Skip to main content

Save up to 8% on your gas bills, by turning down the 'flow' temperature


Jo Alsop

Heating Hero

The Heating Hub

Turning down the 'flow' temperature of your heating water can save around 8% on fuel bills, Jo explains why and how to do it


A recent report by the Heating and Hot Water Council (HHIC)* found that households can save around 6-8% on their gas bill just by turning down the flow temperature on their condensing combi boiler. To be clear, this is not turning down your thermostat.

In this guide we help you understand flow temperatures, whether you have the right sort of boiler to reduce the flow temperature and how to do it.

1) What does 'turning down the flow temperature' mean?

Most gas boilers are set up to operate at what is called 80/60 flow and return temperatures. This means the boiler heats up the water it sends around your radiators (called ‘the flow’) to 80°C. The water returns to the boiler after travelling around all your radiators (called ‘the return’) at 60°C, having given off 20°C to the room.

However 80/60 flow and return temperatures are too high for a condensing gas boiler to achieve the 92% efficiencies shown 'on-the-box'. Condensing boilers only have the potential to reach these higher efficiencies; in other words they are not A-rated out of the box.

The installer must reduce the flow temperature to give the boiler a chance to run in what is known as 'condensing mode' as much as possible. This is when it can recover the heat that was previously lost through the flue on old style boilers.

At 70/50°C the boiler will start to operate in condensing mode. Only when the flow and return temperatures are 65/45°C or lower will the boiler recover enough heat to reach its efficiency potential.

2) What type of boiler do I need?

You will need to have a condensing combi boiler, i.e. a boiler that produces your heating and hot water. You will have a combi boiler if you do not have a hot water cylinder. It will definitely be a condensing boiler if it was fitted in the last 16 years. A condensing boiler will also have a white pipe that runs to an inside or outside drain. If you have a boiler the works with a hot water cylinder, see section 11 below for more and note 'warning' under section 3.

3) How do I reduce the flow temperature?

If you have a condensing combi boiler you can reduce the flow temperature on the front of your boiler. Most combi boilers have two dials: one for heating (usually with a little radiator icon over it) and one for hot water (with a little tap icon).

This is not your room thermostat, this sets the flow temperature for the boiler. NB some boilers have a digital display next to the dial, some have an up and down button, some just have vague markings so its not actually possible to know what temperature you have lowered it to. Turn this dial down and see how you get on.

SAFETY WARNING: If you have a hot water cylinder we do not recommend attempting to reduce the flow temperature yourself. Your heating and hot water temperatures are tied together and reducing the flow temperature for heating can risk legionella in your hot water cylinder. See section 11 for more on boilers that work with a hot water cylinder and help you can get.

SAD FACT: if you have a hot water cylinder set up with a common heating and hot water temperature it will almost NEVER condense! The boiler needs to be set up with separate heating and hot water temperatures and the scope for doing so depends entirely on the boiler.

4) How low can I go?

How low you can turn your flow temperature down will depend on the size of your radiators and how much energy you use to heat your home.

Lower flow temperatures need more surface area of radiator to meet your desired room temperature; hence why we need much larger radiators for an air source heat pump. The good news is that most radiators are bigger than they need to be as the industry has sized based on guesswork or used basic online calculators that almost always overcompensate.

For homes with modern double glazing, cavity walls and good levels of loft insulation you should be able to drop your flow temperature down with no impact on comfort. Even older homes, particularly terraced houses, will be able to do this if they have been modernised and draft proofed.

5) Can I use trial and error?

Yes you can. Without an accurate heat loss calculation and radiator sizing exercise, it will be a matter of turning down the flow temperature and testing it to see if your house stays warm. You can turn it down little by little, or turn it down a lot and notch it back up if it doesn’t seem warm enough.

6) Will I notice any changes in my heating system?

Yes. Your radiators will not feel so hot. This is a sign that the system is running a little cooler, which is what you are trying to achieve. Providing your home does not feel cold and you do not feel cold, the boiler and system are working correctly and more efficiently.

If you have your heating programmed to come on for a very limited amount of time, you may notice that it will take longer for your home to heat up. Many households are accustomed to short heat up times because they run so hot. Lower flow temperatures will mean slightly longer heat up times.

For those that have the heating on much of the day, you will move to lower temperatures for longer periods which will have a positive impact on fuel bills.

7) What if I have a health condition?

We would caveat the above advice for those with health conditions. Staying warm is very important and we would not wish you to risk your well being by adjusting your heating system on a trial and error basis.

8) Where can I get more help with this?

For more information on your boiler you can look in the instruction manual. You can usually find your manual on-line if you search for the make and model of your boiler. To be totally sure you have the right manual, find the boiler's 'GC' number. Every boiler has one and its usually on the boiler's data-plate. It will start with GC followed by a 7 digit number.

Alternatively you can ring the manufacturer's technical helpline for customers. They will need the model or preferably the GC or serial number to help you with your particular boiler.

You can also ask your installer when you next have the boiler serviced. NB not all installers are familiar with condensing boiler technology, but should be able to assist with this function. A training video is due to be released to help installers in these concepts and we will post the link here when it's available.

9) Why have I not been advised about this before?

A vast skills and knowledge gap (that dates back to 2005 when condensing gas boilers become mandatory) means 99% of installers do not understand how condensing gas boilers work and therefore cannot set them up to run as they were designed to. Installers have been let down and so have UK households. We have simply not benefited as much as we should have from this leap forward in boiler efficiency.

Part of our work is to educate households on their heating systems and help them get higher efficiencies from their existing boilers. The over-replacement of boilers is a problem in the UK. We want to see more existing boilers running optimally with small changes that reduce energy bills and our carbon footprint. A 15% efficiency improvement across all UK households is enough to heat every home in Wales for a year.

Finding skilled installers is also a struggle. We have spent a lot of time building a highly competent installer network**.

10) Is there anything else I can do to improve efficiency?

Yes lots, but it depends on your home, your boiler and your existing heating system. Every house in a terrace is different and every heating system in that terrace is different. Apart from 'lower the flow temperature' (and only if you have a condensing combi boiler), each home needs a bespoke package of measures. To be clear, this is rarely to change the boiler. Rather to make the existing boiler and heating system more efficient with small cost-effective upgrades.

11) I have a hot water cylinder, what do I do?

This approach does not suggest you throw out your hot water cylinder in favour of a combi. As we transition to low carbon heating solutions we will need our hot water cylinders once again. Rather the boiler needs to be set up correctly with the cylinder to get the best efficiencies for heating.

If you have a hot water cylinder, you will have either a 'system' boiler or a 'conventional' boiler (aka heat-only or regular boiler) with a vented or unvented hot water cylinder. Nearly all are set up on what are known as S or Y plans. (This is how the boiler works with the radiators and hot water cylinder.) In both S and Y plan set ups, the heating temperature is tied to the hot water temperature and therefore cannot drop below 70°C.  This means that boiler almost never condenses, but reducing the flow temperature risks legionella in the hot water cylinder.

Most system boilers and a few heat-only boilers can be set up differently to separate the heating and hot water temperatures (this will work with vented and unvented hot water cylinders). If you would like to know how to run your heating system more efficiently we can offer bespoke advice as it will depend on the boiler.***.

12) If I can run my heating system low, does that mean I ready for an air source heat pump?

If you can run your heating system at 45°C then your system will work with a heat pump without modification. If you have underfloor heating in large parts of your home; a home built in the last 10 years; or an older home that has been heavily insulated there is a good chance you can run your heating system low enough.

WARNING: many new build homes use 'microbore' pipework (instead of standard 15mm pipework you will have 10mm or even 8mm pipework) and it maybe too small to cope with lower flow temperatures. There are exceptions when it can work however.


Jo & Caroline - Heating Heroes

1-2-1 advice with our Efficiency Audit

Get the whole package of efficiency upgrades with our bespoke advisory service

More reads...

Guide to boiler efficiencies

99% of gas boilers are running below their A-rated efficiency, we explain how.

Learn more

The real cost of boiler inefficiency

Soaring gas prices expose the real cost of poorly performing boilers.

Learn more