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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 I have a Honeywell CM67 but the model has been superseded by the CM907.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