Boilers Controls and Thermostats

someone changing their boiler temperature

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It’s completely normal to be a little baffled by your boiler’s controls and thermostats. 

For instance, why do you have a thermostat in your hallway, a thermostat on your boiler, and thermostatic valves on your radiator? Don’t these things work against each other?

Well, while your house’s central heating and hot water network might seem complex, it’s actually rather simple. 

Plus, it’s a pretty sensible idea to get a good grasp of the timers and controls at your disposal – it only takes a few tweaks to significantly lower your energy bills. According to the Energy Saving Trust, installing a room thermostat and thermostatic radiator valves could save you £75£155 per year, if used correctly.

On this page, we’ll explain the typical controls associated with central heating and hot water, including tips on the optimum settings for high efficiency.

someone changing their boiler temperature

Do you need a thermostat for your boiler?

In short: yes. Your boiler thermostat is essential if you want to be able to control the temperature of your central heating and hot water. If, for some reason, you’ve got a broken boiler thermostat, we advise you get it looked at – pronto.

Plus, since 2018 it’s been a legal requirement in the UK for all new boilers to include time and temperature controls. It’s called the Boiler Plus legislation, and applies to all gas and oil boiler installations. 

However, according to the National Energy Foundation (NEF), more than two thirds of households in Britain have boilers that wouldn’t meet the new minimum legal standards.

Boiler and central heating controls explained

There are four devices any homeowner needs if they want full control over their central heating and hot water. 

Of course, there are other things that can help (e.g. smart thermostats), but these four are the bare necessities. Just to be clear, these aren’t things you should have to go out and acquire. Rather, if your household doesn’t have all four of these things, you should sort it out. 

  • Boiler thermostat (i.e. boiler temperature controls for central heating and hot water)
  • Room thermostat
  • Boiler timer
  • Thermostatic radiator valves (TRVs)

Let’s go into each one in a little more detail…

How does a boiler thermostat work?

Your boiler has two jobs: 1) to supply hot water to your radiators, and 2) to supply hot water to your showers and taps. It makes sense, then, that you’d have temperature controls for each of these two functions. This is why a ‘boiler thermostat’ essentially encompasses two sub-thermostats

But where can you find them, and what do they look like? It depends on what type of boiler you have.

Heat-only boilers

There are two types of heat-only boiler: conventional and system. A conventional boiler uses a cold water storage tank and a hot water cylinder, while a system boiler just has the hot water cylinder. It is the hot water cylinder that is responsible for supplying your showers and taps. 

Heat-only boilers come with a central heating thermostat (which you can find on the main boiler unit) and a hot water cylinder thermostat (which you can find on the cylinder itself, near the bottom). Both thermostats look like small dials with a range of temperature options in a circle.

Combi boilers

Combi boilers are much more compact, taking water directly from the mains and heating it instantly. They combine the functions of a cold water storage tank and a hot water cylinder in one small unit – hence their name. 

A combi boiler has two dials on its main unit – one for the temperature of the radiators and one for the temperature of the hot water. If you’ve got a modern combi boiler, it’s likely your ‘dials’ will actually be a digital display with buttons. 

For example, on a Viessmann combi boiler, the two boiler thermostats are represented on a screen by a radiator icon and a tap icon, respectively. You’ll find that most modern combi boilers have a system similar to this.

How does a room thermostat work?

While a boiler thermostat is situated close to the action, a room thermostat is a little more distant. 

It should be located on a wall in a room where you spend a lot of time, such as your living room, and – crucially – not near your boiler. 

It uses a sensor to monitor the temperature of your home, and consequently it keeps an eye on what your boiler is up to. If your room thermostat senses that your home has reached its target temperature, it will switch the central heating off. Once the temperature drops, it’ll fire up the boiler again. 

Without a room thermostat to help it, your boiler would just keep pumping hot water through your radiators without really knowing when it should stop. In fact, it wouldn’t stop. 

Room thermostats are either analogue or digital. While analogue thermostats are useful for elderly homeowners who rely on simple technology, digital thermostats are much more accurate and look nicer on your wall. 

It’s recommended that you set your room thermostat’s daytime temperature to around 19-20°C, while at nighttime 16-19°C is adequate.

How does a boiler timer work?

All boilers come with an electronic timer, which enables you to programme your boiler to turn on at specific times in the day. You can decide when your boiler turns on and for how long, with most homeowners typically programming their boiler to turn on for a while in the morning and a while in the evening.

Despite what some people think, it isn’t true that boilers work best if they’re left on all the time. If you live in a home with decent insulation, you can have a bit of heat in the morning, leave your home to cool down gradually throughout the day (while you’re at work), and then set your heating to come on about an hour before you get home.  

More advanced electronic timers have a seven-day function, which enables you to set different instructions for your boiler during weekends. 

Some boilers – especially combi boilers – also have a mechanical timer. This timer is usually a round dial with a 24-hour clock face printed in the middle. Each hour of the day is broken into 15-minute slots, and there are little grey notches for each 15-minute slot. If you want a burst of heat outside of your boiler’s usual programming, you just pull the relevant notches down.

person adjusting radiator thermostat

Thermostatic radiator valves

While your boiler thermostat and room thermostat are ways to control the temperature of your entire home (and hot water), thermostatic radiator valves (TRVs) allow you to manage the temperature of individual rooms. 

TRVs are those nozzles located on the bottom of radiators, usually with numbers 1-5 and an asterisk (*) on them. The higher the number, the greater the flow of hot water running through the radiator, and so the more heat the radiator will give off. 

It’s recommended that you set your TRVs to number 3, which is about 20°C, although any bedroom radiators should be slightly cooler, set to number 2. 

It’s not recommended that you have a TRV on the radiator in the same room as your thermostat, as a tampered radiator will mislead the thermostat and consequently mess up your central heating.

What’s the best temperature for a boiler thermostat?

Generally speaking, it’s recommended that you set your radiators to 75°C (no higher than 80°C) and your hot water to 60°C (no higher than 65°C). 

If you set your hot water to anything below 60°C, you risk contracting Legionnaires’ disease. If the water in your cylinder isn’t hot enough, bacteria can grow and put your health at risk. 60°C is too toasty for bad bacteria, and not too scalding for you. 

As you’ve probably observed, your tap/shower water should always be a lower temperature than your radiators

Meanwhile, we recommend that you set the temperature of your room thermostat to around 19-20°C, and your thermostatic radiator valves to number 3.

Top tips for using boiler controls and thermostats

To summarise what we’ve been through above, here are a few quick tips for using your boiler controls as effectively and efficiently as possible.

 

  • Place your room thermostat away from your boiler, away from a radiator with a TRV, and ensure it’s unobstructed by furniture

 

  • Programme your boiler to come on in the morning and the evening; it doesn’t need to be on in the daytime

 

  • Make sure you don’t set your hot water temperature any lower than 60°C, otherwise you risk contracting Legionnaires’ disease

 

  • Turn down your heating by just 1°C to save around £80-85 each year (according to the Energy Saving Trust)

 

  • Set your TRVs to number 3, but set your bedroom TRVs to number 2

 

  • Set your room thermostat to 19-20°C, but no lower than 16°C for households with elderly people in

Replacing your boiler

So that’s that. If you’ve always had boiler controls and not known how to use them, we trust this article has shown you just how useful they are. If you don’t even have adequate boiler controls, now might be the time to get them. With UK energy prices destined to keep on rising, you should be doing all you can to keep your bills low. 

If your boiler seems to be on the verge of conking out, we can help you find reliable replacements for a good price. Simply pop your details in this quick form, and our professional installers will give you a call. Happy heating!

Charlie Clissitt Content Manager

When it comes to all things eco-friendly and economically savvy, Charlie is a spectacularly woke millennial. A university-educated solar panel scholar with an eye for detail, when Charlie isn’t writing, he’s tucked up in bed watching his favourite black and white film.