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Blog Archive

Friday, October 31, 2008

Great Gadgets : Honeywell CM67 programmable Thermostat

A gadget which I think goes largely unnoticed by consumers and plumbers alike is the programmable thermostat. I have a Honeywell CM67 but the model has been superseded by the CM907. Worcester do a similar model- I'm sure there are others as good. You can get them for around £50 online at plumbing shops.
There is a range called Heatmiser too, you can find these on Amazon. 

These devices replace the room thermostat, which in many cases is the classic "clunk-dial" based on a bi-metallic strip. These old style stats are based on 19th century technology. Their strip is made up of 2 thin layers of different metals, bonded and when it heats up the strip bends due to the differing rates of expansion of the two metals and this forms the pole of a switch. Because of the phenomena of hysteresis, these types of system are slow to respond, and take too long for the stat to switch on or off. To compensate , they actually have a tiny heater in them and actually consume
some power. The heater is a wire wound resistor next to the strip which attempts to warm the strip to make it more responsive. It's an inaccurate, old and ugly solution, but still widely used as it's cheap, reliable and I have to say is popular with the innate 'conservatism' (small c) of typical British Plumbers/Builders in my experience.

A programmable stat does the same job but also a lot more. First off it uses a thermistor to measure the temperature which is a much more accurate way of telling if the house is at the right temperature. Of course both systems suffer from the problem that the placement of the unit is where the measurement is taken , which might not be typical for the whole house.

The programmable part is that you can set the device to achieve specific temperatures at different times of the day. So I can set the unit to achieve 19 degrees in the morning for getting the kids up, drop back to 17 during the day, go back up to 20 for home time and climb to 21 in
the evenings, for example. There are 6 temperatures per day and 7 days, each of which can be entirely different.

In addition there is an optimum start feature which uses the previous days 'data' to see when the system needs to come on to achieve a specific temperature at a specific time. This takes the guess work out of starting the system early . It also means that while fluctuations in weather will tend to catch it out, it will gradually adapt to colder/warmer weather and switch on earlier or later as the temperature trend changes.

Finally there are various exception programmes (holidays, work at home days, party mode !) where the programme is overridden for a specific number of hours or days. If you go on Holiday you can specify that the temperature drops to a lower one, but comes on in time for the house to be warm when you walk in, n days later.

I actually use this as my main CH programmer, so the CH timer beneath the boiler is permanently set ON all day/year round. This doesn't mean the heating is always on , as the stat controls the pump & boiler circuit, it just means that the decision as to when the heating comes on is based on a more specific set of conditions. I use the stat to programme , which means that it picks up a cold day in summer, but conversely doesn't come on for those sunny days we sometimes get in October.

While these devices are not specifically 'Green' i.e they may increase your consumption,my experience is they decrease it . The house is heated much more accurately , and not based on arbitrary click-on/click-off based on who last walked past the stat. The different modes mean that I can have the system throttle back for holidays or simply the odd few hours when I might change the pattern of what I day (work at home, or in the office)

The programming can be a bit complex due to all the options, but it's not too hard. Finally wiring one up is pretty simple, the main complication is that if your remove an old room stat you have 4 wires to deal with whereas these units only need 2. The reason being they don't need to have mains power for the tiny heater element I mentioned.

I should add that my unit has an additional module on the right hand side. The CM67 had a few bolt-on units, as is apparent from the pictures. Mine has the clock module which sets the time every day from the UK Time Service broadcast on Long Wave. This means that :

1) This clock is always accurate, so acts as a useful time source

2) I don't have to re-programme the stat when we change with daylight saving time

I have also seen a preview of new internet-enabled programmable thermostat called the ecobee on the web. It's being made by a company in Canada and uses on-line weather forecast information in it's optimisation. So for example if the forecast predicts a sudden cold spell the stat factors this in it's calculations and , for example, starts to heat the house earlier if there was a sudden cold spell. This is a great idea, though the device is predicted to cost around $400


Ed said...

Is this a direct replacement for an existing bi-metal thermostat? Does it require batteries or does it power itself off the mains?

Mr Ives said...
This comment has been removed by the author.
paul_tanner said...

There are a couple of more sophisticated heating controllers around (in the UK). I hear that the one I've been trialling will be launched at EcoBuild.

I think the Nest device may need to be adapted for the UK market as the signalling between it and the boiler relay may be non-standard. I have scoured the Nest site for more details but have not found any.


Alison said...

A friend with an oil powered boiler has no controls apart from on and off. Can this be retro fitted?

Mr Ives said...

Thanks paul, would be very interested to know about other type of smart stat available in the uk. I can see that external temperature sensors would help optimise the optimisation, as would a measure of sun light. Long range weather forecast data would also help.

Malcolm said...

This kind of logic seems to be built in to ground source heating systems, at least as far as sensing the outside temperature and heating the home accordingly.
One more reason for getting more than 3 kw of heat for 1 kw of electricity.
We find it very effective.

Mr Ives said...

Hi Alison,

The honest answer is 'probably'. In most gas fired central heating systems the room stat works as a switch, turning on/off the pump.mif your friends oil fired system pumps hot water around radiators, then I see no reason why you could not use such a stat. The wireless model would be useful, you wire it in place of the switch and place the wireless stat where you are in the house. Placing the stat near the boiler is not a good idea as that is probably untypically warm compared with the rest of the house.

Mech said...

I have a CM61 which is the same as the CM67 but with just a 24-hr program rather than 7-day.

I've disabled the 'optimum start' feature because it came on at crazy times at night. Even if it worked correctly it would save a mere fraction of a percent on fuel, so I actually prefer the more deterministic system of using only the timer.

The heater found in some mechanical thermostats is called a 'heat anticipator'. I have never seen a domestic UK central heating system that actually used one. They need calibrating and it's just not worth the effort.

Whilst a digital programmer is probably more flexible than a mechanical timer / bimetal roomstat combo, it's really just a gadget. Both systems will regulate temperature equally well in a typical domestic installation and the mechanical solution won't require a new battery every few years. Both only have the option of turning the boiler on or off. The Cm61/67 only registers temperature changes in half-degree steps anyway, so it's unclear that it has an advantage in accuracy over an analogue stat.

Weather compensation... well that's pretty pointless unless you have a condensing boiler and the programmer can vary the system's water temperature. Otherwise you're still limited to on/off control.

Anyone with no central heating controls would be just as well served with a mechanical timer and room stat. If one already has that, then the next step is thermostatic radiator valves... and they're practically always mechanical!

Mr Ives said...

Thanks Mech for the input.

I have to say I disagree on each point, but I'm pleased to get your input.

If your optimum start is coming on in the middle of the night that suggests that the house is never achieving the desired temperature, and the optimser is resetting itself earlier and earlier ? or possibly that you have one of the different daily programs set high in the night ? There are also some additional offset parameters you can set in the configuration.

Mine varies by maybe an hour ? hard to say. My priority is that the house is a specific temperature at the time I want it to be i.e when I get up , and mostly it achieves this. What time the heating comes on I leave to the stat. It's more noticeable in the afternoon as I'm around , So maybe at present the heating comes on at 2:30 to raise the house from 18 c to 20 c for 3:30
when the kids come home.

certainly the drayton stat I replaced had a metal strip and a large wire-wound resistor inside, that appeared to always be in circuit, so I assume it's function was to act as a heater to compensate for the hysteresis of the strip.

I know that it's typical to have a north facing external temperature gauge on a condensing boiler as you describe. My thinking was that the optimiser in the CM67 only uses the previous 24 hours data. hence it will get caught out by cold snaps or sudden sunny days. A forecast could be incorporated into it's calculation where maybe it tracks the 7 day trend rather than just the previous day ?

I absolutely agree with you that both old and new stats are fundamentally a switch. Both could be replaced with a light switch and be a person could determine exactly when the system came on or off. The only difference is the criteria they use.

Finally I have to disagree on the TRV issue. I have always found them to be utterly useless, as their ability to react to the room temperature is compromised by the fact that they are thermally attached to the primary heat source. I have removed all of mine as they simply remained permanently on or permanently off , before finally leaking.

Sorry but I guess we will have to agree to disagree on this subject

AVisitor said...

I've had my CM67 for about 10 years and it has suddenly started having a blank display due to losing its power. I can only assume that it is either a connection problem or a circuit board failure. I am planning to replace the unit, but found it has been superseded as you said. Do you know if the newer CM907 will slot straight in to the current wiring for the CM67? I imagine the backplate will be slightly different and will require drilling a couple of new holes. That's fine

Mr Ives said...

Dear AVisitor, sorry about your blank display. Mine too is a bit over ten years old. The battery is just to persist the settings in case of a power cut AFAIK so clearly not related. As you say, sounds more like a bad circuit board, given the connections don't just come loose.

perhaps you could ring Honeywell Controls ? I recall originally phoning them with a programming question and got through to a very helpful chap. They could certainly advise if the mounting was the same.

I think I'd be tempted to get the wireless model and move the stat around the house depending on where I was in the house. My wife might suggest I could then wear it as a medallion ;)

AVisitor said...

Hi Mr Ives

Thank you for your prompt and humourous reply. I must say I had not considered the wireless version. I wonder how that copes with all of the other wireless devices in the vicinity. I've lost count of how many times my wireless doorbell has randomly sounded. I will take your advice and call Honeywell
Many thanks again

STC Technologies said...

Good Job!!!STC Technologies

Anonymous said...

i have read your post stating you disabled the optimum start on a cm67 could you tell me how to do this thanks

Mr Ives said...

Hi Anonymous,

No I definitely have not disabled optismart - it's one of the features I really like about the CM67. One of the other commentators stated that they had. Its not so hard to do but is only described in the installation Guide, rather than the simple user guide. the full installation guide can be found here :

Anonymous said...

Hello, just to clarify the batteries (2 x AA)drive the unit, the contacts are volt free in that they only switch and the unit is definitely not mains powered in any way, this is how it is possible to utilise the control on a variety of different appliances that operate on varying voltages. TRVS are very effective but as the saying goes you get what you pay for, quality units react effectively and allow independent zone control for occupation areas where differing temps are required, say bedrooms or work areas. Indeed, these are now a requirement on new build and retrofit upgrades on domestic heating systems with boiler replacements etc. I have 2 x cm67S operating a combi boiler with 3 port valve that allows varying programming to the main house and annexe, and you are correct with the anticipator heating resistor in older mechanical type stats being widely utilised across the
UK. Also stated by a poster was the fact that this stat does not alter the flow temp of the system to accommodate varying weather conditions, nor should it. That would require a compensated heating control with internal and external sensors to calculate the level of heat input required for a given occupancy value (0c) and frankly suitable for larger domestic or commercial applications and would serve little purpose in a typical domestic installation in a cost/benefit ratio. As an installer of some 33yrs experience I would suggest simple is best, not only for use but also maintenance. Finding a knowledgable installer with the capability to specify appropriate energy saving controls is very difficult as true tradesmen are becoming few and far between and too often poorly trained installers are all you can find. Unfortunately the old apprentice training system has been replaced with a poorly implemented training scheme and this is why the construction industry is in such poor shape ability wise in the uk, I trained for 4 years + and even then was considered only an "improver" able to work on my own to gain experience. I came across a guy who started to question me on potential earnings after he had finished his 6 month college course when he intended to start as a sole trader! He became upset when I explained he was unrealistic in expecting to earn the same as me with such limited experience and lack of any on site experience and in my view would be robbing people if he called himself a tradesman, rant done, spock out!!

Mr Ives said...

Thanks Mr Spock for that clarification and tips. I managed to get on a UK trial of the Nest device from the USA , which is interesting. It does some of the same things as the CM67 but also weather compensates as its an internet connected device and can be controlled from a phone or tablet. Its definitely interesting and its feature which tries to figure out if anyone is at home seems to work well. Ill keep you posted

Anonymous said...

Can someone help I have a Honeywell cm67 its worked fine for 7 years and has suddenly stopped it wont turn ny central heating on at all now I have put new batteries in still dosnt work but it still has power everything is on the screen as normal just wont turn heating on any help would be greatly appreciated thank you

Mr Ives said...

Hmm, well are you sure the problem is with the stat ? Could the pump have failed or another switch. The stat is just a switch which turns on the CH pump and boiler ?

My CM67 stopped working after 11 years and only ran the inbuilt demo programme,e , I could not get it to register new programmes and it started to freeze up too where I could not control the display.

I don't know if you have an old bi-metalic stat you could test with ? As the Honeywell is basically a switch with two wires leading into it, If you turn off the power and temporarily replace it with a lighting switch just to prove the problem is the stat and not another break in the pump circuit ?

D Hopkins said...

Mr Ives, I never post comments on any forum but saw your response to Mech in 2012 when you disagreed with him about the optimal heating. Well I too had this problem. My boiler was fitted 14 months ago and the first Honeywell stat was faulty so I had a new one fitted, a CM67NG. That September, 2013 was a warm month but so I had programmed the heating to come on at 7.30 at 18 degrees. 3am and we were awoken by the unbearable heat. So the plumber came around and called the Honeywell Tech help who told us that if you want the house to be 18 degrees at 7.30 then the optimal function will do this. Now I'm guessing it was already about 12-13 degrees at 3am so my heating needed to fire up nearly 4 hrs early to raise the temp by 5 degrees?? I don't think so! So we turned this option off which slightly narked me as it was only days old. Last night, yes, you guessed it, 3 months out of warranty and the heating comes on without being set and the stat will not work, there is no display, the batteries have been changed 3 times. I should never have accepted this stat last year when it was so obviously not right. I would love to hear your suggestions Mr Ives. Oh and my house has cavity walls and is very warm. Thank you.

Mr Ives said...

Hi D Hopkins,

I'm sorry you have had a few issues and finally a dead unit. Mine failed in a similar way but only after 10plus years of effective use. Perhaps as you say there was a problem with the unit from the start. The optimum start worked well for me and Typically caused the heating to come on an hour earlier than the time I specified. There are a lot of settings for offsets etc and I wonder if one of these was set at the start ?

Mine finally failed in that I could no longer programme it and it ran the default programme. For the last year or so I have been testing the Nest programmable thermostat which has worked well. Many of its functions are similar, though it also attempts to learn your preferences over time. It's also wifi enabled so incorporates weather information . The main area that works well is the thermostats ability to predict when no body is at home , auto away, which works well for us.

My experience of the CM67 was a good unit for our fully pumped CH system in a house that was part old 1930 and part new extension. I'm sorry your unit proved less so