The thermostat is an essential part of the boiler system, but what exactly does it do? In this piece we’ll cover the basics and then take a deeper dive into how they work and what different kinds there are.

What is a thermostat?

In terms of central heating, a thermostat is essentially a switch, but instead of a person switching a handle or pressing a button, it is switched on and off by changes in temperature in the room.

A lot of thermostats use the fact that materials expand and contract when heated and cooled to physically make and break electric circuits. Common materials are bimetals and wax. With bimetals (two metal strips joined together along their length), they work by the difference in expansion rates between the two metals (say, steel and copper) making the strip bend. With wax, it’s pure expansion physically pushing on a valve.

There are also thermostats that have no moving parts in them, detecting temperature using thermistors, semiconductors and thermocouples (like a kitchen thermometer that you plunge into your cake or chicken).

Not all thermostats have a simple on-off function. Some open and close gently to control the flow of fluid through them. The principle remains the same – they are reacting to heat – but they are designed to balance temperature in a much more controlled way. Gas ovens work by controlling the flow of gas in response to the temperature inside; and cars’ cooling systems allow just the right amount of water through the engine to keep its temperature optimised. It’s also the principle behind thermostatic radiator valves, which we cover below.

Main thermostat

In a central heating system, the main thermostat is an essential component. Without it you would have to turn your heating on and off multiple times a day to try to keep the temperature in the home reasonably constant. You’d probably end up going to bed with the heating on full for 8 hours every night!

It’s important to remember that the thermostat is reacting to the temperature in the room, not in the radiators or the boiler. If you set your thermostat to 18 °C on a hot day, there’s a good chance the heating won’t come on at all, as the sun is doing all the work. In the depths of winter, however, it will turn the heating on and off as the temperature drops and rises in the home.

Traditionally, the thermostat is connected to the boiler by wires running through the walls of the home. It is often located somewhere neutral, like the hallway, but it can often be in the main living room of the house, where occupants spend most of their time. The location of the thermostat is very important, as the following examples show:

  • Imagine you placed it somewhere warm, like a utility room that contains a washer/dryer, and set the desired temperature to 20°C. Because the utility room might frequently be warmer than 20°C, you might find that the central heating never turns on.
  • Now imagine you place it somewhere cool, like a porch. In winter, it’s unlikely that the porch will ever reach 20°C, so as far as your thermostat is concerned, the heating needs to be on constantly. Your radiators will never be off, even if the temperature indoors is well over 20°C.

Wireless thermostats

Nowadays, wireless thermostats mean they don’t need to be wired into the walls, and that they can be carried from room to room. Many new boilers come with wireless technology included, but it’s often possible to convert your existing boiler to wireless technology with a simple kit.

The advantage of the wireless thermostat is that you can make sure it’s keeping the temperature regulated right where you need it. If you’re spending the night in the living room, who cares what the temperature in the dining room is? If you’ve got a baby, a thermostat in their bedroom will keep that room at the ideal temperature.

The only drawback is that they require re-charging or replacement batteries from time to time.

Radiator thermostats

Thermostatic radiator valves (TRVs) are the devices you often find next to the inlet pipe of a radiator. They are larger than the standard on/off valves and have a rotating head with numbers showing.

Just like the main thermostat, they measure the temperature in the air (not the radiator). Unlike the main thermostat, however, they don’t have a simple on/off function – they regulate the rate at which hot water passes through that particular radiator, so can be off, low, medium, high or full, depending on where you set it.

TRVs are useful for when you want to have different temperatures in different rooms. Note that the main thermostat will still decide whether the heating is on or off, so if that says it’s off, no amount of adjustment of the TRV will change the temperature in that radiator.

Wi-Fi connected and smart thermostats

The newest innovations are “smart” thermostats. These are usually Wi-Fi connected so you can control them from anywhere in the world via a phone or computer – useful if you’re coming home unexpectedly early or late. But some models actually go further, and learn what times of what days you normally need heat, and can switch on and off to match.

All thermostats can be overridden, so you can have the heating on or off regardless of temperature. But as a way of maintaining a comfortable level of heat whilst staying as efficient as possible, they are invaluable when used correctly.

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