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Honeywell DT90E wiring

Postby seanxb » Wed Oct 31, 2012 4:46 pm

I am replacing an old dial honeywell room stat (42003840-001) with a digital DT90E.

I am unsure about the wiring compatibility and wondered if someone could clarify please?

Currently, I have 3 wires - Blue, Yellow and Red.

The blue is inline with a resistor and then tied to the yellow, which is then connected to what I assume is the thermistor. Indicidentally, Blue is in terminal no.2 and yellow in no.3.
The red wire is in terminal 1 and this runs to the other side of the thermistor.

The new unit has three terminals only, A which is marked as Live, B which is marked as neutral via a resistor symbol and C, which is not marked but on the diagram, it look like it is internally connected via a switch to A.

Question being, where do my Blu, Yellow and Red wires go?

Any help would be most appreciated.

Thank you,
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Postby Atticus Finch » Thu Nov 01, 2012 3:18 pm

seanxb wrote:I am replacing an old dial honeywell room stat (42003840-001) with a digital DT90E.

Blue is in terminal no.2 and yellow in no.3. The red wire is in terminal 1

Connect the red wire to terminal A
Connect Yellow wire to terminal B
Blue wire is not required and should be made safe, e.g connect to a small terminal block
Terminal C is not used.
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Postby Russ072 » Mon Oct 21, 2019 3:40 pm

I’m doing the same model. But I don’t have a yellow cable. Just have the Earth, two reds and a black. Any ideas anyone? The video on YouTube reckons you only need two wires. I tried that and it’s just on constant.
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Postby ericmark » Tue Oct 22, 2019 8:57 pm

It is better to start a new thread, however there is no colour code as such, the previous guy had explained very well what wire was connected to which connection.

The old thermostat had a resistor that heated the thermostat when on slightly and so reduced the hysteresis, and the description allowed Atticus Finch to work out which wire was the neutral, but the neutral was often left disconnected anyway.

There are two reasons for fitting a thermostat, one is to control room temperature, the other to stop the boiler cycling as the weather improves, with the latter a large slewing range of temperature it is actually better. So some times only two wires used.

So to work out what wire goes where we need some info as to how connected to the old thermostat was connected.
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Postby M_arnold218 » Tue Oct 29, 2019 9:05 pm

I have a dial thermostat with the following 3 wires (according to the connections in it): N, L also looped through to COM, ON and no wire in OFF.

I'm not sure how these should connect to the DT90E A,B,C connections.

Help appreciated.
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