Combi losing pressure - how to locate leak?

A friend's combi-driven CH system is losing pressure. After refilling, the pressure drops to zero within two hours. No obvious leaks upstairs, downstairs or in the unheated cellar where the downstairs pipes are visible.
He thinks it must be leaking under the relatively recent concrete-floored extension.
Is that a reasonable conclusion?
If so, how can the leak be pinpointed? He's hopeless at DIY and I'm at the other end of the country but in general terms, is there a way to prevent the entire floor being dug up in the search?
He's had a plumber in who pumped in some leak sealer to no effect.
What should he do next?
And can anyone recommend a reliable plumber in E17 / Walthamstow? (He's away for Xmas anyway so he's not requiring an emergency call out on Xmas eve.)
Thanks for any pointers.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

In case everyone else is down the pub, I offer this:
Turn off all lock shield valves on all radiators (or if he knows the pipe routing, just on the first rad on each leg) then repressurise. If it still leaks, the leak is between the last bit of visible pipe(s) and one or more of the rads on one or more of the legs.
If it doesn't leak, open lockshield valves in some kind of sensible sequence until it does leak - then the leak is between the one just opened and the one just previously opened.
This advice is worth exactly what you've just paid for it.
IANAP.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

If I understand you correctly you seem to be suggesting that the radiators are plumbed in series - that's not normally the case, they are in parallel.
Peter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 21 Dec 2007 18:57:15 GMT, "Peter Andrews"

Ahem. It's a good job IANAP, then. (blush)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
mike wrote:

It's possible although unlikely. Mine does the same, but it's the blow off valve ( the high pressure relief thing) that is causing it - I've purchased a new one and fitted it but the problem remains the same, however, in the past 2 days I have managed to keep it pressurised and I did this by accident. I turned up the roomstat from 20 to 25 degrees, this made it too warm but the thing stayed full, so to compensate, I turned down the heat setting on the boiler....I've no idea how this 'fixed' it, but it's not required refilling since Wednesday, whereas before then it lasted no longer than an hour or two

Ask him to check the copper outlet outside next time it loses pressure, he'll probably find that his hot water is not far away.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 21 Dec 2007 17:58:22 +0000, mike wrote:

I suggest you read the SealedCH FAQ.
If the leak is not obvious it may well be the releif valve.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Agree with all that.
Also, if there has been a recent extension then the current expansion tank may not be large enough. I havn't had a relief valve fail (and they usually fail if they leak) since I added a bigger tank.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In an earlier contribution to this discussion,

As others have said, there may not be a conventional leak at all, but the system may be losing water through the PRV (pressure relief valve).
Your friend can check this by locating the pipe which goes out through the wall from the PRV, and placing a container under the end of the pipe to catch any water which comes out.
If water *is* being lost through the PRV, the valve *could* be faulty - but it's more likely that it's simply doing its stuff and preventing an excessive pressure build-up - as can happen when there's a problem with the expansion vessel.
The expansion vessel contains air (or sometimes nitrogen) which is separated from the water by a diaphragm. When the water in the system gets hot and expands, it compresses the air in the expansion vessel, causing just a modest increase in pressure. However, if the air leaks out and the expansion vessel is completely full of water, any expansion causes a much higher increase in pressure - with the result that the PRV opens and releases it. When the system cools, the pressure is all lost.
If this is the problem, the expansion vessel will need to be re-charged with air or - if it is completely shot - replaced.
--
Cheers,
Roger
______
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks for all the replies and thanks for your explanation Roger - that really helped to clarify what Ed has listed in the FAQ.
He's taped a bag over the pipe tonight and will report back on any water collected tomorrow.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Another place to look is under the caps of lockshield valves. Water can leak through the gland when warm but is evaporated by the heat.
When I converted my system to sealed operation I changed all of the valves and there were three of the originals that had leaked as evidenced by staining and gunk around the upper part of the valve. There was nothing on the chrome body part.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ah a sensible post on this topic from me - had a similar (but less noticeable) problem with my systeme wher a bathroom towel-warmer type rad was leaking from the TOP joint ... never noticed it until the powder coating blew ...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.