Honeywell CM67 Thermostat Problem???

[sanj2000]

Hi all,

Just a quick question I have a Honeywell CM67 Thermostat connected to a Vokera Linea boiler (cant remember the model but I think its 75). After using the program function on the thermostat and selecting a temperature for any day or time range the slide switch is moved from prog to auto. If required the boiler will fire up to the reach the requested temprature. However if it is required to increase the temp then this only happens for a approx 5 seconds and then the flame light on the thermostat will go off even if the required temperature has not been obtained. If I manually increase the temperature using the temp button whilst in auto mode the flame light remains on and required temperature is obtained.

Any ideas, is my thermostat faulty or does it need some kind of manual reset??

Thanks in advance for your help!

[sanj2000]

Hi all,

Just a quick question I have a Honeywell CM67 Thermostat connected to a Vokera Linea boiler (cant remember the model but I think its 75). After using the program function on the thermostat and selecting a temperature for any day or time range the slide switch is moved from prog to auto. If required the boiler will fire up to the reach the requested temprature. However if it is required to increase the temp then this only happens for a approx 5 seconds and then the flame light on the thermostat will go off even if the required temperature has not been obtained. If I manually increase the temperature using the temp button whilst in auto mode the flame light remains on and required temperature is obtained.

Any ideas, is my thermostat faulty or does it need some kind of manual reset??

Thanks in advance for your help!

[James Salisbury]

What is the displayed temprature, and what temprature shows on the display when you push I ?

[sanj2000]

The displayed temp is: 20 I temp is: 21.5

(just had a baby so trying to keep the house warm.....lol if you were wondering on the high temps!)

[John Stumbles]

Is it the wired type or the wireless (CM67RF)? The wireless ones can get their knickers in a twist which a factory reset may sort out. IIRC you go into engineer mode and one of the options is to set factory defaults.

Also with the wireless ones the manual says that the sender unit sents a proprtional (percentage) 'demand signal' to the relay box, rather than a definite on/off, and that for temperatures close to the set point the sender may show the flame symbol but the relay unit may not switch the boiler on. Bloody daft IMHO.

I don't know if the same is true of the wired type.

[sanj2000]

No its the wired type

[Matt]

I can't see any reference to that type of operation in any of the Honeywell literature, either the really old ones, the ones they were selling a couple of years ago nor the latest ones with the new bigger relay boxes.

If the burner segment on the LCD is "lit" then the green LED on the relay box is also lit and the relay inside it closes to fire the boiler. The relay box is essentially dumb, all the controller parameters/proportional functionality is contained within the thermostat (i.e remote/wireless) box and not in the hard wired bit.

[James Salisbury]

Ok,

if the two values are closem such as in this case, the demanded temp is 21.5 deg c and the room temp is 20 then the system will start to modulate on and off so it doesn't overshoot too much.

Several things can be done, first is the the thermomter in babys room?

You can reduce the temprature band for the modultuion control. To do so take a look at

the instructions for changing the proportinal bandwidth to 1.5and check that it is set to 6 cycles per hour.Then check radiator in baby's room that it is turned full on. Check back forfurther info.

[Matt]

The 5 seconds seems abnormally low. The proportional aspect of the controller means that while it may rapidly switch off after an initial step change you might find it kicks in for a longer "burn" a few minutes later. It might be worth increasing the minimum on time of the thermostat. I think it defaults to something like 1 minute whereas by that time a lot of boilers are only just getting up and running and the thermostat closure will have no impact on the room temperature - and you would expect no response from the radiators so you'd expect the thermostat to continue requesting heat but sometimes it doesn't ! I found 3 minutes a more useful setting. Make sure that there are no local influences on the thermostat, sunlight falling or a nearby halogen lamp can make a huge difference.

I've recently had a problem with a very early CM67 - so early it's missing quite a few of the features missing in the literature (such as the party function) Despite using non volatile memory the program and parameter storage were getting Alzheimer's. I did a full reset and everything seems to be working ok at the moment.

I'd be tempted to check all the parameters, write them down and then do a full factory reset.

[lairdy]

M3 2.

I had a similar Danfoss unit, where the band for proportional operation was 2C. At 1.5C below, it would fire for 3/4 of a time unit (10 or 20 mins iirc). 5 seconds sounds like a malfunction.

Something I never liked about the Danfoss programming was that proportional control only kicked in once the temperature fell *below* the set-point. So, if the ambient conditions were such that 25% of full boiler output was needed, you had to wait until the temperature settled at 1.5C below set-point. Careful positioning of the stat was necessary to ensure it was in an area where the temperature actually varied by enough to make it work as intended. In the end, I switched that function off and used it simply as a digital stat, albeit with

7-day multiple programmes. It worked out just as economical (compared to the ancient analogue unit it replaced) and kept the house at a more even temperature. Possibly a modulating boiler would have done a better job, reacting to the increased return temps as the TRVs settled, but the boiler was a long way from needed replacement.

I am shortly about to add a wireless digital programmable stat to the system in my new house, and will probably go for the Honeywell equivalent - a CM67RF. It seems to be available at a more reasonable price than most suppliers quote at: