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What does a thermostat actually do?

A thermostat is a temperature controlled switch. Mainly used to turn on the heating, and keep it running until the room reaches a preset temperature. Once its warm enough, the stat switches off. When the room cools slightly it switches back on, and thus runs the heating periodically to maintain the set temperature. The temperature setting is usually user adjustable.

Do I need one

All central heating systems need one. If you don't have one, there is no way to turn the heating completely off once the desired temperature is reached (the building regs refer to this as a "boiler interlock"), or to control room temp accurately.

Some heating systems use more than one room stat. This allows independant control of different areas of the property.

But I have Thermostatic Radiator Valves, do I still need a stat?

Yes, even with TRVs the stat is needed to stop the boiler from running where there is no need. Also TRVs are generally only partially thermostatic, and don't maintain as even a room temp temp as a standalone stat.

How many should I have

Central Heating:

  • Most domestic CH systems have one room thermostat.
  • Large houses sometimes have the heating split into zones, each of which requires its own room thermostat.

Hot water:

  • If your gas or oil heating system also stores hot water, the hot water cylinder needs a cylinder stat. These typically have wide hysteresis and no compenation heater. Some are strapped onto the outside of the cylinder
  • Electric stored hot water usually uses an immersion heater with a bimetal thermostat built into the same housing.
  • Most solar hot water systems use a differential stat to run the solar system pump when the panel is hotter than the stored hot water. Some just use a solar panel to power the pump when the sun shines, and no thermostat. Less often solar systems just rely on gravity circulation, with no pump or stat.

The boiler itself has a thermostat to control the primary circuit temperature.

  • Old boilers use a user adjustable bimetal stat mounted on the boiler
  • Modern boilers tend to monitor primary circuit temp electronically, and sometimes adjust it to maximise efficiency
  • Weather compensation schemes choose the primary circuit set temp according to the outdoor temp. Primary temp is maximised in cold weather to maximise heat delivery, and reduced in mild weather to maximise boiler efficiency.

Systems fitted with Under Floor Heating may have additional room stats to control the UFH heating zones. Wet UFH also uses a thermostat (a TMV) to control the UFH loop water temp, which must be kept well below primary circuit temp. Electric UFH uses a thermostat to limit its temp.

Solid fuel boilers typically use a primary circuit that turns on a radiator when it gets too hot, to dump heat & prevent the system boiling.


Central heating

Normally either a bimetal stat (with compensation) or a programmable electronic stat.

  • Bimetals are cheaper, simple to use, and more reliable.
  • Electronic non-programmables are less reliable.
  • Programmable electronic stats offer different temp setpoints at different times, and include all the fuctionality of a CH programmer except hot water control.


Non frost-frees usually use a capillary stat. Frost free freezers and fridge freezers mostly use a few electronic stats, but bimetals have been used too.

Plugin heaters

Thermostatic ones generally use a non-compensated bimetal stat, providing poor performance.

Small appliances

When thermostatic, they usually use an uncompensated bimetal stat


These use a wax capsule thermostat, or an electronic sensor linked to the engine control unit. Less often, viscous coupling has been used.


These use a wax capsule

TRV capsules are affected as much by the water temp as well as room temp, so they don't provide properly thermostatic control of room temp. TRVs with a well ventilated head respond more to room temp, but only to an extent. TRVs improve temperature imbalance between rooms, but they can only go so far in this.

Thermostatic showers

Non-electric ones use a wax capsule. Electronic showers use electronic temp sensing, usually with a faster responding sensor.

Heating tape

Electric heating tape wraps around pipes to prevent frost damage. They now tend to use a resistive compound that is inherently thermostatic, ie its resistance falls when it gets cold. The older type uses a bimetal stat for control, typically set to 3-5C.

Types of thermostat


Bimetal or mechanical stats use a bimetallic element (2 sheets of different metals bonded together) that bends with temperature change. This operates a switch.

Being mechanical they are pretty reliable long term, and easy to understand. They typically control the temperature to an accuracy of half a degree. They lack the extra features of electronic programmable stats, such as ability to lock the temp control, and change temp settings through the day.

The user interface is a dial, making them trivially easy to operate. (Though this interface is analogue, the control system is still digital.) Good stats have plenty of turn per degree C, making adjustment easy. Less good stats can be mildly fiddly to adjust by a small amount.

Compensation or acceleration

Mechanical CH stats include a tiny compensation heater (a small resistor eating less than a watt) that comes on when the stat switches "on," heating the thermostat very slightly. The purpose of this is to reduce the hysteresis of the bimetal stat from around 3 degrees C to around half a C.

This resistor requires a neutral connection to work; without this connection the room temperature would swing up and down by a few degrees, which is not satisfactory.

(It is possible to miswire a bimetal stat so that the compensator is on at the wrong times. If this happens, a wide range between switch on and switch off temps will be seen)

Mechanical stats are usually designed for operation in mains voltage control systems. The 240v compensation resistor won't work on a lower voltage.

Uncompensated bimetal stats

Cut price bimetal stats found in equipment such as portable heaters often lack compensation, resulting in poorer performance, with room temp varying.

Sometimes such stats are affected by the heater rather more than is ideal for compensation, making them unable to maintain a steady room temp.

These issues don't apply to central heating stats, which have well designed compensation.


Electronic thermostats use some electronics plus a choice of possible sensor components, such as a thermistor or diode. These can maintain a slightly closer temperature control than bimetals, to within 1/4-1/2 a degree C.

Central heating electronic stats

Programmable thermostat

Rather than employing a rotary knob and temperature control, electronic CH stats usually have an LCD readout.

With programmable electronic stats, the preset temp can be set to automatically switch to different levels at specific times of the day. Programmable stats can in effect act as programmers as well as stats. (On combi boiler systems where there is no need for a cylinder stat, the system programmer can be dispensed with altogether.)

The downsides of electronic thermostats are:

  • less long term reliability
  • cease working when the battery runs out
  • can be frustratingly complex to operate
  • a battery leak can kill them
  • greater cost than bimetals

The advantages are:

  • settings can be locked
  • can act as controller as well as thermostat
  • can set different temps at different times of the day
  • can maintain slightly closer control of temperature, reducing fuel use by a very small amount

Electronic stats in cars, appliances etc

Most of these have no user interface, and are a fixed temp setting only. Frost free freezers use a few of them.

Wireless CH stat

Wireless CH thermostats avoid the need to run a mains wire to the stat, and are battery powered. They can be moved from one room to another. They stop working when the batteries run flat.

Tamperproof CH stats

These are stats which once preset, can't easily be altered by casual users.

  • Many programmable stats can be "locked" to achieve this
  • Tamperproof bimetal stats have the control knob accessed after removing screws


Fridges and freezers mostly use a capillary stat for control. These consist of

  • a metal bulb of a material that expands with temperature
  • a very thin capillary tube that connects this to...
  • a sprung switch with user setpoint adjustment

The purpose of these is to enable the user control to be in a separate position to the sensing element.

Capillary stats were also used in early central heating systems, which are now long obsolete.

Wax capsule

A capsule contains wax & aluminium or copper powder. The wax expands with rising temperature, and pushes out a pin, moving a diaphragm to control water flow. The metal powder conducts heat to speed response, but with thermostatic showers its not really quick enough when someone turns a tap on.

Car wax thermostats are usually fitted to the engine block, though stats fitted externally in the water piping are also found.

Wax stats are used in TMVs, to limit hot water flow according to temp, blending it with

  • cold water in the case of HW feed to taps
  • lukewarm water in the UFH circuit in the case of wet UFH

Viscous coupling

Some cars have used viscous coupling to control the radiator cooling fan. The fan is driven by from the engine when hot, but when cool its almost uncoupled and barely turns. The control element is a fluid that changes viscosity with temperature.

CH Thermostat features

Volt free contacts

This means the switching contacts aren't connected to the supply the thermostat runs on. This makes it possible for the thermostat to be run on one supply, but switch a different supply.

When a volt free stat replaces one without this feature, you'll normally need to add a wire from live to one of the stat's switch contacts so you get switched live out. But note there are systems using low voltage control where live must never be connected to the thermostat's switched output, or bad things happen.

Changeover contacts

Some thermostats have 1 pole 1 way switching. These switch on when heat is needed, and off when not. They can't therefore control air conditioning, as they only turn power on when the room's too cold.

Two way switching stats, switch either on or off as temp rises, depending on where you connect the wires. Thus they can control heating or cooling equipment. A lot of basic stats are 2 way.

Frost protection

Frost protection is a 5 degree C setting designed to avoid pipes freezing. Many bimetal stats can be turned down to 5C rather than switching the system totally off. Electronic stats normally incorporate frost protection.

Aux switching and contacts

Some stats have additional contacts available for switching extra equipment along with the heating.

Proportional / Modulating stats

These are more sophisticated stats that are able to either report the actual temperature back to the boiler, or indicate the amount of heating required. These are often bespoke items that need matching to specific boilers that understand how to use this type of stat, and are often included in "weather compensating" thermostats. This can allow for more efficent operation of a boiler by adjusting the flow temperature of the boiler water to match the actual needs. Hence the radiators will run hotter on cold days, rather than just longer, and cooler on mild days, to achieve better efficiency.

Optimising stats

These are programmable stats that will attempt to more accurately meet the changing temperature demands of the stat as it switches to each new temperature setting throughout the day. To do this they can turn the heating on before the demanded time to ensure that the temperature requested is met at the start of the time.

Hence if you say have the heating set to tick over at 15 degrees over night, but then want it to be 21 degrees between 7 and 9am. Assuming it is not particularly cold over night, a non-optimising stat will wait until 7am before calling for heat. This means the desired temperature is not actually reached until sometime after 7am. The optimising stat turns the heating on some time before 7 to ensure its 21 degrees by the time 7am arrives.

Some optimising stats will attempt to automatically learn the response of the house, others may need to be set manually to adjust for the typical lag of the building.

Optimising stats are used in large commercial premises.

What does the thermostat control

On a typical single zone domestic system, the wire the stat switches on power to fire up the boiler and switch on the pump. The zone valve is controlled by the controller.

In more complex systems, depending on the type of stat and the type of zoning used, the stat may connect to one of more of the boiler, a zone valve, and a pump. See the Central Heating Controls and Zoning article for diagrams of how everything is wired up in the commonly implemented central heating systems.

Which is best?

Each type of thermostat has advantages and disadvantages.

  • If you only want one temperature setting, bimetals are more reliable and much cheaper than electronic stats. Ideally pick one with well spaced out temp markings so adjustment is easy.
  • If you want temp settings to change through the day, only programmable electronic stats give this feature
  • If you want a simple user interface anyone can use, bimetal stats don't require any learning
  • If you want maximum energy efficiency and more accurately controlled performance than the above types, eg for a large building, then the cost of more complex control systems involving features such as proportional control, anticipation, electronic learning and so on are justified. Don't assume these will work with a standard domestic system, often they won't.

How to wire a Central Heating stat

Type of cable

Fixed house wiring should be implemented in flat cable:

  • 1mm2 3&E is the best option. It provides a neutral, enabling all the main types of stats to be used
  • 1mm2 T&E can be used when no neutral is needed, but it does makes it impossible to change to a more reliable bimetal stat
  • Larger cable is also ok, and 2x T&E can be used if no 3&E is in stock.

Earthing or lack of

Earthing of the stat is usually needed where metal parts are touchable. This includes old plastic cased stats where the knob can be pulled off, uncovering a metal nut or spindle.

No earth connection is required for double insulated stats. This is typical for battery operated programmable stats. When an earth wire is present but not wanted, simply sleeve it fully and either fold it out of the way, or park it in a spare unused earth terminal. Do not park it in an unused switching terminal.

Note is it not acceptable to re-purpose an unused earth wire for some other purpose, such as supplying a neutral.

What to do with spare neutrals

If neutral is not needed (ie with battery powered electronic stats), it is simply not connected. Put the stray wire end into a lone screw connector to stop it touching things it shouldn't.

Some stats that don't need a neutral have a parking position for a neutral wire. This can be used instead of the above. If your stat doesn't have this, don't connect N to an unused connector position unless it is specifically marked for the purpse, or spitzensparken and fusenpoppen may result, and this can kill the thermostat's relay.

What else can be controlled with a thermostat

  • Switching electric space heating directly - but thermostat current rating is often limited
  • Controlling winemaking heaters, yogurt heaters, propagators etc
  • over temperature alarms
  • freezer failure alarm
  • frost protection heater


Most thermostats operate at mains voltage, which raises the usual safety issues when wiring.