Room Thermostat with no Neutral.

[melodymaker]

I have inherited broken room thermostat that has only 3 wires running ino it. A red, black and earth. Obviously there must be no Neutral so I need to know what kind of room thermostat I need that doesn't accomodate a neutral input for a gravity boiler system (an old Vulcan)

Thanks.

 

[Space cat]

A standard room stat is just a switch but there is generally an extra bit in there. It's a small resistor tucked in somewhere near the bi-metal strip and it's job is to make the thing cycle on and off. Here's how:

The room is too cold so the switch closes. The heating comes on. That resistor is connected between the load side of the thermostat and neutral so it heats up the bi-metal strip. The switch opens. The resistor cools down and so does the bi-metal strip. The switch closes. And so on.

This cycling on and off happens slowly. The proportion of closed time to open time varies with room temperature and so you get smoother control. It's a form of pulse width modulation.

With no neutral connection you have nowhere to connect that resistor. If you leave it floating, the stat will still work but your room temperature will swing up and down, possibly more than you would like. Note that this will also happen if you wire a stat in the wrong way round with the resistor on the supply side.

There may be stats that can do the job but I've never seen one. Maybe somebody else on this forum can help.

 

[breezer]

There may be stats that can do the job but I've never seen one. Maybe somebody else on this forum can help.

any modern digital stat will work, since they are digital.

mine runs on batteries, (to operate the electronics) but it only has live switched live and earth

 

[melodymaker]

any modern digital stat will work, since they are digital.

So it doesn't require neutral?

 

[RF Lighting]

Correct.

Just a permanant live and a switched live