So far all the normal tricks don't seem to be doing it!
Short version:
There is a leak somewhere in my central heating system - its losing pressure regularly, and needs topping up with at least a couple of litres a day.
Longer version:
The pipework is of questionable vintage, but I am guessing 30+ years old. Almost exclusively copper apart from about 3m of speedfit for one particularly awkward pipe run. Largish S Plan+ system - unvented cylinder and about 21 rads (split into separate upstairs and downstairs zones).
The system was pretty good - only needing a small top up at most once or twice a year. However over the last year its got worse. It got to the point where it needed a top up every other week or so. So I carefully checked around any areas where there have been changes in recent times.
I found one radiator tail in the downstairs loo where I had needed to do a "live" pipework change and had used a cuprofit fitting that was knocking about in the plumbing kit. That had a very slight indication of a weep from around the base of the fitting. So I chopped it out and re-did it properly. After that, no improvement or the rate of loss seemed to get even worse! Even though there is definitely no evidence of a leak in that location now.
So I checked the PRV drain pipe on the system boiler. That's not letting by. Checked the drain point tap which is outside - again its dry.
Seems unlikely its the coil in the indirect cylinder since it is an unvented cylinder, and the water pressure inside it is higher than that of the CH primary - so if that leaked the CH would fill up!
So I had a careful look about. No indications of staining on the ceilings, and it seems unlily one could lose that much water by evaporation from the leak point. so I thought ground floor would be more likely - there is loads of space in the under floor void where you could lose water without it showing up anywhere.
However over the last few weeks I managed to get eyes on pretty much all the GF pipework (sometimes indirectly by bribing second born to climb through a hole in the floor and romp about in void and look for any signs of leaks or damp, or corroded looking pipes etc). Got some more boards up today and looked at the last two rad pipe runs. Again all ok.
Any bright ideas?