Problem with a replacement room stat

[bogblock]

Morning all

I recently moved into my present house which has a Danfoss 102E programmer. The room 'stat was a Switchmaster. Both these items are dated 1994, when the house was built. The roomstat had two wires connected. One is permanently live (when the programmer is set to heating on) and the other is a switched wire which goes live when the room stat calls for heat.

I replaced this room stat with a programmable Honeywell CM907 ( CMT 907A1041) wiring it as shown in option E of the installation instructions as a straight two wire replacement.

When I switched the mains back on, the stat behaved as expected, firing up the boiler and pump when calling for heat and switching them off when satisfied. Deep joy, or so I thought.

With the new stat in 'Auto' or 'Man' mode, when it is satisfied (room temp above target temp) the boiler and pump fire up at ten minute intervals. After about one minute the boiler shuts down.

Edit. The stat is fairly close to the hall radiator, but the boiler cycling remains even when the target temperature is well below the room temperature. The system is fully pumped with a three port motorised valve.

I have reset the stat to the factory defaults but it made no difference.

I have re-fitted the old stat and the problem has gone.

I emailed Honeywell about this but haven't received a reply.

Any suggestions before I replace the programmer and rewire the lot?

TIA

Dave

 

[rpevans]

Did you buy this new or used?
Have you checked the Installer Parameters?
Richard

 

[bogblock]

Hello Richard

Yes it's new.

All the parameters should be at their factory default settings.

Cheers

Dave

 

[rpevans]

I have a CM901 and I have noticed that the boiler and pump will run even when the room temperature is reached. I think that this is to do with the 'cycles per hour' setting in the CM907. I think that this is normal.
If the room temperature is controlling accurately then I wouldn't worry.

If you want to check then you can change the cycles per hour setting temporarily to 3 then 12 and observe the effect. Remember to put back to 3 (I am assuming you have a gas boiler).

I also have mine set with optimisation enabled. This function works very well, delaying the startup of the boiler until the last possible minute to reach the room temperature by the set time.

Hope this helps.

Richard

 

[D_Hailsham]

With the new stat in 'Auto' or 'Man' mode, when it is satisfied (room temp above target temp) the boiler and pump fire up at ten minute intervals. After about one minute the boiler shuts down.

I have re-fitted the old stat and the problem has gone.

There is nothing wrong with your thermostat, it is doing exactly what it is supposed to do.

However, if the room temperature is rising above the target temperature you are getting overshoot. Try increasing the Proportional Bandwidth.

Read FAQ TPI control

the boiler cycling remains even when the target temperature is well below the room temperature.

What target and actual temperatures are we talking about?

Have you checked the temperatures against another thermometer?

Is the room receiving heat from another source, e.g sunlight, cooking in an adjacent room etc?

Does the 'flame' show on the display when this happens?

 

[bogblock]

Thanks for the replies.

Richard. What bothers me is that the pump starts at these ten minute intervals and it is in the airing cupboard next to the bedroom. I am a light sleeper.

D.

What target and actual temperatures are we talking about?

When I noticed this happening, the room temperature was 19. I set the target temp. to 14 and it still happened.
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Have you checked the temperatures against another thermometer?

Yes. A digital Danfoss stat which reads one degree higher than the Honeywell.
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Is the room receiving heat from another source, e.g sunlight, cooking in an adjacent room etc?

No.
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Does the 'flame' show on the display when this happens?

I'll have to check when I refit it.
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Thanks for the link about TPI. What I don't understand is that this ten minute cycle happens when the room stat is satisfied (by five degrees). It makes sense as the room temp approaches the target temp, but surely it should stop if the stat isn't calling.

I'll scroll through all the parameters and make sure it's properly defaulted and try it unconnected to see what happens.

Cheers

Dave

 

[D_Hailsham]

When I noticed this happening, the room temperature was 19. I set the target temp. to 14 and it still happened.

Check parameter 12:tO (temperature offset) is set to 0.

A digital Danfoss stat which reads one degree higher than the Honeywell.

But that could be inaccurate. You really need to use a mercury or liquid thermometer to get an accurate value.

What I don't understand is that this ten minute cycle happens when the room stat is satisfied (by five degrees). It makes sense as the room temp approaches the target temp, but surely it should stop if the stat isn't calling.

Yes, that is unusual, which is why I asked about the flame. If it is not showing it means that the boiler is being lit from somewhere else. Do you have a hot water cylinder?

 

[bogblock]

Right

I've had the new stat on the bench and tested it. It seems to work OK. Once it's satisfied it stays that way until it calls again.

Looks to me as though some condition is coming off the existing wiring to cause this problem. The wiring itself is a bit of a rat's nest, so I'll tidy it up when the weather gets a bit warmer.

Cheers

Dave