Hi,
I'm new to this forum, but am at my wits end with my central heating system at the minute, and hoping some of you guys can help.  I'll expain the situation briefly:

Gloworm 30cxi condensing combi boiler, installed in July 2007.  I live in a 3 story mid terrace (basically, two floors plus a basement which includes the kitchen and cellar).  The boiler is installed in the kitchen, but there is no central heating down there, so no pipework under concrete floors or embedded.  All the CH pipework is under suspended floors leading on first and second floor, plus exposed in ceiling of cellar.

Problem started in April - noticed the boiler pressure had dropped to 0.6, but refilling loop was jammed.  British Gas came out and changed it, refilled, etc - all fine.

CH turned off for summer, and no problems.  5 weeks ago, i turned the CH on and boiler started losing pressure again, topping up once every few days.  I've had BG out 4 times now to check and they've done the following:

- replaced PRV
- inflated expansion vessel (apparently it was flat)

However, it seems that since the expansion vessel was pumped up, the pressure loss is now daily, going from 1.5 bars to 0.6 bars and off within 15 - 20 hours.  it seems to hold a lot better when the heating is on (i.e. raising by approx 0.5 bar) but drops within a few hours when off.

I isolated the CH flow and return valves on the boiler 3 times overnight - pressure held in boiler, and decreased by at least 0.3 bar as soon as the valves were opened.  This points to a leak in the system.  However...

there is no water marks or signs of a leak on any of the ceilings, which i would expect such a large leak to create over a month-long period.  Also, a leak detection company came out and confirmed this without charging me, which i was really grateful for.  I've even had all the floorboards up in the one room which i thought could be the offender, but nothing.

I've checked all visible pipework for a leak, including radiator valves, bled radiators, even left whilte paper under then to see if this would identify any leaks - and nothing!

I'm at my wits end here - noone seems to know what the problem is - however, this is more than an inconvenience, as I'm aware that refilling the boiler every day is introducing oxygen into the system which is not good in the long term.

Any advice would be gratefully received.  If it's not the boiler or the pipework, i really don't understand what else it could be!!

Try tying a bag over end or PRV pipe to see if anything is coming out?

It really is looking like a leak, you need to do a 100% check of all pipes under all floors.

If you can not find source of leak then you can disconnect the boiler and rad valves, cap all ends and pressure test the pipework

unfortunately, the system only has 1 isolator for the flow and return for CH at the boiler.  Obviously, I could isolate each radiator - but I'm assuming that this literally isolates the radiator and not the actual pipework that leads up to it.  

I'm assuming you mean isolating for example the upstairs and downstairs - which unfortunately I don't think my system has the capacity to do.  If not, how do you mean 'pressure testing each section'?

thanks for the advice.  From a novice point of view, is it quite simple to add isolation valves to the pipework?  I'm a bit nervous about having to cut the pipes to fit them!  is it best to leave this to an experienced plumber?

Just to add to Toms brilliant video.

The arrow simply points to the end that can be safely disconnected when the pipework is under pressure.

The leak is unlikely to be upstairs as it would show on the ceiling, so I would lift a couple of boards under each rad, where the pipes could get kicked, knocked with hovers etc.

You should find the boards are screwed down from when the system was installed and quite simple to get up.

Have you checked the condensate pipe, whether it is running when the boiler is off, could be an internal leak.

What is it pipes in plastic or copper pipe or a mixture of both.

Have you behind the rads and pipes with a mirror for any sign of staining.

Maybe stating the obvious but are you checking with everything cold..

What is the incoming mains pressure.