Makes we repair
What temperature to set a combi boiler to
A combi boiler's radiator setting can usually come down to about 60°C. Combis work best heating the radiators' water to 60°C or below. Many boilers run hotter than a home needs, often still at the maximum they were set to when fitted.
Flow temperature is how hot the boiler makes that water before it goes round the radiators. It comes back cooler, at the return temperature, and a lower flow temperature brings the return down with it.
Keep it higher for anyone vulnerable to the cold. A home that gets too cold can be dangerous for older people and anyone with a health condition. The same goes for anyone in a home that's already underheated. A vulnerable household may need it a little above 60°C.
With a hot-water cylinder, the boiler stays at 65°C or above, so these steps are for a combi. Boilers with a cylinder, and why.
1. Find the radiator setting
The radiator setting is the one to change. A combi has one setting for the radiators and another for the hot water. The radiator one often has a radiator symbol beside it. Changing it leaves the temperature at the taps and the shower as it was. Your boiler's user guide shows which dial or menu it is. On Ideal's Logic Combi ESP1, for one, it's the knob with the radiator icon. Ideal's manual has you set it to its "e" mark or lower, for the best efficiency.
2. Note the setting as it is
A photo or a note of the settings means they can go back as they were. The change is quick, free and easy to undo. Get the room thermostat, the programmer and the radiator valves set well before turning to the boiler.
3. Turn it down to about 60°C
About 60°C is a sensible starting point in winter, and 50 or 55°C may be enough in mild weather. Baxi's guide turns the dial up to its maximum, then slowly down until the display shows 55°C.
4. Turn it up if the rooms stay cold
A home that stays cold at the lower setting needs it turned back up a few degrees at a time. Very cold weather can call for a higher setting for a while.
The settings the advisers and the makers give differ, so your own boiler's user guide is the one to follow. Here is what each says for a combi's radiators:
Nesta
The setting for a combi's radiators
60°C in winter, 55°C from the end of February and 50°C from the end of March, in its trial with households. Its campaign suggests 50°C in October, 55°C in November and 60°C on the coldest days
The Energy Saving Trust
The setting for a combi's radiators
About 60°C, found by trial and error. 50 or 55°C in mild weather, and higher when it's very cold
Baxi
The setting for a combi's radiators
Down until the display shows 55°C, then up a few degrees at a time in very cold weather
Vaillant
The setting for a combi's radiators
60 to 70°C for a modern combi. 50 or 55°C in spring and autumn, and in some homes through the winter
Worcester Bosch, to installers
The setting for a combi's radiators
About 50°C to try, raised for a while in especially cold weather
Ideal, the Logic Combi ESP1's manual
The setting for a combi's radiators
The "e" mark or lower, and towards MAX in winter if the home needs it
Viessmann
The setting for a combi's radiators
75°C, with the settings left as they are
Viessmann's 75°C is the one that differs. Viessmann also says the water coming back to a condensing boiler must be about 55°C or cooler for it to keep its efficiency. A lower setting helps with that.
Sources: Nesta, the Energy Saving Trust, and the makers' guides and manuals: Baxi, Vaillant, Worcester Bosch, Ideal and Viessmann.
Boilers with a hot-water cylinder
A system or regular boiler that heats a hot-water cylinder stays at 65°C or above, so that it can heat the cylinder to 60°C. Stored hot water should be kept at 60°C or more, against Legionella bacteria. On these boilers one setting does both jobs, the radiators and the cylinder. A setting below 65°C leaves the cylinder too cool to be safe.
Some sources say to leave it alone altogether. Baxi says not to turn the flow temperature down at all where a system or regular boiler heats a cylinder, because of the Legionella risk. Nesta's campaign gives no advice to lower it in a home with a hot water tank. Nesta's evidence report says 65°C is safe with a separate cylinder, and advises asking a heating engineer.
Not sure which kind of boiler you have? Ring us on 020 8064 0067 and we'll ask about it. Combi, system and regular boilers, and how they differ.
Unvented cylinders, and what goes wrong with them.
Sources: the HSE, the Energy Saving Trust, Baxi and Nesta.
Worth knowing
A cylinder at 60°C is hot enough to scald, so take care with the hot taps.
Radiator valve settings
A thermostatic radiator valve, or TRV, holds a room at the level you set. It senses the air around it, and lets more or less hot water into its radiator. It can shut the radiator off when sunshine warms the room. The numbers usually run from 0, off, to 6, fully open. They don't stand for the same temperature on every model, but lower is cooler and higher is warmer.
- Where to start: a middle setting, then down to the lowest that keeps the room comfortable. Try 3 to 4 in the rooms you use most, and 2 to 3 in bedrooms and rooms used less. Viessmann puts 3 at about 20°C, a figure that varies from valve to valve.
- Rooms used less: some background heat helps keep condensation and mould away.
- Radiator covers: best avoided, as they trap warm air around the valve and turn the radiator off too soon.
- The room with the room thermostat: a TRV goes in every heated room but that one. A TRV doesn't switch the boiler off, but the room thermostat does.
- A new boiler: since 15 June 2022, an installer replacing a boiler in England must fit room-by-room temperature controls, TRVs in most homes. Valves that work needn't be replaced.
A radiator that stays cold, and what fixes it.
Sources: the Energy Saving Trust, Viessmann and BEAMA.
Is it cheaper to leave the heating on low all day?
Leaving a boiler's heating on low all day uses more energy and costs more. With a boiler, it's more efficient to have the heating come on when it's needed. A heat pump is the other way round, which is where the idea comes from.
- The programmer: on about half an hour before you get up and off about half an hour before bed, and off while nobody's in.
- Very cold days: bring the heating on a little earlier, rather than turning the thermostat up.
- A setback temperature: a programmable thermostat can hold a lower temperature overnight or while you're out, so the home doesn't get too cold. Keep that minimum higher where someone is older, has a health condition that makes them vulnerable to the cold, or is at home a lot.
Sources: the Energy Saving Trust.
Other ways to reduce heating bills
Heating bills come down further with a few more changes, from the Energy Saving Trust's own advice, besides the flow temperature and the programmer above.
Set the room thermostat lower
The room thermostat saves energy set to the lowest temperature you're comfortable at, 18 to 21°C for most people. Turning it up won't heat the home any faster, and it works best clear of curtains, furniture and radiators.
Let the radiators give out their heat
Radiators give out more heat with the space in front of them clear. Bleed any that need it, and reflector panels behind them help, particularly on walls with no insulation.
Draught-proof the gaps
Gaps around windows and doors let warm air out, and blocking them is draught-proofing, though a home that's well draught-proofed still needs its vents.
Wrap the hot-water cylinder
A hot-water cylinder keeps its heat better in a thick jacket. Choose an 80mm British Standard jacket over a thin, loose 25mm one.
Have the boiler serviced every year
A yearly service keeps the boiler running properly and at its most economical. Turn the heating on before winter, to be sure it works. What a boiler service includes, and booking one.
Smart controls
Smart controls save money or not, depending on how the heating is controlled now. Some change the boiler's flow temperature with the temperature indoors and out. Smart thermostats, and having one fitted.
What boiler efficiency means
A condensing boiler recovers heat from its flue gases. It does that best when the water coming back to it is cool enough. Nesta's testing found below 54°C, and Viessmann gives a dew point of about 55°C. The lower the flow temperature, the cooler the return, and the more efficiently the boiler runs. Vaillant says a combi set above its 60 to 70°C range won't go into condensing mode.
- Is yours a condensing boiler? A plastic flue, or a plastic pipe from the bottom of the boiler to a drain, points to one. A gas boiler fitted after 2005 points to one too.
- The ratings: boilers were graded on the SEDBUK scale until 2009, and on the ErP scale since. A new boiler generally scores A or A+. The Building Regulations set a minimum efficiency for a new gas boiler in an existing home.
- What the research found: in Nesta's tests at the Salford Energy House, each step down in flow temperature made the boiler more efficient. The house took longer to warm, though. It took 37 minutes at a 60°C flow, against 23 minutes at 80°C.
- How to make a boiler more efficient: a lower flow temperature, a service every year, and weather or load compensation. That's a control an engineer fits, which changes the flow temperature by itself as the weather or the home's need changes.
The saving from a lower setting depends on the home, so no figure is given here.
Sources: Nesta's Salford Energy House testing, Viessmann, Vaillant, the Energy Saving Trust and Gas Safe Register.
When a setting won't fix it
A home that feels cooler after the change may need the setting nudged up rather than a repair. Rooms can feel cooler in a draughty or poorly insulated home. Radiators feel cooler to the touch at around 60°C, and the home takes a little longer to warm. Turn it back up a few degrees at a time.
- Radiators sized for hotter water: Worcester Bosch asks installers to check whether a home's radiators were sized for high temperatures. It also asks whether insulation or double glazing has been added since.
- An older boiler: Baxi says not to turn the flow temperature down on an old, standard-efficiency boiler that doesn't condense.
- A fault, not a setting: some signs mean the boiler needs a repair. They're heat or hot water that comes and goes, kettling, a leak, pressure that keeps falling, or a boiler that switches itself off. Boiler repairs, and how the visit goes.
A yearly service keeps the boiler at its most economical. Booking a boiler service.
If the radiators won't warm, the boiler shows a code or it keeps stopping, ring us on 020 8064 0067. We'll ask what it's doing. If you smell gas, leave the boiler alone and open the gold "Smell gas?" bar on this page for the free emergency number.
All our boiler and heating guides.
Sources: Nesta, Baxi, Worcester Bosch, Gas Safe Register and the Energy Saving Trust.
Do you cover my part of London?
Yes: we service and repair boilers all over Greater London, in every borough north and south of the Thames. Pick your side of the river to see the areas.
North of the river
Every borough north of the Thames: Barnet and Enfield at the top, Hillingdon in the west, Havering in the east, and central London in between.
South of the river
Every borough south of the Thames: Richmond and Kingston in the west, Bexley in the east, and down through Croydon to Sutton and Bromley.
Questions about condensing boiler settings
Will a lower flow temperature damage the boiler?
Turning the flow temperature down with the boiler's own dial won't damage it. The dials and buttons on the front of a boiler are there for you to use, Nesta says. Nesta's research found it's high flow temperatures that make a boiler switch on and off more often, which adds to the wear. The exception is an old boiler that doesn't condense, whose setting Baxi says to leave as it is.
Why is my hot water cooler after the change?
Hot water that runs cooler after the change on a combi points to the hot-water setting turned by mistake. The radiator setting doesn't change the taps, and turning the wrong one is one of the slips to watch for. With a cylinder, the one boiler setting heats the cylinder as well. Below 65°C it can't bring the water up to a safe 60°C, so turn it back up to 65°C or more.
Does it work with old radiators?
Old radiators often still heat the home at a lower flow temperature. They give out less heat, though, and the rooms take longer to warm, as Nesta's tests found. Radiators sized generously can heat rooms at lower temperatures, Worcester Bosch says. It depends on their size, then, and on any insulation added since. On very cold days, turn the setting up a little.
Who can set it for me?
You can set it yourself where your boiler's user guide shows how. The dial on the front is one of your own controls, Nesta says. With a cylinder, Nesta advises asking a heating engineer. Weather or load compensation, which sets the flow temperature by itself, is fitted by an engineer. Ring us on 020 8064 0067 about a service, and we'll ask about the boiler. A yearly boiler service, and what the engineer checks.
Don't know what has failed? You don't have to.Ring and say what you can see or hear. Finding the fault is our job.
Calls answered 24 hours a day
A real person answers and gives you the rate on the call. No work starts until you say yes, and repairs are guaranteed for 12 months: parts, labour and any return visit.
