CH Underfloor pipe leak

[Chris Bartram]

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 darker.

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....

[Hawi:]

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.

[Andrew Gabriel]

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 through it.)

[Roger Mills]

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!

[Chris Bartram]

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 wet ceilings.

Thanks for the suggestions.

[Chris Bartram]

Not easily. The piping is the main flow/return for the ground floor, and nearly all of it is below concrete.

No, nothing for several months anywhere near it. I've painted a ceiling, but that seems unlikely ;-).

No isolator valves anywhere :-(.

I'm thinking of heat gunnning the floor to dry it a bit, then see if the dampish look comes back.

[Fredxx]

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.

[Hawi:]

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!

[Roger Mills]

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!]

[Tim]

Is it a condensing combi? If so, a leak in the heat exchanger could be going down the boiler's condensate drain unnoticed.

Tim

[Roger Mills]

Good thought - but probably not condensing if it's the same age as the pipework.

[Chris Bartram]

Now that *is* a good thought. The boiler is a condensing combi, much newer than the pipework. It's a 3 year old Alpha HE CB33. I'll take a look at the conensate drain.

[Chris Bartram]

Nothing coming out of the condensate drain :-(.

[Roger Mills]

Each time the pressure falls, how much water do you have to put in to top it up?

[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 pressure vessel].

[Chris Bartram]

I'd say 10 sec or so of the tap open, so it's a fair amount.

[Roger Mills]

OK, so probably 1 or 2 litres - which suggests that the pressure vessel is probably ok.

However, if this volume *keeps* escaping, it ought to be visible somewhere!

[Hawi:]

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 very shortly!

[Chris Bartram]

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.

[Roger Mills]

Yes, you are!

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!

[Chris Bartram]

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.