Honeywell CM 927 Programmable Thermosat problem

[martin4533]

Have had the wireless Honeywell CM 927 for nearly a couple of years now. Works well and generally pleased with it but there is one thing that puzzles me and I'm not sure it's working correctly. Whenever the tempaerature is hovering around the programmed temperatures or set points I have noticed that the boiler cuts in and out for very short cycles ( 1-2 minutes). The length of time the boiler is working there is definitley no impact on the room tempertaure the stat is in itse,f so why does it come on and go off ? Boiler is Baxi heat only, hot water controlled by cylinder stat. CM927Stat is in temperature stable environment ( not in draughty location). Everything else fine . On start up/set point changes stays on until up to correct tempertaure no problem . Only happening when maintaining temperature but I know the hot water hasn't even made it to the rad, let alone change room temp to cause the switch off, so is it a false switch on reading? Any one any ideas?

 

[mikely]

See this thread which describes what the unit is doing: //www.diynot.com/forums/viewtopic.php?p=1482999#1482999
It should be sensing that your room temperature is stable once up to the required set point and be adjusting the boiler on/off cycle times accordingly, so is probably working fine.

If your system is slow to respond, you could try increasing the minimum boiler on time in the installer parameters system settings. By default this is set to 1 minute. If you increase this, you should find that the unit will cycle the heating at a lower rate.

 

[D_Hailsham]

The length of time the boiler is working there is definitely no impact on the room temperature the stat is in. So why does it come on and go off ?

Don't forget that the control unit only has a discrimination of 0.5C, so when it reads 20C it could be anywhere between 19.75C and 20.25C. My guess is that the sensor can measure to a much closer tolerance and is therefore turning the boiler on to maintain the temperature more closely.

 

[martin4533]

Thanks Mikely, have read the thread and sounds reassuring. I was concerned that it could be damaging the bolier and/or using gas unnecessarily,but if it's doing what it should be then hopefully not the case.

Interestingly my pump runs for longer than the boiler as the thread you referred me to but I know this is because there is a pump overun timer on the system to help dissapate the heat as my boiler is positioned in the loft space and without can cause problems with the heat exchanger apparently

Thanks again.

 

[martin4533]

Thanks D hallisham. Is this connected with the Proportional Band Width ? I read this as somehowa latering the sensitivity of the unit but I'm not sure. If I wanted the boiler to come on less but cycle for longerwhen it does come on do I increase the band width or decrease ? same with the temperature range you mentioned ?