I have a new roomstat, correctly wired in an S plan system after a boiler change and updating of the heating control.
The stat seems to ask for heat but within a few minutes of the pump and boiler cutting in it cuts off as satisfied and then asks for heat again a few minutes later. This has the boiler operating on a fairly short cycle. The rooms are warming up OK, but should the stat be operating on such a short time cycle? I know they're in the house somewhere, but I've mislaid the installation instructions at the moment. I seem to recollect something about the stat needing the heating resistor to give a proper temperature differential.
Yep. Most stats come with an optional 'neutral' as well as a live and switched live connection..tying the neutral down is supposed to add hysteresis..or does it reduce it...not sure.
Make sure the stat is not near a radiator too.
But shortish cycling is not a HUGE problem. If its not too short. At this time of year not a lot of heat output is required, and the boiler will fire up for a couple of minutes and then take a ten minute break, typically.
Make sure that what you think you are seeing IS what you are seeing..i.e. most boilers will leave the pump running, but cut the flame once the CH circuit is up to temperature. Thats donw to the temp control on the return side of the boler.
If it is an analogue thermostat, chuck it out and fit a proper programmable one. A decent one will have good control logic built in, including manually setable anti-cycle settings, should you so desire.
I assume that it's a mechanical stat with a bi-metalic strip switch rather than an electronic programmable jobby?
Mechanical stats - in their crudest form have a lot of hysteresis (temperature difference between switching off and switching back on again)
*and* are prone to overshoot. By overshoot, I mean that they turn the heating off when the set temperature is reached but, because the radiators are red hot, the room goes on getting hotter for a while, reaching an even higher temperature. So the heating stays off for a long time until the room cools right down to the point at the other end of the hysteresis, where the stat switches back on again.
Many mechanical stats are fitted with an accelerator heater in an attempt to reduce the overshoot - though it can't do much about the hysteresis. The heater is simply a small resistor - connected between switched live and neutral - which produces a small amount of heat when the CH is running, and makes the stat think that the room is warmer than it really is - so that it switches off a bit early.
The problem with having a lot of hysteresis is that, if the stat is set such that the room never gets too cold for comfort, it actually gets too hot before the system switches off - and you waste energy. To maintain tight temperature control, you actually *need* the system to turn on and off in shortish bursts and pauses - rather than long on/off periods.
Electronic stats can achieve much tighter control than mechanical stats because they have logic which anticipates what is going to happen. They don't need accelerator heaters, and they don't have any inherent hysteresis. Many of the control parameters are user (or installer) configurable - enabling them to be optimised for a particular system.
Err, I don't think so. Not with S plan wiring. The boiler cuts out when it's internal stat is up to temperature. There's also a nice red 'demand' light on the boiler, which only comes on when either the H/W or C/H valves open, and goes out after the short time I mentioned.
Ok..didn't know how clueful you were. Well the stat is doing what it should..but rather too little hysteresis. Ive got the opposite problem..the UFH came on when it was cold and windy, and took a couple of hours to get teh room up to temp..and then cut off..then the wind dropped and it stayed off, with the room temp climbing..stabilised a degree too hot - oh well.
Older types did indeed have a shunt accelerator. Which needs a neutral present. If you don't have a neutral best way is to change to an electronic stat - usually a programmable type - which is battery operated and provides more accurate control anyway.
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