The free diy home improvement guide with answers to your questions on a wide range of do it yourself projects.


Room Thermostat – Replace a Room Stat

Old mechanical room thermostats offer limited functionality in controlling the central heating system in your home. With ever increasing concerns about saving energy and reducing bills, a digital programmable room thermostat can make a difference. These programmable units vary in functionality but the basic concept is to add time control as well as temperature control. More expensive models will have more features – for example being able to set different temperatures and on / off periods for weekdays and weekends.

Research a suitable programmer to determine which make and model can be used with your existing wiring most easily. Many manufacturers supply detailed information on the wiring connections when replacing a basic room stat with their digital programmable models. Some, such as Danfoss Randall also give detailed tables showing the wiring conversions in relation to a number of other manufacturers units. If you cannot find a suitable replacement, or do not fully understand the wiring details, consult a professional electrician.

Removing the Old Room Thermostat

Isolate the power for this circuit at the consumer unit. Carefully remove the cover of the old stat. The cover may have retaining screws on the underside which need to be undone. Lift off the cover to reveal the base plate and wiring connections. Before touching anything, double check that the power has been isolated.

Label the Wiring Connections

Check the markings on each wiring terminal and label each wire accordingly with a small piece of tape. You will need these details later. If, on inspection, there are no markings on the wiring terminals, you will need to consult an electrician rather than doing it yourself. Disconnect all the wires and remove the back plate from the wall by unscrewing its fixings. As you remove it, pull in gently over the wires so that you don’t dislodge the labels.

Fix the New Thermostat Back Plate

Remove the back plate from the new unit and feed the wires through the central cut out. Hold the back plate against the wall and mark the new fixing positions. If you’re lucky one (or two) may line up with the originals. Otherwise, mark the new fixing points and remove the plate again. Drill the new fixing points and plug the holes with suitable plastic plugs. Re position the back plate and secure with appropriate screws.

Wiring the New Thermostat

The wiring for the new thermostat will be clearly detailed in the manual. Read this carefully and identify which of your wires will be connected to which terminal. If you have any doubts at all, do not proceed. Instead, you’ll need to get a professional electrician to advise. Connect the wires to their appropriate terminal following the manufacturer’s details. Old room stats may have a neutral which may not required for the new unit. If so, cap it off properly by fitting a terminal connector block and position it out of the way. Similarly, many modern thermostats also do not require an earth so, if you have an earth wire that is not needed, this will need to be sleeved with green and yellow sleeving and capped with a terminal connector block as well, and positioned out of the way.

Connecting the New Thermostat

Fit the batteries for the new room thermostat – checking that the polarity is correct. Mount the unit on its back plate. This is often done by a hinging arrangement where the top of the unit is secured and then hinged down against the back plate and clipped into position.

Reconnect the power to this circuit at the consumer unit and test the operation of the new thermostat. Study the instruction manual for complete information on setting the time, on / off periods and temperatures.

T&S Heating Limited