Thermostat Wiring for new Honeywell CM900 Replacing old Danfoss RMT230


Postby kensitp » Fri Aug 28, 2015 9:45 am

I need to replace my house thermostat that controls the central heating boiler. Having replaced one before I thought no problem, but when I removed the plate I found it had two cables going into it rather than the expected single cable with a live and neutral. I'm planning on installing a Honeywell CM900 to replace the old Danfoss RMT230. The Honeywell has just 2 terminals for a Live and a Neutral, any advice on how I move the wires across, as per the attached photos? Much appreciated.
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Postby kensitp » Fri Aug 28, 2015 3:54 pm

Sorry I dont know why the photos have been rotated. From previous posts I think I know what needs to be done, apart from the fact that one of the two cables has both the red and black going into the white connector/resistor(?) from which a single red goes to the No. 1 Live terminal. Do I maintain that and just move that to the Live on the new thermostat and take the red wire that goes to No. 2 as my new Neutral and tie back the black wire that goes to No. 4?
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Postby ericmark » Sat Aug 29, 2015 6:59 am

Today thermostats have batteries to work the electronics old types did not. The old thermostat had a problem in that the temperature between switching on and off was rather large around 2 degrees where today it is around 0.5 degrees. To get around this problem a heater was installed so when it switched on the heater would warm the thermostat to around 1.5 degrees above the room temperature so reducing the difference between on and off temperature back to 0.5 degrees. To do this it needs a neutral connected to the resistor. With battery powered models this neutral is no longer required and just needs insulating and left disconnected.
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Postby kensitp » Sat Aug 29, 2015 2:15 pm

ericmark wrote:Today thermostats have batteries to work the electronics old types did not. The old thermostat had a problem in that the temperature between switching on and off was rather large around 2 degrees where today it is around 0.5 degrees. To get around this problem a heater was installed so when it switched on the heater would warm the thermostat to around 1.5 degrees above the room temperature so reducing the difference between on and off temperature back to 0.5 degrees. To do this it needs a neutral connected to the resistor. With battery powered models this neutral is no longer required and just needs insulating and left disconnected.


Thanks for the reply. I've attached a diagram of the current wiring as I'm still unclear what wires need to be moved across to the line and neutral terminals on the new thermostat, would you be able to advise on this?
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IMG_2391.JPG (21.79 KiB) Viewed 941 times
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Postby ericmark » Sun Aug 30, 2015 1:35 pm

Simply don't connect any neutral wires they are not required.
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Postby kensitp » Mon Aug 31, 2015 7:24 am

Thanks Eric, so I remove the resistor , tape back the black neutral and the black wire on terminal 4 and then move the two reds across to the new thermostat? Does it matter which way round the two reds are on the new stat? Thanks again.
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