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Thursday, April 11, 2013

Great Gadgets : Honeywell CM927 wireless programmable thermostat

My existing programmable thermostat , a Honeywell CM67 started failing. Rather odd actually, it would happily run the default programme but would not let me reprogramme. Every time I put the unit into programme mode, it would let me make a few changes then freeze completely. I tried changing the battery, but no help. Oh well its lasted me 10 years and done good service.

I was trying to decide what to replace it with. I looked at the heatmiser range, which have some interesting models including some that can be controlled over wifi i.e from your computer or externally from a phone. Turning up the stat while travelling home on the bus seems a nice idea. However the heatmiser range have to be counter-sunk into the wall , whereas my honeywell unit sits on the wall.

I also have been in contact with the UK operation of Nest, the American Smart stat. It has many interesting features, and possibly I'll get on a trial, but that is some way off. I had a pressing need for a new stat, especially as the UK weather is awful at present.

In the end I went for the CM927. the modern equivalent of the CM67 is the CM907, which is a different shape and has a backlight, but appears exactly the same

the CM927 is a wireless stat. NOTE: this is nothing to do with WiFi. Wireless stats mean that the programming unit , which measures the temperature , is a seperate unit you can place anywhere in your house. It runs off two AA batteries (supplied). A small box fits to the wall where the original stat went.

the attraction of this model is that I can move the stat to where I am. In reality it has remained on the mantle piece in my study as this is where I mostly work and its been doing a good job.

Programming the unit is very similar to the CM97, and the style of options is familiar. It has the optistart feature where the unit figures out when to come on to try to reach a specified temperature at a given time.

This unit can also control other Honeywell devices, for example it can control electro-mechanical Thermostatic radiator Valves. I'm not a fan of TRV's generally but having them controlled centrally seems a good idea. I might do something with this feature in the future, but for the time being its running normally.

I did notice some persistent negative feedback on Amazon about this model and the possible failure of the LED display after a few years. I will keep an eye on that.

I also noticed that Honeywell in the US sell a WiFi stat, which would have been preferred but this model is not available in the UK

Wiring the CM927 was more complex than I expected. The CM97 and I *believe* (please do check with the supplier) CM907 are voltage-less units i.e the stat is powered from a battery and simply switches the Live and switched Live connections.

the CM927 wall mounted unit requires power i.e a live and neutral to function , and a live and switched live to control the Central heating pump. Fortunately  the neutral was present in the wall, and had been safely capped off with a connection block. Wiring the unit using the diagrams provided was pretty easy once you understand that you have to use the second live connection point in the unit to provide Live to the switched side. You need a short wire link to do this.

IF IN ANY DOUBT please consult an electrician, these units switch mains electricity so always be careful, switch off and if in doubt consult a professional.


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