When I moved to my present home I inherited a 6 year old Thermecon oil boiler supplying 16 radiators. I have replaced all the 40 year old radiators and converted the previously very antiquated system to a fully pumped one with boiler interlock. It occurs to me that there must be quite a bit of heat wasted at the boiler when the room stat reaches temperature and the boiler shuts off, and that it might be worthwhile arranging for the pump to overrrun so that the already heated hot water is pumped round for a further 10 minutes or so rather than just wasted heating up the utility room that houses the boiler (at present the pump switches off the moment the stat reaches temperature).
Any comments please? Would this be worthwhile, or is it a crazy idea? How would one arrange it - what electrical equipment would one need - please specify exactly, I have little technical know-how about these sorts of things.
I did just this to my old boiler - which was a basic electrics gas one. But I'd say 10 minutes is too long. Think I ended up with three. I'm not sure if there's anything available commercially to do the job - I made my own. I've still got it somewhere - it's surplus to requirements since the current boiler does all this itself. But like all this sort of thing installing it requires a degree of knowledge to modify the wiring.
I would fit a pipe stat. Just set it to pump until the temp drops below a useful level. Wire it across the permanent live and the pump live. About a tenner.
I was also considering doing something like this as my boiler is located in a boiler house, and I would prefer the boiler residual heat to be used in the house. The boiler is an old cast iron design that doesn't need a pump run-on circuit itself.
Unfortunately the circuit is quite complicated as I have three zones, and each zone's motorised valve closes when its associated thermostat is satisfied. The auxiliary contacts on the zone valves control the boiler and pump, so all three zone valves have to be closed before the boiler (and pump) shuts down.
I need the circuit logic to recognise when all of the zones have been satisfied, and then keep the last zone valve open whilst the pump run-on operates (no point pumping against a closed valve!). And then, if the last zone to be satisfied is the Domestic Hot Water, would I want the pump run-on to pump through the DHW circuit? Probably not, as it might overheat the tank.
And what about during summer, when the CH is off? Where will the pump run-on send the residual heat from the boiler then?
At the moment I have a gravity circuit that brings the residual heat from the boiler into the bathroom towel rail. Maybe I should stay with that.
As you say, with an S+ Plan system, it is the closing of the zone valves which switches the boiler off, so during any over-run period, the valves are all - by default - closed, thus preventing any circulation to dissipate the residual heat.
How about deciding which zone you want to dump to, and plumbing in an automatic by-pass valve in parallel with the appropriate zone valve? You'd have to adjust the by-pass valve so that it *only* opened when all the zone valves were closed - much the same as a normal by-pass installation except that the return is via a zone-worth of radiators rather than direct.
Using my suggested solution, via whichever zone you've selected. OK, you don't *want* the radiators to get a bit warm, but it doesn't really matter. [It relies on having at least rad open, of course - but presumably the one near the room stat will be, and won't have a TRV on it].
As far as achieving pump over-run is concerned, you can do it with one of these.
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's intended for keeping bathroom fans running for a while after you switch the light off, but works just as well with a CH pump. I'm using one which is set to about 2 minutes on my system, and it works fine. [My Baxi Solo MkI
*does* require pump over-run, but implements it rather crudely by initially switching the pump off with the flame and switching it back on again when the water temperature gets up to the stat setting - by which time it is sometimes too late, and over-heats and trips. My system keeps the pump running for a couple of minutes whatever, and ensures that the residual heat can get away *before* the temperature builds up to trip point. I have an automatic by-pass valve between flow and return in the airing cupboard (a long way from the boiler) so there's plenty of scope for dissipating the heat].
Sorry to reply so late but I clicked the link and saw that it is a Manrose timer from TLC. How have you got on with this?
I've been sent a few of these but found them wildly inaccurate. When set to 20 minutes on the front I get at most 7 minutes on. YMMV. I suppose if you ignore the markings on the front and set it to the required time and leave it alone it's ok, but it makes setting a new delay a pain as it is trial and error, or was in my case. If the legend on the box matched the time delay it would be so much easier.
It seems to do the job. I agree that the scale isn't very accurate - and is very small, with less than one revolution of a smallish knob covering a range of 1-60 minutes (I think). So it's a bit "suck it and see" when setting it - by nudging it a bit and then using a stop watch to see how long it over-runs. This is not too bad, since I've set it to about 4 minutes - so you're not waiting about for too long. Once set, it seems to be pretty consistent.
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