replacement thermostat Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[tomahawk_pa38]

I want to replace my old thermostat with a Drayton Digistat 3. My old stat has two wire (red and a yellow) there is also an earth and an unused blue wire.

My Digistat has 3 terminals shown as C, Call and Sat (satisfied ?) How do I connect it please. I've tried lots of different combinations with the two wires but none seem to work properly. The nearest I've got is the stat calling for heat but then it won't switch the heating off. Should I use the blue wire as well ? Which ones should the red and yellow go in ?

[jondeau]

Be careful what you connect to what.......your old stat is most likely just a glorified on/off switch.

So your red and yellow wires are merely the live feed to it, and the switched live back to the boiler/controls.

The blue wire is probably not connected to anything, most heating engineers used to carry only four core cable to make things simpler. It was used a lot for some control systems.

[Paul Pepper]

what brand is old stat?

[sparx]

most room stats require a neutral to a resistor inbuilt called an 'anticipater'
it is used as a mini heater to cut down the hysteresis curve or dead - band. without it connected the switch on / off points would be around 2-3 deg, & as human body can feel 1 deg change we would be constantly feeling too hot or cold before stat operates.
This is the reason we put in 3core & earth cable. If it's required drawing that came with stat or is inside cover will show N connection; your blue wire should/could be connected to Neutral at junction box (near 3 port v/v usually).
As stated on previous reply the red is normally supply from programmer to stat. com. & yellow is switched line to call v/v to heating position,
regards SPARX