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

Finding simple solutions to cutting your heating bills can be tricky at times. 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 will slash your heating bills straight away.

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 probably 60°C.

See also: What are the different types of boiler?

Does lowering boiler 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. 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.

Actual savings 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.

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.

The best way to get the right settings for you 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 hot to lower the risk of Legionella bacteria. 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. 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 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.

On the Nesta website, you'll find an interactive boiler temperature tool for combi boiler owners – it explains the process of changing flow temperature settings for your heating.

What flow temperature should I use?

We see different recommendations all the time.

For example, Vaillant and Viessman both suggest combi boiler owners have a 75°C flow temperature. But Octopus Energy has previously recommended combi owners set a much lower heating flow temperature of 50°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 as you get a feel for the effects on your home.

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.

Our expert guide on getting the best boiler service reveals what to expect 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 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 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 to quickly kill 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 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 – 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.

Our research reveals the best boilers

The results of our annual boiler survey are backed by over 10,000 people and 198 heating engineers who 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, plus 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, consult our reviews.

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