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What is a smart thermostat?

A smart thermostat is an app-controlled product that remotely controls your home's temperature. Not only can you switch your heating on and off via your smartphone, tablet or computer, the app can determine when you are on your way home and turn the heating on for you.

Likewise, a smart thermostat can use this facility, known as geofencing, to turn off your heating when you leave your house if you have forgotten to do so, saving you money.

While these functions could be achieved via a non-smart programmable thermostat, it would require the user to input information manually and have a very predictable routine!

Smart thermostats could knock 10% off your heating bill

A number of manufacturers claim that smart thermostats can cut heating bills by an average of 10%, and Jo Alsop from consumer advice website The Heating Hub says that various studies have backed this up.

However, Greg did find some more bold claims, including from Tado, who say their smart thermostats can save up to 28%, and that this is "scientifically proven". However, Jo says that saving was largely made through the geofencing feature and was based on a house where the thermostat was set and left at 20 degrees for most of the day.

Savings could take years to show

Greg gives Jo the example of listener Wendy's home – she has good cavity wall insulation, a 12-year-old condensing boiler and thermostatic valves on the radiators in each room of her house (following Sliced Bread's episode on Portable Heaters), and she has a wood burner in her lounge.

As Jo explains, a smart thermostat would automate what Wendy does manually already. Of the geofencing facilities, Jo notes: "She might benefit a few pounds from the algorithms and the sensors." However, Jo says: "Assuming she saves perhaps £50 a year, the value of a £200 smart control plus installation is going to take six years to pay back."

Turn down the flow temperature of your boiler

Most modern boilers – be they combi boilers or hot water cylinder boilers – are condensing boilers that capture waste heat before it leaves the boiler and recycle it back into the system.

They can achieve up to 90% efficiency, as opposed to a rate of 60-75% for non-condensing boilers. To help ensure 90% efficiency from a condensing boilers, Jo offers a tip: "Many people have their flow temperature set to about 80 degrees, but if you lower it to 60 degrees on a boiler, efficiency would improve and that can save you money."

Compensation control is the real efficiency driver

While smart thermostats are essentially turning a boiler on and off, there are compensation controls that actually maximise boiler efficiency. They do this by monitoring flow temperature, and also by preventing the thermostats from overshooting the set temperature. This happens because the residual heat in the radiators can be higher than the boiler temperature due to the time-lag between the boiler going off and the water flow stopping.

Compensation control doesn't require smart thermostats to work, although a combination of the two would provide the easiest and most efficient system. Unfortunately, anyone who has a boiler older than eight years won't be able to get a compensation controller to communicate with it – Wendy is a case in point.

TPIs offer another heating control option

There is another option for Wendy, and for energy users like her, Time Proportional and Integral thermostats – or TPIs – that use algorithms to change flow temperature. Crucially, these are available in smart and non-smart thermostat types, the latter being a cheaper option.

The cost of smart thermostats can be a 'rabbit hole'

The fairly modest savings that can be achieved with a smart thermostat are going to be even more modest now that energy prices have dropped, however a cheap smart thermostat can start from around £50-£60 (more expensive options range between £100-£150 says Money Saving Expert), and so, for a one-bed flat, for example, this still might be a worthwhile investment.

Of course, it's an economy of scale. "If you have got this large house with a more complicated system," says Harry Kind, consumer affairs journalist at Which?, "you've got multiple heating zones, so you can be spending £200-£300. And then, if you start adding smart thermostatic radiator valves into the mix, you can be adding more money. There is a rabbit hole that you can go down."

Being 'smart' also means being tech-savvy

While smart thermostats are supposed to be easy to install, it might be "better safe than sorry", says Harry, to get a trusted trader to make sure they fit with your electrics and plumbing. Another reason to get a professional installer is to help troubleshoot technical issues and queries.

Before installation itself, it's worth checking the security level of the smart thermostat. "You might find that if you get one of the cheaper brands it might not have updates available to the app,” says Harry. This could make your software vulnerable to hackers. Choosing a trusted brand is important in general, given that the app will have access to your location and perhaps even contacts and images.

Find out whether Wendy thinks she'll be spending the money on a smart thermostat, or going for one of the other options intead, by listening to the episode in full here.

The information contained in this article was correct at the time of broadcast on 26 October, 2023.