replacing mechanical roomstat > programmable

[Gangley]

Hi,
I have a mechanical honeywell room stat which has 4 wires running to it.

1 is live feed
2 ? Black and never live wherever the microswitch is - but looped to 3 via a resistor, and both 2+3 are looped to 1 via a capacitor.
3 Return Yellow live when the micro-switch closes to demand on
4 Neutral Blue live when the micro-switch makes to demand off
I know the microswitch makes and breaks by flipping a live feed to either 3 OR 4 and back

How come this old stat has 4 wires to it and what are they all doing?

It is part of a dual valve, pumped system which is almost an S Plan but some odd wiring evidently ( last plumber info)

I have bought a honeywell programmable room stat which only has call for two wires- Live and Neutral.
Please can you help me to avoid blowing up my new toy by telling me where to shove all of the wires- ho ho - so tempting eh! Surely it cannot be so difficult to provide a simple conversion diagram and instructions?
Thanks,

 

[IanMichael]

i think it is more than likely that you need to re-wire your junction box (near the cylinder) as the honeywell stats are voltage free.

 

[Gangley]

Thanks Ian,
I am not sure what "voltage free" means since the new thermostat has Live and Neutral going into it. It has A - Live,B- Neutral and C terminals, where A is Live and B is Neutral. C seems to be the switch internal to the stat to choose between C and B.
I guess by wiring at the Cylinder you mean the PCB on the boiler?

 

[IanMichael]

no, not the pcb. you have a hot water cylinder?? thejunction box near that - where the pump and motorised valve run to

 

[mikeyd]

On your old stat, 2 is probably a neutral feed which is used when the stat turns on to put 240v across the resistor which warms up and somehow improves the operation of the thermostat. Newer stats work differently and do away with the need for the resistor. The 2 capacitors I guess just suppress any arcing of the switch contacts to make it last longer. 4 is not a neutral, its a live return when the stat is not on. On my boiler it's not connected and I don't know about anything more complex than connecting to a boiler which only needs a live when on so can't help there.

Are you sure the new stat has only 2 connections and marked live an neutral? sounds odd unless its a remote stat transmitter, but you would then also need the receiver which goes next to the boiler.

 

[flameport]

The thermostat you have is a changover type, which is why there are 4 wires.
If the new one only has 3 wires, it will just be an on/off switch. Not compatible with what you have got, and the only way to make this compatible will be to significantly alter the wiring to the rest of the system.

 

[Gangley]

The thermostat you have is a changover type, which is why there are 4 wires.
If the new one only has 3 wires, it will just be an on/off switch. Not compatible with what you have got, and the only way to make this compatible will be to significantly alter the wiring to the rest of the system.

Thanks Flameport. I guess I have bought the wrong kit unless I enlist some heavy help. I did try to keep it simple but I could not find any 4 wire mechanical stats, or even the honeywell replacement. I really wanted a simple electronic stat but that came with the same problems.

Thanks to all who sent help. Stay warm!

 

[Gangley]

hi Y'ALL!!
Thanks for all the tips. I sorted it out after a long trawl through the net. I think I had a T6360B but had bought a badged honeywell programmer stat from Plumbase at half price (£30) in the sale. A call to honeywell techies allowed me to connect the live and the return from T1 and T3 into A and B respectively. I had to put a terminal block on the other two wires to make them safe since they are still live at times.
Anyway, the boiler is on 24/7 call for heating and the room stat now controls the temperature along with the rad valves. It should save some dosh.
Regards,
Gangley