What Temperature Should I Set My Boiler? Heating and Hot Water

The three settings people mix up

Most homes have three different temperatures that can be set, and they do different jobs. Knowing which is which solves most of the confusion.

• The heating dial on the boiler, often marked with a radiator symbol. This sets the flow temperature, meaning how hot the water leaving the boiler for the radiators is.
• The hot water dial on a combi boiler, often marked with a tap symbol. This sets how hot the water coming out of your taps is.
• The room thermostat on the wall. This sets how warm the room gets before the boiler stops heating it.

Why around 60 degrees suits the heating on a condensing boiler

Almost every gas boiler fitted in the last twenty years is a condensing boiler. It gets its extra efficiency by cooling the exhaust gases enough to recover heat from the water vapour in them. That only happens properly when the water coming back from the radiators is cool enough, roughly below the mid fifties. If the boiler sends water out at 80 degrees, the return is too hot to condense much and the boiler runs less efficiently than it could.

Running at around 60 degrees keeps the return water cooler, so the boiler condenses more of the time. The radiators feel warm rather than scorching, and the room takes a little longer to warm up. Many households notice little difference in comfort, and the boiler also cycles on and off less often.

When to turn the heating dial higher

A lower flow temperature is not right for every house. Turn it up in steps of five degrees if any of these apply, and turn it back down in milder weather.

• The house does not reach the thermostat setting on cold days, even with the heating on for hours.
• Radiators are small for the rooms they heat, which is common in older London houses where rooms have been knocked through or extended.
• The weather is very cold for a spell. Some people run 60 in autumn and spring and 70 in the depths of winter.
• The boiler heats a hot water cylinder as well, which is covered below.

What temperature should radiators be?

With the boiler at around 60 degrees, a radiator will feel hot to the touch at the top but you can keep your hand on it. At 70 to 80 degrees you usually cannot hold your hand on it for long. Radiators are rarely all the same temperature, and a small difference from top to bottom is normal because the water cools as it gives up heat.

What is not normal is a radiator that is cold at the top and warm at the bottom, which means trapped air, or warm at the top and cold at the bottom, which usually means sludge settled in it. A radiator that stays cold while the others are hot often has a stuck valve. None of those are fixed by turning the boiler up.

The hot water dial on a combi boiler

A combi heats water only as it flows to the tap, so there is no stored water and no Legionella concern from a lower setting in the boiler itself. Around 50 degrees is a good starting point. Most people then mix a little cold at the shower or basin anyway. Going lower reduces scale building up in the boiler, which matters with London water, and reduces the risk of scalding. If the hot water is not hot enough in winter, when the mains water arrives much colder, turn it up a few degrees.

If the combi gives no hot water at all, or the water runs hot and then cold, the setting is not the issue. Our guide on no hot water from a combi boiler covers the usual faults.

If your boiler heats a hot water cylinder

System boilers and older regular boilers with a cylinder work differently. The boiler heats the cylinder through a coil, and a thermostat on the cylinder stops the heating once the stored water reaches its setting, normally around 60 degrees. That figure matters for hygiene. Our answer to what temperature should a hot water cylinder be set at explains why it should not go much lower.

Heat only flows from hotter to cooler, so the boiler must be running well above 60 degrees for the cylinder to get there. If the boiler is turned down to 55 or 60 to save gas, the cylinder may take hours to heat, or never reach its thermostat setting at all, and the boiler keeps running trying to get it there. On these systems the hot water side is usually best left with the boiler at 65 to 70 degrees or more. On many regular and system boilers the one dial controls both heating and cylinder, so this also limits how far you can lower the heating. If your hot water has become lukewarm since someone turned the boiler down, that is very likely the cause, and our guide on water not getting hot covers the other possibilities.

What to set the room thermostat to

Between 18 and 21 degrees suits most people in the rooms they use. At least 18 is the figure usually given for health, especially for anyone older or unwell. Each degree lower on the thermostat reduces how long the boiler has to run, which is where the real saving in a heating bill comes from, far more than moving the boiler dial.

Set it and leave it. Turning a thermostat up to 30 does not warm the house faster. It just means the boiler keeps running long after the room reached the temperature you actually wanted. If you want the heating on at different times, change the timer rather than the thermostat, and our guide on how to set a heating timer explains how.

When the setting is not the problem

If you have set the dials sensibly and the house still will not warm up, or the hot water will not get hot, something in the system is not working properly: a pump, a valve, a thermostat, a blocked heat exchanger or sludge in the radiators. Turning the boiler to maximum hides these problems for a while and makes the running cost worse.

Call us on 020 4652 5111 or send an enquiry and we will find out what is actually wrong. We have 20 engineers across London, including G3 qualified engineers for unvented hot water cylinders. Our rate is on the pricing page and agreed before we attend.