Cm 907 thermostat

[Lawrie Davidson]

Hi, can anyone help please.I have a Honeywell CM 907 thermostat in the hall. It has worked perfectly well for a couple of years. Recently however, I have noticed on several occasions that either the house seems cold and when I look at the display, it has a much lower actual temp compared to target temp. This morning however I went downstairs and found it very hot. The target temp was 21.5 and actual was 24. I have tried new batteries to no avail. Obviously the boiler is not turning off when it should. Has anyone any idea of what it could be. I can only assume the CM 907 has gone wrong. Should I just replace it. Any help would be appreciated, Lawrie

[Jim Chisholm]

I've a similar one, which threw a couple of wobblies.

Taking battery out and putting it back in cured it.

The first time I attempted to get the supplier to replace it but one never appeared.

Once it never came on, and once it never went off...

I'd be interested if others have had similar issues?

Fitted one at sisters house years ago and it has never given trouble.

Jim

[bert]

In message , Lawrie Davidson writes

The thermostat display will tell you if its calling for heat when it shouldn't be. The tolerance is +- 0.5C. Check the air flow through it isn't blocked. If it's set to 21.5 and it's still on above 22 then it is u/s.

[fred]

In article , Lawrie Davidson writes

I don't think these have a proper factory reset but try:

Reset Parameters to Factory Defaults which is Category 1 (Systems Settings) number 19 in the installer menu. You set this param to 1 and then it will reset on exit.

This is available when in installer mode, if you don't have those instructions handy then have a google, they're readily available as a pdf.

Before doing this however, take a note of all the current programmed parameters as you will need to completely reprogram them following the reset.

After this, there's not a lot you can do but you would have nothing to lose in popping the case to take a look inside. The mode switch rubs contacts over gold plated PCB lands so clean those up with alcohol. The relay may be desensitised or sticky but you'll never find a matching part so total replacement is then the way to go. Flick the selector between manual and off to see if the relay switches correctly. Like all battery powered stats, these use magnetically assisted (latching) relays so the relay coil is only pulsed to change its state and so if it misses the pulse, the relay will continue in the old state. Again not much you can do except check that the connections to the relay coil haven't gone dry. The relay is exceptionally small and may not survive taking the lid off but this would give access to clean up dirty contacts.

[Roger Mills]

The thermostat *may* have gone wrong - but, equally, whatever it is controlling may have gone wrong!

Have you got an S-Plan system, with a 2-port zone valve controlling the heating. If so, I would have a close look at that. The valve opens when told to do so by the thermostat. Once open, the valve's auxiliary contacts switch on the boiler and pump. If these fail to close, the house will be cold. If they stick in a closed position, the house will be too hot - particularly if the hot water is being heated at the same time. It sounds as if either or both of these things could be happening intermittently.