On Sat, 18 Aug 2012 10:17:57 +0100, Roger Mills
wrote:


I'm not totally clear as to the layout of the system - any chance of a
diagram?

Good point. I'll try to knock something up.

I'm assuming that it's gravity HW and pumped CH - is that correct?

Nearly, it's a fully pumped system and always has been since it was
converted from a fully gravity system.

If
so, it seems that there's not enough gravity flow in the HW circuit to
keep the boiler happy - despite what you say about the return getting
hot. Converting to a fully pumped system would probably fix it.

See above. ;-)

Meanwhile, have you tried turning down the boiler stat when you only
want hot water? That would give a wider margin, and make it less likely
that the overheat stat would trip.

Another good idea / point. 'Mum' has always had the boiler stat set on
2 (out of 10) and I think we have tried it on 1 with no discernable
improvement on the issue.

BTW, a 30-year-old boiler ain't going to modulate!

Ah, well .... after posting last, a mate who is Corgi Gas Safe
popped round. I'd already put the heat exchanger back on and re-filled
the system so was able to show him the system in action from cold [1].

And of course it worked perfectly, running just the HW (CH isolated on
the return with a gate valve), running hard / full burner initially
then modulating down a bit till it brought the h/w up to temperature
then cycling on / off thereafter (the pump runs continuously, along
with the boiler, via the time switch).

In fact, he suggested it was 'running perfectly' and followed that
with 'I want one'. ;-)

So, I'll have to repeat the experiment but 'now' ... ?

Cheers, T i m

[1] *Maybe* it's more sensitive to overtemp if I run the boiler up
from cold but with the HW cylinder already up to temp via the
immersion? I was thinking along the lines that the boiler stat might
not be able to modulate the boiler fast enough?