One simple way to adjust your boiler to lower your heating bill

When it comes to cutting the costs of your heating bill, easy answers are few and far between. Turning down the thermostat is the obvious choice, but if yours is already as low as is comfortable, there's something else that can help.

Flow temperature is a tool in the arsenal of combi boiler owners that many of us haven't used yet. And whether it's gas, oil or electric – changing this one setting on your combi boiler will cut your heating bills straight away.

Below, we explain whether this is suitable for your central heating system and how to get started.

To what extent this will work is highly individual — some people can use low flow temperatures while others need to keep theirs high. We explain this too. For people who own a heat-only (regular) or system boiler that stores hot water in a tank, there are limits to how low you can safely go.

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What is boiler flow temperature?

Most homes are heated by wet central heating. That means water is heated by a boiler and then sent to radiators and back again in a closed-loop system. The heating flow temperature of your boiler is the temperature at which water leaves your boiler on its journey to your radiators. 

It's different to the setting on your thermostat. For example, if you set your thermostat to 19°C, your boiler then heats water much hotter than that and sends it to your radiators to warm your home until the air around the thermostat reaches 19°C.

By default, many boilers are set to heat this water to 75-80°C. But many homes with condensing combi boilers can be suitably warmed with heating flow temperatures of 60-65°C or lower. Lowering that temperature means your boiler uses less gas and operates more efficiently – meaning immediate cost savings for your energy bills while still keeping your home warm.

If your boiler was installed after April 2005, it will be a condensing boiler.

Combi boilers also let you select how hot the water supplied to your taps is. So you can adjust both your heating (radiator) and hot water temperature separately. 

Water comes back from radiators into your boiler when it reaches a certain lower temperature, ready to be heated up again. This is called the return temperature. If you have a flow temperature of 80°C, the return temperature is probably 60°C. 

Read about the different type of boilers to find out what combi, system and regular boilers are and how they differ.

Does lowering your boiler's flow temperature save money? 

It can, yes. A study published by the Heating and Hotwater Industry Council (HHIC) in its Heating Up to Net Zero white paper shows that you can make a saving of 6-8% on your gas bills by lowering a combi boiler's radiator flow. And home heating advice service The Heating Hub estimates that you can save another 5-10% if you have a Worcester, Vaillant or Ideal combi boiler by turning off your pre-heat. 

But the actual savings – and how your heating system is affected – vary depending on your home. Reducing flow temperature means your radiators won't be as hot to the touch and it'll take longer for your rooms to get up to temperature. 

As long as your home is reasonably well insulated, your boiler will run more efficiently and you'll be using less energy to achieve the same room temperature, which will save you money. If your home heats up very quickly when the radiators are on and often overheats, you're probably in a good position to lower your flow temperatures.

But for homes that are not well insulated or have small radiators (also known as 'undersized'), the benefits might not be as good. In these situations, your central heating might struggle to work properly with lower flow temperatures. This can be counter-productive as your boiler will be using gas while your home fails to reach the temperature you've set on your thermostat. 

The best way to get the right settings for you is to book in a boiler service. A boiler engineer will be able to assess your home and recommend a flow temperature that's suited to your requirements. 

Can I lower the temperature of a heat-only (regular) or system boiler?

For the most part, these types of boiler don't let you choose a radiator flow temperature that's different from the hot water temperature without modification.

This is a problem for boilers that store hot water in a tank or cylinder, because it does need to be hot. This is to reduce the risk of the development of Legionella bacteria which thrives in standing (stored) water and causes Legionnaires' disease.

For this reason, don't set the flow temperature lower than 65°C without asking an engineer first. 

Combi boilers let you set a different temperature for heating and hot water. They also don't store water, heating water straight from the supply on demand instead.

So if you have a combi boiler, contamination is much less likely.

Small savings can still be made with boilers that use a hot water cylinder, but not as much. Modification to your boiler might enable you to set a separate heating and hot water temperature, but this needs a boiler engineer's guidance. 

How do I change my boiler's flow temperature?

The Heating & Hot Water Industry Council (HHIC) recommends that people adapt their boiler settings with the advice of a boiler engineer. 

This is particularly true if you have a system or regular boiler that keeps water stored in a tank. Because stored water needs to be heated a certain amount to avoid Legionnella bacteria, you should only change settings with professional advice. 

However, if you have a combi boiler, you've made sure it's safe and you've checked your boiler's technical manual, you can adjust these settings yourself. 

This setting is accessible to anyone and it can be changed using your boiler controls. The flow temperature for heating is generally symbolised by a little picture of a radiator, and for hot water, a picture of a tap. Up and down arrows will change the temperature settings.

Nesta has created an interactive boiler temperature tool to walk combi boiler owners through the step-by-step process of changing flow temperature settings for your heating. 

What flow temperature should I use?

We see different recommendations all the time. Some manufacturers tend to suggest a high flow temperature. For example Vaillant and Viessman both suggest that combi boiler owners have a 75°C flow temperature close to the maximum which guarantees scorching hot radiators.  

But Octopus Energy, in its popular Winter Workout campaign, recommended that people with combis set a much lower heating flow temperature of 50°C for heating to try and cut bills. 

Realistically, the best heating flow temperature for you should be as low as possible without reducing your boiler's ability to heat your home properly. Newer homes built in the last decade or ones with retrofit insulation can probably use lower temperatures. Older homes that haven't been renovated are more likely to need high temperatures.

Your radiators may be designed to work with high flow temperatures only. New installation of low flow temperature systems often involve installing new, larger radiators. 

A combi heating setting of 65°C, as a midway point, can be a good place to start. Once you know how to do it, you can adjust this down lower as you get a feel for the effects on your home.

Note that in very cold winter weather, you might find you need to boost your flow temperature back up again temporarily if you find your heating is struggling to cope. 

If you have a very recently fitted boiler with a modern thermostat that has weather compensation or an enhanced load compensation thermostat, these will adjust your temperatures automatically based on your heating demand and the outdoor temperature. 

Find out exactly what to expect from a boiler service and how much it should cost in our guide to getting the best boiler service.

What are the downsides of reducing flow temperature?

Studies have shown that, when flow temperature is reduced too much for some people's homes, there are adverse effects. A too-low flow temperature can mean your boiler and radiators struggle to bring your home to your desired temperature. 

In particular, there are some factors that make it likely a low flow temperature won't be recommended:

• A home with old and small radiators won't get on well with low flow temperatures. Efficient heating systems generally rely on larger radiators with larger surfaces to radiate plenty of heat from.
• Homes in need of extensive insulation work will struggle to retain enough heat to run an efficient heating system. If your home becomes cold quickly, intense bursts of very high heating is needed to counteract this.
• If you have a regular or a system boiler with a hot water cylinder then you'll need to seek advice first, but a setting of 65°C is generally the minimum choice. These boilers can't run at the efficiency potential of combis. 

In need of insulation? Our insulation advice will help to get you started.

Should I also reduce my hot water temperature?

If you have a combi boiler that runs different radiator and hot water temperatures, reducing the water temperature can cut your bills too, especially if you find your hot taps get scorchingly hot.

A hot water setting of 60°C for combis will suit most people. We've seen even lower settings recommended elsewhere – Octopus Energy recommends 55°C for combis, while Viessmann recommends combi hot water settings between 50°C and 60°C. Again, it's a case of using trial and error to see what suits your home.

Combi boilers carry a very low risk of water contamination because they take water from the mains on-demand and don't store water for any length of time.

But heat loss does occur in between your boiler and water outlets, so the water that comes out of your taps will be a little cooler than your boiler's setting. 

If you have any kind of boiler with a hot water cylinder that stores water for later use, a minimum water temperature of 60°C within the tank is recommended to quickly kill Legionella bacteria. 

Again, we don't recommend changing these settings for regular and system boilers without consulting an engineer.  

Are there other ways to run my central heating more efficiently?

Regulations for new boiler installations introduced in 2018 include a handy list of add-ons that make a central heating system cheaper in the long-run.

Two of them relate to flow temperatures, as new boilers now have automatic thermostats that can adjust radiator water temperature depending on the heating need. 

New boilers have to be installed with at least one of the four recommended efficiency measures below. But current boiler owners can buy and add them, too. All of them will cut heating bills.

• Connecting a smart thermostat and radiator valves to give zonal heating control that saves energy in specific rooms when you're not using them. 
• Enhanced load compensation these thermostats modulate the flow temperature of your boiler to achieve stability rather than relying on a traditional on/off approach which will over and undershoot your desired air temperature. 
• Weather compensation this control monitors the outdoor temperature to determine the flow temperature of your boiler, only using the energy necessary to ensure comfort and stability. 
• A passive flue gas heat recovery device which recycles heat in the waste gases from boiler combustion for water heating. 

See our smart thermostat and smart radiator valve reviews for the best models.

The best boilers for 2023

We've just published the results of our latest annual boiler survey. This year, more than 10,000 people and 198 heating engineers offered an assessment of the boilers they live and work with. 

From this, we've found the Best Buy boiler brands you can rely on, as well as the low-scoring brands with boilers that break more often and which experts rate poorly.

If you're in the market for a gas or oil boiler you can rely on, or you want to see how good the brand you invested in is, then read our reviews.

See the Best boiler brands for 2023.