If your current thermostat has been chugging away for years, it will be inefficient and costly compared to today’s more efficient thermostats. This guide will show you how to replace an old thermostat with a new digital thermostat yourself, safely, step-by-step, with a more energy efficient modern digital thermostat, ensuring you can enjoy your more heat-efficient home asap.
WARNING: If you’re replacing an old thermostat with a ‘smart’ thermostat, you must not attempt this yourself. They’re not compatible voltage-wise and the process is too complex and dangerous to be attempted by a DIYer.
How to change a thermostat
Changing a thermostat is a fairly challenging job for a moderately capable DIYer. It requires second-to-none electrical safety. Alongside this guide, please read the instructions on your new thermostat and incorporate your boiler manual too.
Please note, before disconnecting the old thermostat or buying a new one, you need to find out if your potential new thermostat is compatible with your existing wiring. The new one may require additional wiring or alteration of existing wiring. It is essential that you understand the voltage supplying your old thermostat, it was probably fitted with a 240v power supply along with a switched live, however it could be 24 volts! If you make a wrong connection this could seriously damage your boiler.
Tools you will need:
- Approved voltage tester
- Mini spirit level
It’s vital to be safe around electricity. You’ll potentially be working with a 240-volt electrical circuit that supplies your boiler and thermostat, so you must make sure that you isolate it correctly and prove that the circuit is dead using appropriate safe isolation equipment and procedures.
If you don’t know what you’re doing, there’s no question that you should employ a qualified electrician to do this task. Accidents with mains voltage electricity can be deadly.
Safe electrical isolation procedures
- Assuming you have a boiler that provides heating to your house, it should have a fused connection unit (FCU) or socket next to it and this should also be the FCU that isolates the thermostat, as the heating circuitry should all be isolated from a single point.
- A good way to test this while the boiler is switched on is to go to your fuse box or consumer unit and switch on the circuit breaker on the ‘sockets’, cutting the electricity supply to all the sockets in your house.
- If you can see that your boiler is now switched off, and when you test the thermostat with an appropriate safe isolation equipment and procedures, you can see that it is dead, you can be sure it’s on the ‘sockets’ circuit.
- The above point is no substitute for switching and unplugging at the socket next to the boiler – do both.
- Now use your appropriate safe isolation equipment and procedures to confirm that you have really isolated the thermostat wiring before starting to work on it.
Types of thermostats and wiring:
Wired thermostats are connected with fixed wiring from the heating circuit and typically the supply will be 3 core and earth cable.
The wires are usually colour-coded as follows:
Switched Live: Grey
Earth: Yellow and green
The live and neutral provide the power, and the switched live output allows the thermostat to switch the power on and off as it reads the ambient temperature.
Some thermostats are battery-powered and don’t need power, so only the live and switched live are used. Some digital thermostats use the mains as the power supply, therefore they have a neutral terminal.
Please note: Don’t assume your wires are colour-coded correctly. It’s very important that you take a photo or make a note of which wires are connected to which terminals in your old thermostat so you can repeat the wiring in your new one.
How to install a thermostat
- Use the safe isolation procedures listed above. You are dealing with a 240V power supply.
- Remove the thermostat frontplate and the dial.
- Open the transparent cover and expose the wires and connections.
- Before you touch anything, use your voltage tester to double-check that all the connections are dead.
- Take a photo of the connections, or even better write them down or label the wires exactly as they were connected. Eg. if the blue wire was in connection B, write ‘B’ on the piece of tape and wrap the tape around the wire. Label any loose or unconnected wires as well. You’ll need to know this info for wiring the new thermostat, or just in case you have to temporarily rewire your old one. Don’t necessarily trust any coloured sleeves on the wires, as it could have been installed by a non-professional.
- Disconnect the wires from the terminals, then remove the thermostat from its backplate.
- Disconnect the earth wire from the backplate, then unscrew and remove the backplate from the wall.
- Make sure the cable and wires stay hanging out of the wall. You don’t want to lose them behind the wall. One tip is to tape a pencil to the wires – it’s just enough weight to keep them there.
- Fix the backplate of the new stat to the wall – some thermostats come with a larger backplate to cover up bare plaster or paintwork so you don’t have to repaint. You may need to measure up and drill new holes in the wall for the new backplate, so use a pencil and a mini spirit level.
- Now you can start connecting the wires to the new terminals. Use your photo, notes or labels to reconnect the wires. Check the new thermostat manual for how exactly to reconnect the wires. If you’re in any doubt at all, call a professional electrician.
- Now you can clip on the frontplate to the backplate.
- Now programme the timer.
- Once you have followed all these steps, go back to your fuse box and switch back on the electricity supply to the boiler and thermostat.
- Check everything’s working correctly.
How do I know if my house thermostat is faulty?
If you’re getting faulty thermostat readings and it’s much colder or warmer than your thermostat is telling you, it’s most probably because your thermostat needs replacing. A thermostat that’s not correctly reading the temperature will usually cause your central heating system to overwork, which is bad news for your heating bills.
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Can I replace my thermostat myself?
Replacing a thermostat is a challenging and potentially dangerous task. You must be comfortable with carrying out safe electrical isolation procedures and wiring an appliance. Some thermostats may require additional wiring that your home doesn’t have, so check the thermostat manufacturer’s information before tackling the job yourself.
How do I replace an old thermostat?
1. Disconnect your thermostat and boiler from the mains electricity supply.
2. Remove your current thermostat, labelling your wires with the terminals they were connected to.
3. Fit the new thermostat, connecting the correct wires to the correct terminals as before.
4. Programme the thermostat then reconnect the electricity supply.