DIY Doctor

3 Colour Thermostat Wiring When Replacing Honeywell Thermostat

Postby antk » Sun Mar 15, 2020 9:27 am

Hi, i'm trying to replace an old Honeywell thermostat with a more modern one. The old one just has pins marked 1,2,3,4 connected with red=1, blue=2, yellow=3. The new thermostat has two pins, marked W and Com(mon).

I'm trying to find which wires to connect on the new one. Unfortunately i've only been able to find information in forums, and i've found two different answers, so i'm not sure what is correct. Do you know which I should connect, and even better is there any information about it, like if this is a standard. I could also take pictures of the inside of the old thermostat if it helps, and potentially could try and trace colours to the pins on the boiler controller, although there's quite a few wires coming out of that one.

thanks
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Postby ericmark » Mon Mar 16, 2020 9:18 am

Old thermostats needed line in, line out, and neutral the latter worked a small heater to reduce the hysteresis, the new battery operated thermostats don't need the neutral and unlike the old thermostat it does not matter which way around the line in and line out are.

So you need to identify the neutral, there is no colour code as such, but most seem to used blue for neutral, but better looking at old thermostat and see which terminal is neutral. Once you know which is neutral put that wire safe in a connector block or something and just use other two.
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Postby antk » Sat Mar 21, 2020 4:55 pm

It turns out its a bit more complicated, it looks to me like the old setup is a mains voltage thermostat, and the new thermostat is volt-free.

I got a relay device that allows a low-voltage thermostat to be used on a mains system: https://www.manualslib.com/manual/11515 ... c840t.html

Connecting the thermostat to the relay seems to be fine, it has a light on it to show if it is closed or not. The issue I have now is that the relay state does not seem to control the boiler, the boiler is always on. The old thermostat used to control the boiler ok.

There are 3 wires coming from the boiler, red, yellow and blue. Looking at the old thermostat, the relay seems to open/close the red and yellow wires, but there is some more wiring involved, and I can't really tell which is live and which is the one being sent to the boiler.

So I wired it with the red as live (black on the relay), yellow as send (red on the relay), blue as neutral (blue on relay). It does light up the relay LED correctly when I control the low voltage thermostat on and off, but it never turns off the boiler. I tried swapping red and yellow, but it didn't seem to change and the LED wouldn't come on. I also checked if it was reversed, but whether the relay is on or off the boiler is always on.

Any suggestions? thanks
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Postby antk » Sat Mar 21, 2020 5:07 pm

Here's the original thermostat, the top-left connector in the image near the 12 is red, bottom left is blue, top right is yellow.

Image
Image
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Postby ericmark » Sun Mar 22, 2020 12:15 am

Sorry I would use a meter, be it to measure voltage on wires or work out which is neutral, I tried to trace on the pictures, but could not see which is neutral.

Remember in the time when we used red, yellow, blue the black would have been neutral so there is no colour code.
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Postby antk » Sun Mar 22, 2020 9:01 pm

Actually I've played with it a bit more and I think the thermostat is working correctly, and the problem I'm having is the way the 2 channel boiler works. Even if the thermostat is off, I think the boiler is running to warm the HW when the CH is on, but the radiators stay cold. I wanted to use the thermostat to control the boiler remotely, but if it fires the HW anyway then I'll have to find a different solution. (Replace the programmable controller)
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Postby stoneyboy » Tue Mar 24, 2020 12:34 am

Hi,
There is a possibility that the old thermostat was controlling a motorised valve which depended on a 3 wire configuration to open and close the valve ( and therefore turn the boiler on/off).
Defining what is needed is beyond the remote advice available here and you should get a heating engineer with electrical expertise to sort this out.
Regards S
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Postby ericmark » Tue Mar 24, 2020 7:59 pm

Central heating in the main follows plans, we have C, Y, S, W etc. With each they can have additions, but broadly speaking W and Y have three port valves, S will have two x two port valves, and C might have a valve but often only control is turning pump on/off.

The C plan has changed over the years, it uses gravity or thermo syphon to heat the domestic hot water (DHW) and the pump is only required for central heating (CH) early systems had no control over the DHW other than time, and the programmer has a electrical and some times also a physical switch or turn buckle so you can't select heating without hot water, in the winter the hot water simply ends up same temperature as the circulating water, with some a thermostat was added to the storage tank, but this only works in summer, with gas systems often a motorised valve was added to turn off the DHW but with oil there is a worry the boiler could over heat as there is no cooling cycle.
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