you let the pressure drop to zero, the boiler should trip out leaving
you without any heat or hot water. If you (or other occupants) are
vulnerable to low temperatures and don't have any alternative heating it
is perhaps the lesser of two evils to waste your inhibitor. Explain this
to your plumber and they will test (or more likely replace) your
inhibitor when they fix the valve.
What do you mean by a drip? More than a bucket a day will be
significantly diluting your inhibitor, and also introducing limescale to
your boiler, which is (in general) a bad thing. If it is less than a cup
a day, and you can get a plumber in within a couple of days, you won't
really be doing much harm.
You are continually adding fresh oxygenated water which will cause
accelerated corrosion and limescale deposition in the heating system.
Then the boiler will lock out or just refuse to fire next time it tries
to start up.
Its the primary water that flows though the boiler, and the rads.
If the boiler is a combi, then there will be a separate heat exchanger
for the tap water. (and that secondary heat exchanger HE is itself
heated by the primary water heated by the main heat exchanger - there is
normally a diversion valve in a combi which will direct the flow of
primary water either through the rads, or the tap water HE)
I mentioned in one of the other posts that the problems started after
the combi diverter valve was replaced. So using hot water is going to
be an issue even if I turn the CH off?
PS none of the companies I've contacted today have got back to me :(
You need the primary side filled and at adequate pressure to run the
rads or the hot water. When in hot water mode, the diversion valve short
circuits the primary loop such that it just flows through one side of
the secondary heat exchanger, and then back to the main heat exchanger
for reheating. (The cold mains only flows through the other side of
the secondary heat exchanger).
 Just for extreme pedantry, there are a very small number of combi
boilers out there that actually have the tap water HE heated directly
from the gas just like the primary one, or instead divert the cold mains
water through the primary HE itself in place of the water normally
circulated to the rads.
Fixing the over pressure relief valve is DIYable. (as would be replacing
it) if you can get to it reasonably easily. Chances are its just
acquired a bit of crud / scale / solder etc that is preventing it from
resealing correctly. Just taking apart anc cleaning it will probably fix
Thanks. I have a man coming this afternoon - arose out of a
recommendation from someone I don't know on a local message board I
frequent so that should be fun. To be fair the recommendation is from
someone who says he runs several properties. I'd like to do it myself
but I just know I'll put it all back together and it will be worse.
Whilst I agree that it may be crud the last guy did a few "blow outs"
and it seemed to work for a while. The part costs about ?20 and mine
is a good 10yrs old.
My main worry know is the amount of fresh water I've introduced to the
system as this problem has been ongoing for a few weeks and I
understand a complete flush is a slow process.
Any advice on what I should be getting the service guy to check?
Presumably all radiator fluid above the height of the valve will end
up draining out - fortunately as this is a bungalow that will only be
the pipes that go up and down.
No need for a flush generally, unless the system has lots of sludge in
it, just get them to add new inhibitor when the system is back up and
If its a modern condensing boiler, then it might be worth getting a
magnetic/cyclonic filter (Fernox TF1, Magnaclean etc) fitted on the
return to the boiler if its not already got one. That would trap any
particulate material before it can get into the boiler and block or
damage the primary HE.
There should be service valves on the flow and return to the boiler. So
he will be able to isolate that, and drain just the boiler. The rest of
the system can be left filled and untouched.
Just mention to him that you have had the problem for a while and so are
not sure what the inhibitor level will now be.
Well he came yesterday late afternoon, part in hand, and after a lot
of fiddling with a small allen key replaced the valve, repressured to
1 bar (the last guy went up to 1.5 bar), satisfied himself that the
expansion chamber appeared to be operating and gave me instructions to
check that the pressure didn't exceed 2 bar with the evening's
He seemed quite disinterested doing anything about inhibitor levels,
said he was out of the testing paper (litmus paper?) and I had to push
him to run through with me what I needed to do.
So today I drained about 1 litres off from the drain point below the
boiler, opened the 'drain' tap upstairs where the pipes go from the
back to the front of the house, drained another litre then added 1
litre Calmag Chem Protector, topped up with some distilled water and
tightened everything up, put the pressure back to 1 bar and checked
for leaks. The water that came out was quite clear.
I'll leave it a couple of hours to settle though I'm not sure if
that's necessary then my wife wants a shower.
Not sure if I've done all that is worthwhile. I'd like to just run
the pump for an hour but I don't know how to do that (Baxi 105 HE).
Do I need to bleed the rads? All the rads are on the ground floor of
a bungalow and the top up was done on the pipe above in the attic
which was filled to just overflowing and then cracked open (bled)
after I'd pressurised and water came out immediately.
I guess I'll just check the rads for cold tops and check the top pipe
again but otherwise all seems well.
That didn't last long :(
I bled the rads as suggested the next day.
Yesterday morning the rads were cold, boiler error light on:
Pump Fault or Low Pressure
The pressure was at 2.0 bar.
Switched to off - clicked to reset and working again. Working
pressure around 2.3 bar.
This morning my wife come's screaming in shivering, got into the
shower and the water was cold. Same fault indication - repeated reset
procedure and ok again.
One unsettling thing, when I bled the rads I took the pressure to 1.3
bar - it is now sitting, and not changing, at 2 bar which is a little
on the high side.
If the inlet isn't turned fully off this should surely be creeping up
- so why has it risen and is it a factor in the boiler turning itself
Just a recap:
12/7 Standard boiler service
09/8 Rads not heating, new diverter valve assembly fitted
10/8 Pressure dropping and leaking through the pipe outside
This sort of stabilised until
14/9 no longer stable, dropping and dripping outside
10/10 New prv fitted
11/10 Added Calmag
17/10 Bled system
1/11 Above faults started
Thanks again for your input John.
The filling loop is braided. I'll have a go at detaching it tomorrow
when I can afford to turn the heating off again. But if it was
leaking the pressure surely would just slowly continue to rise and the
pressure relief valve pipe is as dry as a bone.
I went around the bungalow again and checked/bled the radiators.
There is one that spluttered for quite a while and it is the same one
that did the same when I bled them a couple of weeks back after
putting the inhibitor in.
There is a nice description of the pressure system at
I'm wondering now if there is an issue with the expansion chamber
introducing a leak into the system.
Of course the engineer will say it's easy to fit another one but the
costs of this boiler are mounting up - what next? Pump? Circuit
board? Classic dilema of throwing money at it when getting a new one
instead of having it serviced may have been a better option.
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