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, Sean.
Thank you but it doesn't really help with the pairing of wires from the original Honeywell dial stat to the new DT90E wiring. If any one else could advise it would be most useful. Thank you, Sean.
Sean, almost certainly your current Red wire is the 'Live', almost certainly your Blue is the 'Neutral', and almost certainly your Yellow is your 'Switched Live' (the one that becomes 'live' when the 'stat turns on and calls for heat. So, basically, if you touch your red and yellow wires together, your boiler should come on. Not that I'm suggesting you do that. The neutral (blue) is only there to make your old 'stat work more sensitively. Your new 'stat is battery operated so doesn't need the neutral (blue) wire. This wire should instead be terminated inside the back box in a way that it cannot touch anything. You do this by either snipping off the bare wire bit and then folding around a cm of the covered bit over itself and then taping it securely, or else by sticking the bare wire bit into a single insulated terminal connector - and then taping it all over. DON'T cut this wire right back, 'cos you might want it later if you fit a mains-powered 'stat. Ok, if the red is live and the yellow is switched live - which they almost certainly are - then stick the red into 'A' in your new 'stat and yellow into 'B'. If these wires ain't what they say they are, the worst that'll happen is that you'll blow a fuse. But I doubt you will. 'Night.
Just to add to this, I have the same one. But it seems to not listen to the programming. I have my heating on a timer. When I installed the thermostat, my heating was meant to be off, but when I put the temp up on the thermostat, it kicks the heating or the pump on. Does anyone know why this is?
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