Fitting a room thermostat

The price list item is charged per unit, that is per thermostat. It typically includes:

• choosing a position on the wall according to the manufacturer's instructions and the layout of the room,
• fixing the thermostat or its wall plate to a prepared back box or directly to the wall,
• connecting the conductors to the thermostat terminals and to the terminals of the boiler, wiring centre or receiver,
• for a wireless thermostat, fitting the receiver near the boiler and pairing it with the thermostat,
• inserting batteries and making the basic settings for temperature, mode and time,
• checking that the boiler fires and stops on demand from the thermostat,
• explaining how to use it and, for a smart thermostat, connecting it to the app.

The work does not include the thermostat itself, running new cable through several rooms, chasing walls and making good, any work on the gas side of the boiler or boiler commissioning by a heating engineer. These are priced separately. If a new cable is needed, it is best run during a renovation before plastering.

In a new build or renovation, the thermostat cable is run together with the rest of the electrical installation and the thermostat itself is fitted after decorating, once the heating is complete and the boiler commissioned. In an existing home, a thermostat is most often added when the boiler is replaced or when the old thermostat stops working. When a new gas boiler is installed in England, time and temperature controls, at minimum a programmer and a room thermostat, are a requirement, and combi boilers also need one additional energy saving measure such as load or weather compensation or a smart control with automation and optimisation. Where there is no cable from the boiler to the living room, a wireless thermostat is the usual answer because it needs no chasing.

Before the electrician or heating engineer arrives, it helps to have:

• A thermostat compatible with the boiler. Check in the boiler manual whether it accepts a standard switching thermostat, the OpenTherm protocol or the manufacturer's own bus, such as eBUS on Vaillant boilers or EMS on Worcester Bosch boilers.
• A chosen location. Usually the living room or another room where most time is spent, known as the reference room.
• A cable or a plan for a wireless link. For a wired thermostat, a cable from the boiler or wiring centre to the thermostat position; for a wireless one, a supply for the receiver near the boiler.
• Access to the boiler or wiring centre. The thermostat terminals are under a boiler panel, in a separate wiring centre or in the control electronics.
• Wi-Fi for a smart thermostat. A good signal where the receiver or hub will be.

The position decides whether the thermostat measures the real temperature of the room. Manufacturers and the industry association for heating controls repeat the same rules: the thermostat should be at a height of about 1.5 m, with a free flow of air around it, in a room that is heated.

• on an internal wall, not an external wall that is cold in winter,
• at around 1.5 m above the floor,
• out of direct sunlight, which warms the thermostat and switches the heating off,
• away from radiators, open fires, gas fires, televisions, fridges and lamps,
• away from draughts by doors and windows,
• not behind curtains, inside a cupboard or behind furniture where the air does not circulate,
• not in the kitchen, where cooking raises the temperature.

In many older homes the thermostat sits in the hallway, often next to the front door, simply because that is where the original cable came out. In practice it is often placed where an old switch was or where the cable happened to be, rather than where it should be. The result is heating that reacts to sunshine, a log burner or a draught, while the rest of the house stays cold.

Isolating the supply

Before any work on the boiler or heating controls, the boiler is switched off and isolated, usually at its fused spur, and the circuit is isolated at the consumer unit where necessary. The terminals are then tested to confirm they are dead. Turning the old thermostat to off does not isolate it. Thermostat and boiler manufacturers list this as the first step in their instructions.

Identifying the connection type

In UK homes the most common arrangement is a thermostat that switches 230 V: a permanent live, a switched live (the call for heat) and, on electronic models, a neutral. Some boilers instead provide low voltage or volt-free terminals for the thermostat, so the boiler manual must always be checked. Combi boilers often have dedicated thermostat terminals bridged by a link wire, which is removed when a thermostat is connected. On system and regular boilers with a hot water cylinder, the thermostat usually connects at a wiring centre and switches a motorised zone valve (S-plan or Y-plan) rather than the boiler directly. Some boilers use digital communication such as OpenTherm, eBUS or EMS, where a compatible thermostat connects to two low voltage terminals and controls the boiler output smoothly. A thermostat connected to the wrong terminals, for example mains on a low voltage input, can damage the boiler electronics.

Fixing the thermostat to the wall

The thermostat wall plate is screwed to a back box or directly to the wall. Wired thermostats are commonly connected with 1 mm² twin and earth or three core cable. The conductors are terminated and the cover is closed straight away. Where the thermostat needs a 230 V supply, the connection is made by an electrician according to the wiring diagram in the instructions.

Wireless receiver

With a wireless thermostat, the receiver is fitted near the boiler and connected to its terminals or to the wiring centre. It is powered from the mains. The thermostat is then paired with the receiver according to the instructions and the signal strength is checked from the thermostat position. Thick masonry or concrete walls and floors weaken the signal.

Setting up and testing

Batteries are inserted and the time, heating mode and target temperature are set. The set temperature is then raised above the current room temperature to check that the boiler fires. When the setting is lowered, the boiler should stop. On a programmable thermostat, the daily and weekly programme is set up.

A like-for-like swap of a wired thermostat on existing wiring is something a competent person can do, provided the boiler casing is not opened. Remember that most UK thermostats carry 230 V, so the work must be done safely and to the wiring regulations. Procedure:

1. Switch off the boiler and isolate the heating controls at the fused spur and, if in doubt, at the consumer unit.
2. Test the terminals of the old thermostat with a voltage tester to confirm they are dead.
3. Photograph the wiring on the old thermostat.
4. Disconnect the conductors and remove the old thermostat from the wall.
5. Check in the new thermostat's instructions which terminals take the live, switched live and neutral.
6. Screw the new wall plate to the wall or back box.
7. Connect the conductors to the correct terminals and close the cover.
8. Insert the batteries and set the time and temperature.
9. Restore power and check that the boiler responds when the temperature is raised and lowered.

If the old thermostat has more conductors than the new one needs, if new cable has to be run or if the thermostat has to be connected inside the boiler, leave the job to an electrician or a Gas Safe registered engineer.

Basic thermostat

Only the target temperature is set. It is inexpensive and reliable, but it cannot lower the temperature automatically at night, so it is normally paired with a separate programmer.

Programmable thermostat

A programmable room thermostat combines the timer and thermostat in one unit and allows different temperatures for different parts of the day and days of the week. For example, a lower temperature at night and during the day when everyone is at work. This is the most common reason a thermostat saves energy.

Smart thermostat

It is controlled from a phone app, can react to whether anyone is at home, and some models learn the household's routine. Common brands in the UK include Hive, Google Nest, tado, Honeywell Home, Drayton Wiser and the boiler manufacturers' own controls. It needs Wi-Fi and often a hub or receiver. When planning smart control for the whole house, it is worth considering smart thermostatic radiator valves as well.

On/off or modulating

A standard thermostat simply switches the boiler on and off. A modulating thermostat using OpenTherm or the manufacturer's bus tells the boiler what water temperature is needed, and the boiler adjusts its output. The boiler then starts less often and runs more evenly. This only works when both the boiler and the thermostat support the same protocol, and some boilers need an adaptor.

Thermostat and thermostatic radiator valves

If the radiators have thermostatic radiator valves, the radiator in the reference room with the room thermostat should have no TRV or have its valve fully open. Otherwise the thermostat and the valve compete, and the boiler keeps running even though no water is flowing through the radiator.

What the thermostat switches depends on the heat source in the home. The principle is the same across systems, but each is handled slightly differently.

Gas or oil boiler

The most common case in both flats and houses. With a combi boiler, the thermostat connects to the boiler terminals intended for a room thermostat. With a system or regular boiler, it usually connects at the wiring centre together with the programmer, the cylinder thermostat and the zone valves. With a condensing boiler, a modulating thermostat is worth choosing if the boiler supports it, because the boiler then runs at a lower water temperature and condenses more of the time.

Electric boiler and direct electric heating

With an electric boiler, the thermostat connects to its control input in much the same way as with gas. For electric underfloor heating or direct electric panel heaters, different thermostats are used that switch the heating circuit directly and often have a floor probe. They are chosen according to the load of the circuit and are wired into the 230 V installation, so this is always work for an electrician.

Heat pump

Heat pumps usually have their own weather compensated control and a room controller from the manufacturer. A third party thermostat should only be connected to the input the manufacturer designates for it. An incorrectly wired thermostat can switch the heat pump off too often, which shortens its life. Before buying a thermostat, consult the installer who commissioned the heat pump.

Multiple zones

In a larger house, or with underfloor heating in every room, zone control is used. Each room has its own thermostat, which controls an actuator on the manifold or a valve on the radiator, and a control unit fires the boiler whenever at least one zone calls for heat. Such a system is priced as a whole, not as a single thermostat.

• Thermostat in the wrong place. It is not unusual to find a thermostat above a radiator or opposite a sunny window, switching the heating off while other rooms are still cold.
• Thermostat on an external wall. The cold wall cools the thermostat and the boiler runs longer than necessary.
• Wrong terminals at the boiler. The thermostat is connected to mains terminals instead of a low voltage input, or to terminals for another device.
• Incompatible thermostat. A smart thermostat is bought that cannot communicate with the particular boiler.
• TRV turned down in the reference room. The room with the thermostat never warms up and the boiler runs continuously.
• Weak wireless signal. The boiler occasionally misses a command and stops heating.
• Flat batteries. Some thermostats stop switching when the batteries run low. Replacing them once a year before the heating season prevents this.

Connecting a thermostat to the boiler and any work involving 230 V mains belongs to a competent electrician, ideally one registered with a competent person scheme such as NICEIC or NAPIT. In England and Wales, a like-for-like thermostat replacement on existing wiring is not notifiable under the Building Regulations, but a new circuit is, and all electrical work must still comply with the wiring regulations. New wiring should be tested and a Minor Works Certificate issued. Thermostat manufacturers state that installation should be carried out by a competent installer. On a gas boiler, nobody other than a Gas Safe registered engineer may work on the gas side, and if reaching the thermostat terminals means removing a casing that forms the room seal, the job is for a Gas Safe registered engineer. Some boiler manufacturers make the guarantee conditional on work on the boiler being done by an approved installer. Fitting a room thermostat does not normally need any approval, but check with your local authority building control if new circuits are being added.

• the thermostat is on an internal wall at around 1.5 m, away from sunlight and heat sources,
• when the set temperature is raised, the boiler fires within a few minutes,
• when it is lowered, the boiler stops or reduces its output,
• the display shows a temperature that matches a thermometer in the room,
• with a wireless thermostat, the receiver shows a connection from the thermostat position,
• the covers on the thermostat, boiler and wiring centre are closed and no cables are left loose,
• the programme is set and you know how to change it,
• you have the instructions and, for a smart thermostat, access to the app.

If the thermostat shows a different temperature from an ordinary room thermometer, most models allow a temperature offset to be set in the installer menu, typically by a fraction of a degree up to a few degrees.

The labour rate in the NUMIQ price list is €35 per unit for wiring and fitting a room thermostat, plus €1 for small consumables. The price does not include the thermostat itself. Prices vary by region and city.

The final cost depends on:

• whether a cable from the boiler to the thermostat position is already in place,
• a wired, wireless or smart thermostat and the number of devices to pair,
• the type of boiler or wiring centre and how easy the terminals are to reach,
• the need to run new cable, chase walls and make good,
• control of several zones or circuits,
• boiler set-up by a heating engineer.

Related work can be priced separately, for example pre-installation wiring for a smart thermostat, thermostatic valve replacement or cutting a chase for the cable. Further items can be found in the electrical installation work section.