I was called around to my sister's to "replace some radiators because
they're too small to heat the rooms".

In actual fact, the bathroom towel rail rad was straight across the
boiler feed/return and the lock-shield was full open. That adjusted, and
a first crude attempt to balance the rads appeared to solve most of her
problems (even the particularly confusing one of the living room rad TRV
turning itself permanently off, once the room was up to temp... go
figure???, do TRVs require a certain pressure behind the pin-valve to
work properly???)


Anyway, that said, she was explaining that she had to fill the CH
circuit every day or so, and was "advised" by the installer "Anywhere up
to the red line is okay" - that being 3-bar, the pressure at which the
Pressure Relief Valve actuates, and dumps the water out of the relief pipe.

Having educated her in RTFM for the boiler (a Baxi something 80) which
indicates normal pressure 1-2bar, min 0.5, max 2.5 she's agreed not to
go above 2-bar on filling.

But, the question remains, what is causing the pressure drop?

I've now got a container collecting anything from the pressure-relief
valve, to check that that's working ok; I've been under the ground-floor
(I job I really don't enjoy) and checked all the joints on the CH system
- no leaks evident. I've not yet checked under the upstairs flooring,
since that's a much bigger job for another day. I've checked around the
boiler and there's no obvious leak.

So, my question is (and I know there's not much to go on) - is there any
likely "gotcha" on a closed system where I can look for a leak or
anything?; I'm not competent to repair anything, but if I can spend my
time finding the puddle of water, maybe I can help save her some cash
when she needs to get an engineer / plumber in.

As always, any help gratefully received.

Mike