How do I reconnect a wall mounted thermostat that has been removed
without making a note of the connection details.
It has three wires. Whoever installed it originally used twin & earth
cable, using the earth as the third wire by insulating it with a piece
of black tubing. It's not an option to replace the whole cable.
So I have 1 x red wire, 1 x black wire & 1 earth wire sheathed with
An all too common and stupidly dangerous practice, it is likely that the
red wire is the live thermostat feed, the black is the switched live and
I do hope that the misused earth is a neutral used to provide a return
path for an accelerator heater used to make the thermostat more accurate
(assuming this is a manual thermostat, not electronic).
First check that the above is true using a multimeter. If correct, carry
on, if not then disconnect both ends and rewire as follows.
Next, the safe thing to do is to restore the earth wire to its proper
purpose by connecting it to earth at the supply end and the earth
terminal of the thermostat (if present).
Then reconnect the thermostat as 2 wire only at the greenhouse end by
connecting the red to the thermostat common and the black to the
normally open contact. Leave the thermostat neutral disconnected.
Disconnecting the neutral will reduce the accuracy of the stat a little
so perhaps set it a degree higher, which should be of little importance
for a greenhouse application.
As it is likely that the stat and heater are in roughly the same
physical location I'm not sure why the cable can't be replaced with the
right (triple + earth) for the job.
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