I want to replace my old Honeywell room thermostat with a Drayton Digistat 3 programmable unit but I'm not sure about the wiring connections. Both have numbered connectors but I don't know if the numbering convention is the same. On the Honeywell the connections are:
1- Red 2- Blue 3- Yellow Do I just connect these to the ame numbered terminals on the Drayton unit?
No. Old type mechanical thermostats have a mains heated shunt coil and need a neutral. This should be taped up out of the way. If you connect it to the new stat you're likely to blow it up at worst.
Your new stat has a semiconductor temperature sensor which is much more accurate, is battery operated, so only has a line in and switched out.
It *should* be the blue which is the neutral and has to be be taped up, but this isn't guaranteed. Can you check at the boiler, etc?
I assume that the Drayton unit came with some installation instructions and wiring diagrams etc.? Have you *read* them?
You need to determine the *function* of each of the 3 wires going to the original stat, and connect the appropriate wires to the new unit. It is potentially dangerous to make assumptions based just on colour - unless you installed it yourself - so you need to find the other end of each of the wires, and see what it is connected to.
The most *likely* scenario - but not guaranteed - is that the wires are: Red - live feed from the programmer when CH is on Blue - neutral, used as a return for the accelerator heater in the existing stat Yellow - switched live, which turns on a zone valve or boiler and/or pump depending on what type of system you have
If this *is* the case, you'll only need the red and yellow wires, since electronic stats don't use accelerator heaters. The terminals to use are Common plus whichever other one is connected to common when the room temperature is below the set temperature. Do *not* connect neutral to anything - insulate the end of the wire and make it safe. If you *do* connect it to the 'spare' terminal, you'll have a dead short across the mains - and a very big expensive bang - when the stat switches!
Hi, Thanks for this. The Drayton unit did come with instructions but it defines the connections as:
1 - Common 2 - Heating satisfied 3 - Call for heat My problem is I don't know how these relate to the wires going into the Honeywell
Assuming you have found which is the neutral and insulated/isolated it out of the way, the other two to 1&3. Doesn't matter which to which.
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