“Communication was excellent from my 1st contact with Moe to the completion of the work. This included follow-up messages…”
Christine M. · Glasgow
Google review
Published by Mo's Plumbing and Heating
What Temperature Should Radiators Be on the Boiler?
There are usually two dials on a boiler and most people are not sure which does what. The one for the radiators is the flow temperature, and the number most boilers leave the factory on is higher than most homes need. Turning it down is the single cheapest change available on a heating system, and it costs less comfort than people expect.
The Short Answer
For radiators, set the flow temperature to somewhere between 55 and 65 degrees rather than the 70 to 80 many boilers arrive on. The house still reaches the same temperature, it just takes a little longer to get there, and the boiler runs far more efficiently at the lower setting. Leave the hot water dial, if you have a separate one, at 60 or above. Turn the flow temperature down in stages of five degrees over a few weeks rather than all at once, and stop where the house stops keeping up.
Which Dial Is Which
The confusion is reasonable, because the two are usually unlabelled beyond a symbol and they do completely different jobs.
The flow temperature, sometimes shown with a radiator symbol, sets how hot the water going out to your radiators is. This is the one this page is about and the one worth changing.
The hot water temperature, usually a tap symbol, sets how hot the water at your taps is on a combi. On a system or heat only boiler with a cylinder, the equivalent control is the cylinder thermostat rather than a dial on the boiler.
Neither is the room temperature. That is the thermostat on the wall, and it decides when the heating runs at all. The flow temperature decides how hot the water is while it runs. Turning the flow temperature down does not lower the temperature of the house; it lowers the temperature of the water that heats it.
That distinction is the whole point and it is the bit that is worth being sure of before touching anything. A house set to 20 degrees on the wall thermostat still reaches 20 degrees with a lower flow temperature. It takes longer.
The Number to Aim For
Between 55 and 65 degrees for the radiators on most homes, and lower within that range is generally better if the house can hold up.
Boilers commonly leave the factory set at 70 to 80, which suits an installer wanting the house warm quickly on a commissioning visit rather than the household living there afterwards. A great many boilers are never touched again.
The reason lower is better is that a condensing boiler only achieves its efficiency when the water coming back from the radiators is cool enough, which needs the water going out to have been cooler in the first place. That mechanism has its own page in the boiler types guide. What matters here is the practical consequence: a boiler running at 75 is doing much less well than the same boiler running at 60.
The cost of the lower setting is speed. Radiators are less scorching to touch and the house warms more gradually. Once up to temperature there is no difference at all in comfort, which is why the change goes unnoticed in most homes and is noticed sharply in a few.
How to Change It Without Getting It Wrong
In stages, and with the heating running normally in between rather than all in one evening.
- Note the current setting before you touch it, so you can put it back if the house does not cope.
- Drop it by five degrees. Leave it a week of normal use, through a cold few days if you can.
- Ask the practical question: does the house still reach the temperature you want it at, in the time you want it? Not does it feel different, but does it get there.
- If yes, drop another five and repeat. If no, go back up five and stop there. That is your number.
- Most homes land somewhere between 55 and 65. A well insulated modern flat may sit comfortably lower. An older property with big rooms and single glazing may need the upper end, and that is a fact about the building rather than a failure to try.
When the House Cannot Cope
If dropping the flow temperature leaves rooms cold, that is information rather than a dead end, and it usually points at one of three things.
The radiators are undersized for the rooms. A radiator sized for 75 degree water is not big enough to deliver the same heat at 55, so the fix is a bigger radiator rather than hotter water. That is useful to know because it is the direction the whole industry is moving, and a radiator upgrade in the room that struggles is often cheaper than people assume.
The system is unbalanced, so some radiators get plenty of flow and others get little. That shows as one or two rooms lagging while the rest are fine, and it is corrected by balancing rather than by turning the boiler up to compensate.
Or the system is sludged and the radiators are not actually giving out what they should. A radiator cold at the bottom while hot at the top is the classic sign, and turning the flow temperature up is masking it. That has its own page in this cluster.
In every one of those cases, running the boiler hotter is a way of not fixing the actual problem while paying more for the privilege.
What About the Hot Water Setting?
Different rules, and this one should not be turned down to save money.
On a combi, the hot water dial sets the temperature at the taps. Setting it very high is a scald risk without giving you more hot water, so somewhere comfortable is right, but this is not the dial with the efficiency saving on it.
On a system or heat only boiler with a stored cylinder, the cylinder needs to reach at least 60 degrees. That is not about comfort, it is because water stored below that temperature for long periods can allow bacteria to develop in the stored volume. A cylinder thermostat set to 60 or a little above is the standard advice and it is worth checking rather than assuming.
So the summary is: turn the radiator flow temperature down, leave the stored hot water at 60 or above, and set the taps on a combi to something that will not scald.
Why Older Glasgow Rooms Fight a Low Flow Temperature
The advice to run at 55 degrees is sound and it lands differently in this city's older housing, which is worth saying rather than pretending the number is universal.
A traditional tenement room is not a modern room. Ceilings are frequently three metres or more, so there is a great deal more air to heat than the floor area suggests. Original sash windows, where they survive, lose heat steadily whatever their condition. Solid sandstone walls have no cavity to insulate and, in a conservation area or a listed building, external insulation is not an option and internal insulation is a significant intervention. Those rooms need more heat put into them than a modern flat of the same floor size.
So the honest expectation here is that a lot of Glasgow properties will land at the upper end of the range rather than the lower, and a proportion will not get below 65 without the house failing to keep up. That is a fact about the fabric rather than a failure of the method, and it does not mean the exercise was pointless: coming down from 80 to 65 is still a substantial improvement over leaving it where the installer set it.
Where a room genuinely will not hold up, the productive answer is usually a larger radiator in that room rather than a hotter boiler for the whole house. A single oversized radiator in the coldest room lets the entire system run cooler, which is a cheaper change than it sounds and a better one than heating every other room harder to compensate for one.
Top floor flats are the common exception in the other direction. With a heated flat below and often better loft insulation than the walls suggest, they frequently manage the lower end of the range comfortably, and are worth trying at 55 before assuming otherwise.
When to Call Someone Out
Changing the flow temperature is a dial and takes seconds, so this is a customer job. Worth calling if lowering it leaves particular rooms cold, because that is a balancing, radiator sizing or sludge question rather than a boiler setting, and turning the temperature back up hides it rather than solving it. Also worth it if you cannot work out which control is which on your boiler, or if there is no obvious flow temperature control at all, which happens on some models where it lives in a menu. Mo, Muhammad or David cover heating work, and a balancing visit is usually a short job with a disproportionate effect on how the whole system feels.
Heating Engineer in GlasgowCommon Questions
Between 55 and 65 degrees for the flow temperature on most homes, rather than the 70 to 80 many boilers arrive set to. The house reaches the same temperature either way; it takes a little longer at the lower setting and the boiler runs considerably more efficiently.
What Locals Say About Mo
250+ five-star reviews across Google, Trustpilot and Facebook, from real Glasgow customers.
“Our radiator pipe was bent and leaking. They answered straight away… Guy was here an hour later and he was wearing ID, very professional, patient and kind. We just got a new carpet in this room and he done an amazing job on getting little to no water on it. Couldn't recommend enough, price was reasonable.”
Kayleigh A. · Glasgow
Google review
“We lost heating just in time for the coldest day of the year with a baby in the house… When I contacted them they were ready to dispatch that night. Fixed by the afternoon. Quoted that morning and no extra charges were added. Honestly cannot thank them enough.”
Jemma M. · Glasgow
Google review
“As a small business owner, I needed a drainage company that could respond quickly. The team attended first thing in the morning, cleared the blocked drain and carried out a full CCTV survey. All documentation emailed across promptly. Professional, efficient and great value for money.”
Eric V. · Glasgow
Google review
Need a Plumber? Mo's Team Is On It.
07988 59133125 years, 10,000+ jobs, £5m insured, call or WhatsApp, day or night. Clear price, tidy job, guaranteed.