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Suggestions for a mains powered programmable thermostat

Postby docboardman » Sat Sep 20, 2008 10:47 am

Hi Everyone:

I'm trying to upgrade the CH control system in this house. What I would like is a programmable room thermostat. I'd prefer not to have RF and I'd like it to be mains powered rather than battery.

My original choice was a Danfoss TP7000, but it lacks the ability to automatically change between GMT and BST. I've also looked at a Danfoss TP9000, which will auto change between GMT and BST, plus is two channels so could control CH and DHW. My idea was to place it where the old DHW timer is and use a remote sensor to monitor room temp. However the line between the remote sensor position and the programmer position runs along parallel with some power cables and Danfoss says this is not good.

So I'm kind of stuck at this point and was hoping someone might know of an alternative programmer.


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Simply Build It

Postby long_boy » Sun Sep 21, 2008 7:48 am

hi I'm not that familiar with the danfoss u stated it may be better than wot I'm now goin to say. the way I would do it is I'd install a twin channel programer in airing cupboar as this is probaly the easyes place to save routing cables and then as u want a programerble stat I'd use a honeywell rf programerbel stat which are very raliable hope iv bin a bit of help

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Postby doit » Thu Oct 02, 2008 6:45 am

I have the Danfoss TP9000 which I have just installed as a replacement for an old programmer and mechanical thermostat.

It was the only mains powered thermostat I found with a remote sensor, which makes it an ideal choice for retrofit applications. With a three core cable to the sensor your choices are:

Programmable thermostat at the old thermostat location, live & neutral from boiler cupboard, switched live to boiler cupboard. Retain existing programmer for hot water. This makes programming harder, and doesn't let the new programmer do holiday mode/summer time change for hot water. Also, my old programmer had failed.

You could have the programmer thermostat with DWH control at the existing thermostat location, with a new fused supply to the programmer, then CH on, DHW on, and DHW off signals down the thermostat cable to the boiler cupboard, remove the old programmer and disconnect the fused mains supply in the boiler cupboard. This seems risky as the isolation for the boiler would then be at the thermostat, and you couldn't safely presume the boiler was disconnected without removing the fuse at the programmer supply.

A battery powered programmer thermostat with DHW control won't work as you need live in, switched CH, DHW on and DHW off coming back (four cores). Don't even think about using the earth wire as an extra core as this is dangerous.

TP9000 in boiler cupboard, replace programmer, use two cores of thermostat cable for sensor. This also works if you only have two cores to the thermostat (which means you can't have a mechanical thermostat with an anticipator resistor).

As you said, the instructions say:

Remote Sensor to be wired with 1mm 2 core double insulated cable only. Cable length should not exceed 50 metres. Sensor cable should NOT be run parallel to mains cable.

I used the old thermostat cable for the sensor which goes from the boiler cupboard up into the floor of the airing cupboard then under the upstairs floor then down, past the fuse box, to the sensor in the hall. The cable must be running in the same route as the immersion heater cable, and some other cables, and I haven't noticed any problems.

My guess is that provided the sensor cable is not running parallel to a mains cable for a very electrically noisy device (e.g. big motor) for a long distance then you'll be okay. Obviously the longer the parallel run, the closer the two cables, and the worse the interference source the worse things will be, but I doubt you have a 50m run adjacent to a cable supplying an arc-welder.

My other guess is that the reason for the double insulated cable for the sensor is not because the sensor is high power or high voltage, but because the TP9000 doesn't have an internal transformer, so the sensor is not electrically isolated from neutral (or live) so electrically must be protected as much as a neutral wire in the electrical system, and the neutral must be as protected as though it were the live wire.
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Postby htg engineer » Thu Oct 02, 2008 9:03 am

Personally I don't think changing the time twice per year is that much of a hardship, it normally only takes a few seconds. Especially when some of appliances that do change the times themselves do it eventually - sometimes a few hours later sometimes even the next day.

So why not fit the one you originally wanted ?? (Danfoss TP7000)
I'm assuming as this was your first choice it was compatible with the system and would perform all tasks that you wanted it to ??

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