A question for the old CH gurus please.

[T i m]

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

[Roger Mills]

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

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

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.

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

[T i m]

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

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

See above. ;-)

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.

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?

[<me9]

The house I rented 34 years ago had a modulating boiler. It wqs a hugh wall mounted one, IIRC servowarm. It's only control was the modulation to keep water temperature (approximately) constant, the pump was permanently on. I remember little else about it.

[T i m]

That's sounds pretty well like what Mum has then except it's quite small. ;-)

It's also pretty quiet in that you can only really tell it's on when it first fires up.

And if you subtract all the attention it hasn't had over the last ~30 years from any efficiency downsides then she still might be quids in. ;-)

Cheers, T i m