Honeywell cm67 causing poor boiler cycling

[jimc-m]

I have an infuriating problem with a Honeywell CM67 wired programmable roomstat (which is in the hallway, where there is a radiator at the opposite end without TRV’s - connected to a Baxi 105 HE Instant combi boiler in the kitchen).
Allowing for the optimised start, when a timed preset temperature rise (in my case from 16 deg to 21 deg at 5 p.m.) is arrived at, .the boiler fires and cycles as expected until the preset temperature of 21 deg is reached (If anything, this is too hot, but read on!) However, after that the displayed temperature remains at 21 degrees, even though the ambient temperature everywhere in the house falls noticeably. If left unattended the boiler will eventually cycle again upwards of AN HOUR later!! Then the same thing happens.
If (as we often do!!) we get too cold in the meantime, and try temporarily increasing the required temperature (using the “up arrow” button) to 21.5 degrees, nothing happens – the boiler isn’t switched, but after a few seconds the displayed temperature climbs ‘magically’ to 21.5 degrees!! It’s only if I increase the required temperature to 22 or 22.5 degrees that it switches the boiler on again. We then end up too hot, then become too cold as it takes ages for the boiler to cycle again.
I have put up with this since I fitted the stat some 12 or so months ago – yes I should have done something about it before! It may simply be “faulty”, but I have to say that I had previously bought an “Invensys” programmable stat which I’d returned because I had similar problems – long boiler cycling times once preset temp reached - plus it seemed over-complicated!
This all makes me think it’s the boiler somehow, but I’ve tried a standard mechanical stat and the boiler cycles as expected – once every 8mins or so, and the stat is simply switching the 240 volt supply to it.
I’ve tried resetting the CM67 back to the factory defaults – also tried increasing cycle times to 12 and changing the proportional bandwidth and disabling optimisation - to no avail and have ended up using the selectable temperature change buttons to set a low temperature of 19.5 degrees in the early evening then progressively increase at 45 min intervals to 22.5 degrees, but the long cycling problem (and us getting too hot/too cold) still remains at all preset temperatures. Can anyone offer advice or had similar problems?

 

[simond]

Sounds like it is working correctly.

If you site it in an area which loses heat at a slower rate than most of the living areas this is precisely how it would pan out.

You need to resite the stat to a more representative area or buy a radio version eg: Honeywell CMT927 and move that elsewhere.

 

[simond]

Or a simpler suggestion, try almost turning the rad nearest to the CM67 off.

Fully shut then a quarter turn back.

 

[jimc-m]

Thanks for the reply.

I should have made it a bit clearer - it's a bungalow, the hall is open into the lounge/diner and the bedroom doors are always left open. The reduction in ambient temp is just as noticeable IN the hall as in any other room. As mentioned I have tried a standard mechanical thermostat in the hall and this cycles the boiler as expected - every few minutes.

The honeywell is causing very long intervals between boiler operations, thus the chilly feeling in between.

I have already got the hall radiator virtually "off" with just a quarter turn open.

 

[BDM]

Probably too late now, but I have used the Grasslin Towerstat RF for some years & been happy with it, not least of all since it was significantly lower cost than the Honeywell (which I had used an earlier 8000 model for hitherto). Also, the remote display is mobile & can be carried from room to room depending where you wish to be!

 

[D_Hailsham]

If (as we often do!!) we get too cold in the meantime, and try temporarily increasing the required temperature (using the “up arrow” button) to 21.5 degrees, nothing happens – the boiler isn’t switched, but after a few seconds the displayed temperature climbs ‘magically’ to 21.5 degrees!!

Are you talking about the Target temperature or the Room temperature?

It’s only if I increase the required temperature to 22 or 22.5 degrees that it switches the boiler on again.

That's quite normal; you will get the same thing with a mechanical stat. There is always a difference between the on and off points; if not, the boiler would be continually switching on and off.

tried increasing cycle times to 12 and changing the proportional bandwidth and disabling optimisation

Changing the cycle time will, if anything make the situation worse. The same goes for the PB. Optimization has nothing to do with your problem.

The problem is that the stat is located in a place which does not cool down as quickly as the rest of the house. Get a max/min thermometer and monitor the temperature by the thermostat and then in other locations. You will probably find that the temperature near the stat is fairly constant but fluctuates in other locations.