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

Turning down the thermostat is the obvious choice for cutting your heating bill, 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 some of us haven't used yet. Changing this one setting can help cut your heating bills.

Below, our experts explain whether this is suitable for your central heating system.

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What is boiler flow temperature?

Most homes are heated by wet central heating. 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.

Combi boilers also let you select how hot the water supplied to your taps is. As a result, 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 typically around 60°C, although this can vary between systems and boiler set ups.

See also: What are the different types of boiler?

Does lowering boiler flow temperature save money?

It can, yes. There are various studies available which show how much can be saved over the course of a year. A previous study published by the Heating and Hot Water Industry Council (HHIC) in its Heating Up to Net Zero white paper showed a saving of 6-8% on gas bills, other reports estimate you can save more by turning off pre-heat (which keeps water hot ready for faster delivery at the tap) on some Worcester, Vaillant or Ideal combi boilers. 

Innovation agency Nesta suggests that, for a typical household, turning down the flow temperature on your combi boiler from 75-80°C to 60°C or below can knock £57 off your heating bill each year. This also allow the boiler to recover and reuse heat that would otherwise be wasted through the flue.

Actual savings will vary depending on your home. Note that reducing flow temperature means your radiators won't be as hot to the touch and it'll take a bit 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.

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.

If you're unsure what setting is suitable for your home, one of the best ways to get the right settings is to book 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 boilers 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 needs to be kept hot enough to reduce the risk of Legionella bacteria developing in stored water. 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 mains supply on demand instead. If you have a combi boiler, contamination is much less likely.

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?

At a glance: on a combi boiler, flow temperature for heating is usually shown by a radiator symbol and, for hot water, a tap symbol. Use the up and down arrows to adjust the temperature.

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

However, if you have a combi boiler, you've checked your boiler's technical manual, and you can safely access the user controls, you can adjust these settings yourself. This setting is accessible to anyone and it can be changed using your boiler controls.

On the Nesta website, you'll find a interactive Money Saving Boiler Challenge tool which explains the process of changing flow temperature settings for your heating, as well as additional ways to save around the house.

What flow temperature should I use?

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.

The temperature you use will depend on your boiler and your set up.

For example, Vaillant suggests a flow temperature between 60°C and 70°C for a modern combi boiler. Viessmann suggests modern combi boiler owners use a 75°C radiator temperature and between 50°C and 60°C for water. Octopus Energy has previously recommended combi owners set a lower heating flow temperature of 55°C to 60°C 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.

A combi heating setting of 65°C can be a good place to start. Once you know how to do it, you can adjust this down lower towards 60°C or below, as you get a feel for the effects on your home.

Our expert guide on getting the best boiler service reveals what to expect from a service and how much it should cost.

What are the downsides of reducing flow temperature?

A too-low flow temperature can mean your boiler and radiators struggle to bring your home to your desired temperature.

• A home with older, smaller radiators may not perform well with low flow temperatures. Efficient heating systems generally rely on larger radiators with greater surface areas to emit enough heat at lower temperatures.
• 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 are 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 reduce my hot water temperature?

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.

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 for safety reasons, to control Legionella bacteria.

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

Efficient ways to heat your home

Regulations introduced under Boiler Plus in 2018 require new combi boiler installations in England to include certain energy-saving measures that make a central heating system cheaper in the long run.

Two of them relate to flow temperatures, as newer 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 – gives 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 – 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 – recycles heat in the waste gases from boiler combustion for water heating.

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