My Combi boiler is lsong pressure- no visible leaks, and having tried CH
leak sealer at the suggestion of the guy that checked the boiler (the
boiler is on a service contract), it's actually got worse- it now
empties to zero on the guage in a few hours, so no CH or hot water :-(.
I *think* it's a pipe under the kitchen floor, which is ceramic tiles
over concrete (it would have to be, wouldn't it), as there's an area
that feels cold/damp to the touch and the grout lines are looking a bit
Is there any sane way to confirm this for sure before taking a chisel to
the floor? We already used the spare tiles we had when the bathroom
suite was replaced, so this will probably mean new flooring in the
kitchen after we're done....
Can you get to both sections of the pipe before and after it runs
through the concrete? If so, get a couple of lengths of plastic pipe
and some push-fittings, then isolate that section, and bridge it with
the plastic, whilst you re-pressure check it.
Or even just temporarily disconnect and blank-off, if it's not
an essential part of the pipework for the rest of the system.
Has any building/decorating work happened recently anywhere?
That's something else to check on. My brother had a decorator
recently who left a trail of destruction, for which another few
verses could be added to "The gas man cometh". (When I've got
time, and if I can still remember all the cockups, I'll write
it up for amusement.) One of the cockups was putting a screw
through a central heating pipe under the floor, which had much
the same symptoms as your case.
If the system has isolation valves which enable any parts of it
to be isolated, try doing that to rule out those sections. (Don't
run the boiler unless you're sure you haven't blocked off the flow
[email address is not usable -- followup in the newsgroup]
Perhaps it's worth fitting some, isolating sections should give a strong
clue where the leak is, as well as allow you to work on one section without
draining the rest.
With the depth of concrete above the pipes, it's a bit like heating a
swimming pool with a hairdryer. A better bet would be to get a damp meter,
and check different areas of the floor. Even so, any damp in one area will
soon wick to others.
How *likely* is it that there's a leak under the floor? What sort of pipe is
it? How old is it? Are there any joints under the floor? Does the system
have inhibitor in it?
I would start by checking the pressure relief valve. Find where it
discharges to the outside world and hang a container under its discharge
pipe, and see whether it collects any water.
Has the pressure vessel been checked? If that is dodgy - or just needs
re-charging - it could well cause the system to over-pressure and discharge
when it heats up - and of course to lose pressure when it cools.
I would investigate these areas before digging up the floor!
The pipework is copper, about 15-16 (maybe a little more) years old.
There must be joints under the floor as all the GF piping is under
concrete- from the boiler the flow and return go straight up for the 1st
floor, and down for the ground floor- the boiler is situated in a small
lobby between the kitchen and (ground floor) bathroom. No idea about
inhibitor, unitl yesterday when I added some along with the leak stop.
The pressure relief discharges into an old coalshed- no water at the end
of it. I've had that one before :-).
With the boiler off and cold, I can fill the system up to 1.5 Bar (which
is where it would usually operate hot) and it then loses pressure- you
can see it drop if you watch it for a few minutes. No leakage at system
drain tap (outside), no leakage at PR valve outlet mentioned above, no
Thanks for the suggestions.
On Fri, 04 Dec 2009 11:48:12 GMT, Chris Bartram
In that case, I would start by installing four isolators near the
boiler in both the upstairs and downstairs flow and return. At least
then you can turn off each floor seperately and you've narrowed down
the cause of the problem by 50%. It also means that if you DO have a
leak under the concrete, at least you can still keep the first floor
warm, whilst you rip out the ground floor pipework!
If it *is* a leak in the pipework, that should certainly help to isolate it.
What if the pressure *still* falls with both legs isolated? Any thoughts
from Combi experts as to whether there's any possibility of an *internal*
leak inside the boiler between the CH and HW side? [On seconds thoughts, not
all that likely - 'cos the HW side will presumably be at a higher pressure
than the CH side - but I'll ask anyway!]
Each time the pressure falls, how much water do you have to put in to top it
[The answer to that will give an indication of how much is escaping
somewhere or other, and will also give an indication of the health of the
I may be wrong, but, as the top-up loop is actually there to top-up
pressure, rather than replacing a volume of (unpressurised) water,
and, assuming that no significant water loss occurs when the loop has
lost it's pressure, it's not likely to be anything like as much as 1
to 2 litres.
I'm fairly sure that water (like most liquids) is considered to be
virtually uncompressible under low pressures. (Mains pressure of, say
4 bar is considered low). So I'm guessing that a CH system will
compress due to a pressure of 1.5 bar only very slightly (considerably
less than 0.1%). Therefore, if the CH loop contains, say, 100 litres,
it will only increase in volume by a max of 10ml, - but probably much
less, when pressurised.
I'm sure someone here will know the formula for pressure versus
compression, for water at a given temperature. And they will be along
You're overlooking the effect of the pressure/expansion vessel! This is a
container with a capacity of several litres which provides resilience to
stop the pressure rising too high when the (virtually incompressible) water
expands when it gets hot. It has a diaphragm with air (or nitrogen) on one
side and water on the other. When the water expands, it compresses the gas a
bit - with a slight pressure rise.
When you pressurise the system via the filling loop, you have to introduce
*enough* water to compress the gas in the vessel to the required static
pressure. This will typically require a couple of litres. The *same* volume
of water needs to leak out before the pressure falls to zero again.
Here endeth the Physics lesson! <g>
You'd think so, wouldn't you. The kitchen floor still lokks like it
might be damp, and a heat gun on it did lighten the colour of the grout
temporarily, so everything points that way.
I've found I have household emergency cover, so someone is coming to
look today. Assuming the leak *is* under the floor, the best thing sees
to be to isolate the ground as suggested, then re-test, then consider
running new pipes from the first floor to the rads downstairs.
Chap has been out, isolated CH circuit completely, so we have HW again
:-), with the boiler switched to HW only.
Suggested asking house insurance people to see if that covers it,
otherwise it's time to call a plumber.
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