Hi all,

So, Mums New World / Ascot wall mounted, low capacity, copper HE
balanced flue gas boiler has been running fine for the last 30 years.

It was inserted into what was a full gravity, coal fired, cast iron
panel rads CH / HW system by bringing the return back up over the
kitchen and joining the return from the indirect cylinder, before
going back to the boiler.

Apart from a 22mm gate valve on said new return pipe on the CH side,
she only has the rad valves and the boiler stat as any form of
control. She has been happy with all this.

Now, as part of a bit of a bathroom makeover (she's thinking of moving
to a bungalow but not 'just yet') we replaced the big bath and equally
large cylinder with a gravity thermostatic shower and smaller / modern
/ smaller cylinder (she bought all this bathroom stuff in a Homebase
sale in 2005 but Dad was taken ill so ...).

Now, with the heating isolated and the boiler fired up from cold (and
cold water in the cylinder), the boiler runs for maybe 60 seconds
before tripping the (new) overtemp stat (as it did the original).

Open the heating valve and everything runs fine (even with all the
rads turned off ('single pipe' system)).

Now, from the cold scenario above, if you hang onto the boiler flow
and return at startup, you can feel the flow first getting hot
(quickly) and then the return getting pretty hot soon after
(suggesting there is a good flow round that pretty short cct (possibly
4m, not including the cylinder coil) cct).

Crack open the CH valve and it takes *much* longer for the return to
show the same level of heat as the flow (and by which time the boiler
would have modulated etc). Left long enough, the boiler turns off
(pump stays on all the time on the same timeswitch as the boiler feed)
and cycles on / off thereafter, just as expected?

I've currently got the heat exchanger out and have run a hose though
it (ensuring water went though both pathways) and it seems pretty
clear (little backpressure from a garden hose on full). I put some
kettle descaler in there for a while but it made no difference.

OOI, on all of the pipes we opened there has been little sign of any
furring and when filled, run and flushed the water now runs pretty
clear when drained.

The rads quickly get hot top to bottom and the HW gets pretty hot as
well!

So, the only thing I can think of now is we have definitely reduced
the volume of the HW cct and possibly also reduced the effective
capacity of the cylinder (smaller water capacity, smaller coil and of
a smaller diameter pipe). I feel the flow is ok judging by the speed
the hot water gets round the boiler pump (Grunfoss 3 speed, now on 3
and still pretty quiet) cylinder boiler cct, so, is it that the
new cylinder simply can't consume enough energy to give the boiler
stat time to respond to the temperature increase in time to stop
itself overheating?

Looking at the tubular towel rail in the bathroom, it might be fairly
straightforward (if not easy as such) to put the towel rail
effectively in parallel with the HW cylinder (like a bypass) to both
increase the water capacity of the 'HW' cct and give Mum a hot towel
rail all year round (that she would like). However, what are the
chances it would be enough?

Just to recap, with all the rads closed, CH on and just 2 x 1-1/4"
heating pipes in cct the boiler / HW seem fine?

FWIW It never seems to kettle but I may hear some *very slight*
'sizzling just before it trips the overtemp stat (but the burners
might still be on full at that time)?

Cheers, T i m