The mystery leak

[Vikki]

There is a leak somewhere on my central heating. The pressure is taking about 24-36 hours to work down to a point where the combi boiler won't fire. Most of the pipework is under floorboards, but two radiators are on a concrete floor.
After days of searching, I cannot find this leak. Surely if it is losing this amount of water it would be visible somewhere. I have also had two plumbers (one under my service contract, one independent) to look at this, they are no wiser. They're not too interested because it takes time to track it down and no-one wants to deal with a concrete floor. Fair dos.
Views appreciated on the following proposed action plan. If you have any better suggestions for me, tell me quick, you're welcome.
I think I am going to put in a treatment of Leakex (radiator method), see if that will cure it.
If no luck after a few days, I think I will have to cap off the pipes going under the concrete floor, to rule out or in those radiators.
If it still loses pressure, up with the floorboards everywhere (is the leak going to be more visible if I keep the heating on during this?). If it don't lose pressure, I better get myself one of them Kango things!
Any ideas gratefully received. This is bad (or good depending how you look at it) timing - I was going to start laying new laminate tomorrow!

 

[DP]

Get a (reputable) heating engineer who will call and look at the system for a nominal charge. Free estimates equate to costly repairs. One man band/ small business is best to deal with especially when they come recomended by friends and relatives.

What area?

 

[croydoncorgi]

Since it's summer and you don't need heating, try this:

First, look underneath the boiler and identify the isolating valves on the heating flow and return pipes (they'll probably be at least 22mm pipes). If the valves are in poor condition or the boiler is a Potterton Combi 80 or Puma (valves are total crap and tend to leak as soon as they are used!) forget this idea - you might add another leak.

With the boiler cold, turn it off and carefully re-pressurise to an exact value (say 1.5 Bar). Turn OFF the two isolator valves and leave it overnight. In the morning, look at the pressure. If it's dropped significantly, the leaks in the boiler. Then open the isolators. If the pressure drops suddenly, then the leak is definitely somewhere out in the rads or pipework.

Now it's worth lifting boards and renting the Kango (or maybe trying leak sealer first).

 

[MANDATE]

First place to check before any drastic action is the 'pressure release valve'. I gather that if this has been opened in the past due to too high a pressure it may not have fully re seated itself on closing, therefore causing a small leak. The exit pipe for this valve should be outside but it could well be inside at the boiler.

 

[croydoncorgi]

My suggrestion was intended to localise the fault to 'boiler' or 'somewhere else'....