The short answer: What should your boiler flow temperature be in winter? For a combi boiler, 60 to 65 degrees is the sweet spot: hot enough to keep you comfortable, cool enough for a modern condensing boiler to condense properly and run efficiently. Pair that with radiators bled free of trapped air and a system pressure of 1 to 1.5 bar, and you have a free 15-minute winter-ready checklist that trims gas use without touching a single gas setting.

Why most boilers work harder than they need to

When the heating clicks back on in October, most combi boilers pick up exactly where the installer left them. That usually means a flow temperature of 75 to 80 degrees: the temperature of the water your boiler sends out to your radiators. It is the factory setting, it feels reassuringly hot, and for most homes for most of winter it is higher than it needs to be.

The flow temperature is not the same as your room thermostat. The thermostat sets the air temperature you want in the house (the Energy Saving Trust suggests 18 to 21 degrees as the comfortable band), while the flow temperature sets how hot the water leaving the boiler is. Turn the thermostat down by one degree and the Trust estimates you could save around 10 percent on your energy bill. Turn the flow temperature down and something different happens: the boiler itself runs more efficiently.

With the October price cap putting a typical dual-fuel bill at £1,723 a year, two free jobs done in one sitting are the cheapest winter prep there is. Here they are: set the right boiler flow temperature for winter, and bleed any radiators that need it. Fifteen minutes, no engineer, no new equipment. What the October 2026 price cap means and how to cut your bills has the wider picture.

What should your boiler flow temperature be in winter?

A condensing boiler, the standard gas boiler in British homes for two decades, has a trick up its sleeve: it can recover heat from its own exhaust gases. But it can only do that when the water returning to the boiler is cool enough for those gases to condense, roughly below 55 degrees. When the flow temperature is set to 75 or 80 degrees, the returning water is usually too hot, condensation never happens, and the efficiency trick sits idle.

This is why consumer organisation Which? recommends a flow temperature of 60 to 65 degrees for combi boilers: low enough to let the boiler condense, high enough to keep the house warm. Research from the climate charity Nesta found that turning the flow temperature down to 60 degrees improves a boiler’s efficiency by nearly 4 percent, while earlier Nesta analysis suggested moving from 80 to 60 degrees could cut gas bills by around 9 percent. The Energy Saving Trust, which links to the Nesta findings, adds a sensible caveat: how much you actually save depends on what your boiler was set to before, and whether your home was overheated to begin with. The 9 percent figure is a ceiling, not a promise.

One safety rule before you touch anything. If you have a conventional boiler that heats a hot water cylinder, do not turn its thermostat below 65 degrees. The Trust warns that stored water cooler than that raises the risk of legionella bacteria growing in the cylinder. The flow-temperature trick is really for combi boilers, which heat water on demand and store nothing. The Energy Saving Trust’s full flow-temperature guide walks through the risks and the settings for both boiler types.

How to turn your boiler flow temperature down

On most combi boilers the flow temperature is controlled by a dial or button marked with a radiator icon. That is the one you want: the tap icon controls your hot water temperature, and you should leave it alone.

1. Find the radiator icon. It is usually on the boiler’s front panel, sometimes inside a flap. Your boiler manual will show the exact spot for your model.
2. Set it to 60 to 65 degrees. If the dial has no numbers, turn it roughly two thirds of the way down from maximum and check the display.
3. Change nothing else. Leave your room thermostat, programmer and TRVs exactly as they are for now. One change at a time tells you what actually worked.
4. Live with it for 24 hours. Radiators will feel warm rather than scorching, and rooms may take a little longer to heat up. That is normal and part of the point.
5. Adjust in small steps. If a room feels cool, nudge the flow temperature up by 5 degrees. If everything is toasty, try 60 and see how it goes through a cold snap before committing.

You are adjusting water temperature, not gas pressure, so this is safe to do yourself. Never open the boiler casing or touch anything behind it: that is Gas Safe engineer territory.

Job two: bleed the radiators that are cold at the top

Air gets into central heating systems over time, and because air rises, it collects at the top of radiators. The tell-tale sign is unmistakable: run the heating, then feel each radiator. If the top is cold while the bottom is warm, trapped air is blocking the hot water. You may also hear gurgling, or notice a room taking much longer to warm up than it used to. Worcester Bosch lists regular bleeding among its top energy saving tips, and engineers at Vaillant were advising households to bleed radiators at the start of October 2026, saying the process takes about five minutes per radiator.

You need one tool: a radiator key, which costs a pound or two from any hardware shop. Some modern valves accept a flat-head screwdriver instead.

1. Switch the heating off and wait. Let the radiators cool so you cannot be scalded, and so the pump is not pushing air around the system. Vaillant’s engineers suggest waiting about an hour after switch-off.
2. Find the bleed valve. It sits at one top corner of the radiator, a small square pin inside a round nut.
3. Hold a cloth underneath. Water will escape once the air is out, so have an old towel or jug ready.
4. Turn a quarter turn anticlockwise. Fit the key and rotate gently until you hear a hiss. That is the trapped air escaping, and it is exactly what you want.
5. Close the valve when water runs steady. As soon as the hiss gives way to a steady dribble of water, turn the key clockwise to close it. Do not overtighten.
6. Work through the house. Start with the radiator furthest from the boiler and finish with the closest, then check the boiler pressure.

Bleeding lets water and a little air out of the system, which drops the pressure. Look at the gauge on the boiler front: with the heating cold, you want between 1 and 1.5 bar. If it has fallen below that, top it up with the filling loop, usually a flexible hose under the boiler, opening the valve slowly until the needle reaches about 1.5, then closing it again.

The 15-minute winter-ready checklist

Do the jobs in this order on a mild October day, not on the first frosty night when every engineer in the county is booked solid.

1. Fire up the heating for 10 minutes. Feel every radiator, top to bottom. Mark the ones that are cold at the top.
2. Bleed the marked radiators. Heating off, key in, quarter turn, close at the first steady water.
3. Check the pressure. Top up to 1 to 1.5 bar cold if bleeding dropped it.
4. Set the flow temperature to 60 to 65 degrees. Radiator icon dial, combi boilers only.
5. Open every TRV. Thermostatic radiator valves should turn freely. A stuck one feels jammed and will not click through its range.
6. Check again tomorrow. Run the heating and confirm every room reaches temperature. Adjust the flow temperature by 5 degrees if needed.

When bleeding will not fix it

Not every cold radiator is an air problem. If a radiator is cold at the bottom but warm at the top, that points to sludge: a black magnetite debris that builds up from dissolved metal in the system and restricts flow. Bleeding will not shift it, and the fix is a professional system clean.

A radiator that is cold all over may have a stuck thermostatic valve pin, or the system may need balancing, where an engineer adjusts the lockshield valves so each radiator gets its fair share of flow. And if your boiler itself is elderly, inefficient and costing you every winter, two free jobs can only do so much: the Boiler Upgrade Scheme offers £9,000 towards a heat pump for off-gas-grid homes until March 2027, which is worth knowing before you spend money on repairs to a dying boiler. Fabric matters too: free insulation grants are still open in 2026 for qualifying households.

Three things to leave alone

Do not drop a conventional boiler below 65 degrees. As above, stored hot water must stay hot enough to suppress legionella. If you are unsure which type of boiler you have, the combi has no hot water cylinder; if you have a cylinder in an airing cupboard, leave the boiler thermostat where it is.

Do not seal boiler ventilation. Airbricks, flues and the ventilation serving boilers, fires and flues must never be blocked while you chase efficiency elsewhere.

Do not underheat the house. The Trust’s 18 to 21 degree band exists for a reason: cold homes are bad for health and encourage damp. Older residents, people with health conditions and anyone in an already underheated home should be cautious about turning anything down.

And the one job that is never DIY: the annual boiler service. Only a Gas Safe registered engineer should open the casing, and autumn is the time to book, before the first cold snap fills every diary.

Your questions, answered

What should my boiler flow temperature be in winter?

What should your boiler flow temperature be in winter? For a combi boiler, 60 to 65 degrees is the widely recommended winter setting: hot enough to heat your home while letting a condensing boiler condense properly. If you have a conventional boiler with a hot water cylinder, keep the boiler thermostat at 65 degrees or above to avoid legionella risk.

Will lowering the flow temperature make my house colder?

It should not, but rooms may warm up a little more slowly and radiators will feel warm rather than scorching. The Energy Saving Trust notes that savings depend on your starting point: if your home was overheated before, you will notice the difference in the bill, not the temperature. If a room genuinely feels cold, raise the flow temperature by 5 degrees at a time.

How do I know if my radiators need bleeding?

Run the heating and feel each radiator. Cold at the top with warmth at the bottom means trapped air: bleed it. Gurgling noises and rooms that are slow to warm are supporting clues.

Can I bleed radiators myself, or do I need a plumber?

Most people can do it themselves with a radiator key that costs a pound or two. The process takes about five minutes per radiator. Call a professional if water does not appear after the air escapes, if the same radiator keeps needing bleeding, or if you suspect sludge.

Should I bleed my radiators with the heating on?

No. Switch the heating off first and let the radiators cool, ideally for about an hour, so you avoid scalding and stop the pump pushing air around the system. Check the boiler pressure afterwards, since bleeding drops it.

How often should I bleed my radiators?

Check them each autumn before the heating season peaks, and bleed any that are cold at the top. If you find yourself bleeding the same radiators repeatedly, or the boiler pressure keeps dropping, ask a heating engineer to look for a leak or a system fault.

Sources

• Energy Saving Trust: Should I turn my boiler’s flow temperature down? energysavingtrust.org.uk
• Which?: Top tips to reduce your heating bills which.co.uk
• Worcester Bosch: Energy saving tips worcester-bosch.co.uk
• Vaillant engineers via Brit Brief, 4 October 2026: Bleed radiators now to cut heating bills britbrief.co.uk