2 wire room stat

[bobby spray]

I need some advice on a suitable room thermostat. I have a typical Honeywell ‘S plan’. The room thermostat is fed by three wires (twin and earth) from the potterton controller. When the controller calls for heat the voltage across these wires is 240V. I replaced the ancient brown thermostat that was there with a Danfoss RT1. This is causing me major problems. When I wire it up as instructed nothing happens. I get no ‘click’ and the boiler doesn’t fire. I checked the rest of the system by getting the frost thermostat to kick in. I also bridged the bare wires behind the thermostat and the boiler does fire that way. Does anyone know if these thermostats are suitable for 2 wire applications? If not can anyone recommend one?

 

[chrishutt]

Does anyone know if these thermostats are suitable for 2 wire applications?

Maybe the instructions will tell you?

 

[bobby spray]

Damm. I knew i should have kept those!!!

 

[hi-spec plumbing heating]

where are you putting the wires from your programmer and mot valve.
their are 3 numbers on the rear of the stat.

 

[bobby spray]

There are three terminals on the rear of the stat. Neutral, live and switched live. However I do remeber that the instructions I had (before my wife filed them) said that the controller required that a neutral be connected. I guess that as I only have 2 wires and three terminals, maybe this stat isnt right for my wiring.

 

[Diyisfun]

Have a look here for instructions
http://www.danfoss.com/United_Kingdom/Products/ProductFinder/

 

[D_Hailsham]

Have a look here for instructions
http://www.danfoss.com/United_Kingdom/Products/ProductFinder/[/QUOTE]


Better still, here are the Installation and User Instructions for the RT1

From your original description, the old thermostat was wired with a Live, Neutral and Switched Live. This means that the earth wire was possibly being used as the switched live (NOT recommended).

Fortunately the RT1 does not need this, so you can use the red/brown as the live feed to the RT1 and the blue/black as the switched live to the boiler.

 

[bengasman]

All boilers I have come across will work fine with the roomstat on 2 wires. Yellow/green should have only ONE function; earth. Verify is connected to either earth terminal or metal frame. Do not use as live wire in any form

 

[DP]

But not all thermostats will work correctly with two wires and not all boiler are happy getting 240 volts from thermostats- some work to a lower voltage (e.g. 24 volts)

Most (not all) stats have an anticipator. A thermostat such as this must have three conductors and earth to make it operate correctly.

OP, I suspect the two conductors you are using are live and switched live with earth used (depends who wired the controls previously) as neutral- a bad practice as mentioned above.

 

[Malc C]

When I came to replace my roomstat with a digital version it too only uses two wires. It seems a common practice to run standard 3 core mains cable for stat wiring, so you will always have two wires which could be at the wrong potential depending if the neutral and earth is used for the switched live / neutral.

As far as i know ( and I'm no expert so maybe one of the fitters will chime in here) there is no cable that has two reds and one black to cater for live. switched live and neutral.

Also isn't it not the case that similar practice is/was used for the switch wiring in lighting circuits ?

Malc

 

[bones001]

o

 

[chrishutt]

there is no cable that has two reds and one black to cater for live. switched live and neutral. Also isn't it not the case that similar practice is/was used for the switch wiring in lighting circuits ?

3 core and earth cable has long been widely available. Used to have red, blue and yellow conductors (+ bare earth), now I think the colours are brown and 2 greys, or something much less clear than the old colours. I've still got plenty of the pretty coloured stuff so will carry on with it - s0d the regs.

 

[Malc C]

showing my age there with the old red / black wiring !