Central Heating Starting and Stopping

7 Apr 2007
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United Kingdom
Hi there,

I've recently replaced my old mechanical dial type central heating thermostat with a Honeywell CM907 programmalble one.

All appeared to be working fine until my wife noticed that the heating would kick in (thermostat would click, pump run, boiler on, usual stuff) and then within a few seconds would click off again.

There is no was that the heating was on long enough to make any increase to the house temperature and I'm sure that this continual and brief staring and stopping of the heating is in no way economical or good for the components.

I assume I have a tempremental / faulty CM907 or is this some kind of clever design feature. Thinking of going mechanical again.

Heating system is open vented type with an ideal classic FF280 boiler.

Thanks in advance for any help and opinions.
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Is the new stat actually cutting out. ie does the demand indicator go off?
Yes, the thermostat calls for demand, indicator comes on, heating kicks in then about 15 seconds later, demand and indocator go off, heating turns off.

I must have the most efficient and fastest temperature control loop ever, or a dodgy thermostat.
It's a clever thermostat ;)

Read Honeywell FAQ: TPI control

The important things are:

1. Does the house get up to temperature within a reasonable time
2. Does it maintain a stable temperature.

I've had the wireless version of your thermostat for six years. At first I noticed the boiler going on and off, but soon got used to it. The house maintains a rock steady 21C all day yet the radiators are sometimes only lukewarm.
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To answer your questions

1. Yes

2. No

The house, as far as the stat is concerned gets up to the set temperature then turns the boiler demand off. Then later turns it on again for 15 seconds then turns it off and repeats this until the temperature eventually drops enough to put the heating on 'properly' to reheat the house and then repeats.

As far as the TPI control is concerned its all impressive sales bunkum and it is still basically 'on off control' Proportional control is where the controller output is proportional to the difference between the set point and the process variable, or in this case, actual room temperature. I do struggle to see how my heating system can have proportional control when the output from the stat is either demand on or demand off.

I fully understand the principles of proportional control and even if I was to believe my stat had proportional variable output to my boiler, which in turn would need to have a variable firing rate, there is no way that, with the inherent hysteresis in temperature control loops, that the system can make any discernable increase in temperature within the 15 seconds it stays on for before turning itself off.
turns it on again for 15 seconds then turns it off and repeats this until the temperature eventually drops enough to put the heating on 'properly' to reheat the house and then repeats.
Do you have motorized valves on your system? These can cause problems with TPI control due to the time it takes for the valve to move over.

If you are so knowledgeable about TPI controls and don't think they will work with your boiler, why did you buy one? Or didn't you bother to read the spec first?

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