How Thermostats Work in Boilers: A Clear Guide

Thermostats are essential for regulating temperature in different technologies.

They are often found in boilers, where they help to maintain your home’s temperature. 

Let’s take a deeper look at their role in boilers, including:

• What they are 
• How they work.
• What types there are.

And more.

What a thermostat is

A thermostat is essentially the control for regulating temperature in a heating or cooling system, i.e., central heating, air conditioning, fridges, etc. 

(It’s not to be confused with a thermometer, which is a tool to read – not control – temperature.)

Thermostats work to maintain set temperatures. 

For example, if a temperature drops below the set range, a thermostat switches heating on to raise it. 

Once this is achieved, the thermostat switches off the heating to prevent overheating.

Types of thermostat

Mechanical

Mechanical thermostats control temperature using two key components:

A bimetallic coil. 

This is made of two different metals (i.e. bimetallic) laminated together and wound into a coil. 

These metals expand and contract at different rates even with small temperature changes. The coil rotates accordingly.

A mercury switch

This is directly attached to the coil. It’s a small glass tube containing liquid mercury. 

At one end, it contains two electrical ‘contacts’ (bits of metal). When you set your desired temperature, the coil is positioned so that the mercury sits away from the contacts. This keeps the circuit — and your heating — switched off.

If the temperature then drops, the coil unwinds and rotates, tilting the tube so the mercury slides back towards the contacts. 

Once the mercury connects with the contacts, the circuit completes and the heating switches on. 

When the target temperature is reached, the coil winds back, the mercury disconnects from the contacts, and the heating switches off again.

Mechanical vs digital

Mechanical thermostats are not as accurate as digital ones. The temperature can vary as much as five degrees from the target set point. 

However, some prefer mechanical thermostats’ affordability and an easy-to-use on/off switch.

Using a mechanical thermostat

To regulate your mechanical thermostat, simply use the temperature dial that you can adjust to the preferred temperature. This sets the point for the circuit to switch off and on.

Metal strips take some time to expand or contract, so the process can be gradual. 

Digital 

Today, most new thermostats are digital or electronic. 

In boilers, they have internal components that provide accurate, responsive reactions to room temperature.

The electronic sensors read the current interior temperature and fine-tune the heating accordingly. This usually keeps the room within a single degree of the target setting.

This is a broad category that includes basic programmable devices and increasingly advanced smart ones. The best modern thermostats available are like different types of computer!

Programmable thermostats 

Programmable thermostats use a set schedule to run. They provide heating or cooling at certain times of the day, with both time and temperature settings. 

With boilers, you can regulate the temperature at different periods of the day. 

This allows you to save energy by turning off the heating when you’re at work, for example. 

You’ll need to take the warm-up and cool-down times into account when programming your thermostat.

Smart thermostats 

Smart thermostats enable temperature control remotely. In boilers, they often feature: 

• Internet connectivity
• App-based control
• The ability to create heating zones
• Accurate energy monitoring and reports
• GPS motion-sensor detection. 

And more!

Advanced models automatically adjust to your routine and use your GPS location to pre-warm your home at the right time.

Boiler thermostat considerations

Location & position

This type of device needs to be located in an area of the home with a fresh flow of air. 

If you place your thermostat in a particularly sunny area or behind a curtain, it won’t be able to sense the temperature accurately.

Ideal temperatures

To get started, set it to the lowest temperature that you find comfortable. For most people, this will fall in the 18 to 21-degree range.

Don’t treat cold days too differently

While it may be tempting to turn up the thermostat on cold days, this isn’t necessary. 

The idea behind a thermostat is that it will react to the colder weather and make sure the heating is kept on long enough to warm up the house to the set point of your choosing. 

However, because it can take longer to warm up a house on a chilly day, you could program the heating to switch on earlier.

Problems with your boiler?

At Hometree, we offer fixed-price repairs for your boiler.

Our experts are available 24/7. Get in touch and we’ll arrange for one of our local engineers to visit at a time that suits you.

We’ll aim to fix the issue on the spot, or if it’s beyond repair and under seven years old, we’ll cover the costs of a replacement.

And our boiler cover plans provide full protection going forward.