Honeywell CM67 Room Stat: Cycles off after 1 min

[michaeld121]

I fitted a new Honeywell CM67 Room Thermostat last night, but had a problem. I made sure that it should be on (ie set the thermostat to Manual & the required temperature to 5 degrees higher than current) so that the flame symbol ('burner on') was shown on the thermostat display. The boiler came on for a just over a minute (about 1 min 20 secs) then switched off for another five or 10 minutes, before coming on again for a bit over a minute and so on. Obviously the rads didn't get even vaguely warm in that time.

I've looked at the settings and while there's a minimum on time of 1 minute - I can't see why it's switching off.

I refitted the old thermostat and the boiler immediately fired-up and ran for a good few minutes until all of the rads warmed-up.

Any advice on why it's switching off would be much appreciated.

Michael

PS Don't know if it matters, but in the instructions it mentions a higher number of cycles per hour for a 'thermal actuator' and I've noticed a blue plastic box fitted near my boiler that's labelled 'Mid Position Actuator Code'.

[Set Square]

Have you changed any of the control settings from the factory defaults? Specifically, what are the following set to:

• Cycles per hour?

• Minimum on time?

• Proportional band?

When the temperature is within the proportional band (but not quite up to the set point), the stat will cycle the heating on and off to prevent overshoot. [Your old stat won't turn off until it *reaches* the set temperature - whereupon the radiators are hot and go on heating the space to above this temperature, even though the boiler is no longer firing.]

It seems a bit odd that it is cycling when 5 degrees below the set-point - because the max proportional band setting (on mine, at any rate) is 3 degrees.

You could try increasing the minimum on time to 2 or 3 minutes, possibly in conjunction with reducing the cylcles per hour. I'm not 100% sure what "Thermal Actuator" means - but I very much doubt whether that's what you've got!

[michaeld121]

Many thanks for the suggestions.

I haven't (knowingly) changed any of the default parameters - in fact I haven't even been into the parameter settings at all, but I'll have a look just to make sure that I haven't got one with rogue settings.

I'd wondered about upping the minimum on time, but I was thinking that I'd need to increase it to about 20 minutes to get the radiators warmed up to a decent heat. I was thinking that doing that might then give problems when the radiators are up to heat and it tries to do far shorter cycles to maintain the heat.

Regards,

Michael

[Set Square]

Unless yours is different from mine, the most you can set minimum on-time to is 5 minutes - so 20 isn't possible. It should still run the heating continuously when starting up from cold, until it gets within the proportional range (close to the set-point) at which point it starts cycling.

[James Salisbury]

Have you put the CM67 in cooling mode? It can also control A/C!!!

[John Stumbles]

Have you checked what signals the CM67 and the old thermostat are giving the boiler? I.e. with the CM67 when the boiler switches off (and the radiators are still cold) is the CM67 clicking off, the flame signal going off and the contacts opening (taking the power off the appropriate wire, closing the motorised valve etc)? If it's really the CM67 doing the switching (and you haven't changed any of the magic settings on it) then maybe it's borked. Maybe someone has fiddled with it before you got it? There may be a factory-reset you can do on it, otherwise it's a Dead Parrot.

If the CM67 is not changing state but the boiler is switching off then maybe you have some subtle wiring error (can't think what) or a problem with the pump or circulation (which can make the boiler fire up briefly but shut down before it's got much heat out into the system).

Incidentally a Y-line strainer in the return to a boiler is a great way to bring a system to its knees in this way, when the fine mesh strainer gets blocked with crud. Been there ...

[tonkski]

I have a CM67RF. That sounds very much like the default behaviour for when the boiler unit has lost communications with the thermostat.

[michaeld121]

After Christmas, New Year etc I finally got round to looking at this a few days ago. When I re-fitted it I was getting absolutely nothing from the boiler - it didn't even cycle on for 1 minute.

I tried a bunch of things but in the end it seemed to be down to the wiring. I searched on this ng & found someone else who'd had a problem with with the suggesting wiring. In the end I found when I connected the blue cable from my existing thermostat into the 'B' terminal on the thermostat everything sprang into life. It's been working for the best part of a week since, with no problems at all.

I'm a little concerned at why the colours seem to be mixed-up, but I've got an electrician due round to look at something else in the next few weeks, so I'll talk to him about it.

Many thanks for the suggestions,

Regards,

Michael