The effect of reducing radiator temperature on room temperature

Heating engineer experts have been complaining for years that condensing boilers were not being installed correctly to achieve the claimed efficiencies. A gas condensing boiler that is supposed to give you 92% could be giving you 85% or even less, if it has been set to heat the radiators at 70°C or hotter. Now that we have an energy crisis, this is one of the few things you can do to make significant savings for no cost and hardly any effort (if you have a combi boiler). Nesta have just launched a campaign on this. However the Heating Hub has been giving similar advice for some time. Some of the savings are from increasing boiler efficiency and some are from indirect effects on the room temperature. There is usually only a small reduction in average temperature but it will vary from one house to another. If you find you are uncomfortable you can tweak the thermostat up a bit to compensate.

Increased efficiency comes from condensing water vapour in the flue gases.

Improved boiler efficiency is the main source of savings. Increased efficiency comes from condensing water vapour in the flue gases and that only happens when the fluid coming from the radiators back into the boiler is fairly cool - below 50°C certainly, and the lower the better. This is called the return temperature. To get that down you need to reduce the temperature of the fluid going into the radiators. This is called the flow temperature and for a combi boiler you should be have a control to set this independently of the temperature to the hot water taps. Normally the flow temperature would be 70°C or 75°C, but it could be as high as 80°C. You get significant savings at 60°C but more at 55°C or even 50°C.

The room temperature is also slightly reduced.

This chart shows the impact of reducing the flow temperature on room temperature. The overall average temperature reduces, which means you have lower heat loss and additional savings. You don't have to reduce your thermostat to achieve this; it will happen automatically. The chart comes from tests done at the Salford Energy house for Nesta by researchers from the University of Salford. The Salford Energy house is a full size house inside an environment controlled chamber. For this test they ran a typical heating pattern: on for a couple of hours in the morning and then longer in the afternoon. The outside temperature was 4.5°C, typical for winter. 

Chart shows living room temperature over a day, with varying flow temperatures. From [1]. The green line for 55°C SB has a set back temperature. Instead of turning off, the target temperature is lowered to 17°C. 

The Salford Energy House tests simulated a middle aged boiler, with pronounced heating cycles.

The thermostat was located in the living room the whole time and the target temperature during 'on' periods was set to 21°C. If you look at the afternoon period you can see heating cycles. The heating system is simulating a Worcester boiler that is 8-12 years old. More modern boilers have features that reduce the cycling effect so while this may be typical your boiler may be very different.

The peak temperature in the cycle reduces but the minimum temperature is steady.

When the flow temperature is 80°C the peak room temperature is about 22°C. At lower flow temperatures, the peaks are lower: with the flow at 60°C the peak is about 21°C. This happens because when the boiler turns off there is still residual heat in the radiators which continue to warm the room. It looks as though the boiler is actually turning off at about 20°C (this being the lowest peak you can see - the bottom line for 50°C). The amount of residual heat in the radiators reduces when the flow temperature is reduced so the peaks get lower and lower. If this is a problem for you, you can increase the thermostat to allow for it. The minimum temperature in the cycle is about 19.5°C regardless of the flow temperature.

The room heats more slowly when the flow temperature is reduced.

You can also see that the room heats more slowly when the flow temperature is reduced. However this effect is minor down to 55°C, at least in this case. It will depend on the size of your radiators relative to heat loss in the room. 

If you have narrow microbore pipes in your radiator circuit it is possible that they are not big enough to allow for the lower temperature flow and the room will heat much too slowly. In this case, I am afraid you will not be able to take advantage of this energy saving trick. You will also have a problem with a low temperature heat pump,  unless you upgrade the pipework.

There are also thermal mass effects, at all flow temperatures.

The heating cycle peaks creep up over the afternoon. This is because the walls and furniture heat up more slowly than the air in the room - an effect of thermal mass. As the walls and furniture warm up, more of the residual heat in the radiators goes to heating the air, so the room achieves slightly higher temperature. This effect is stronger in the first few cycles after the heating comes on, especially with the lower flow temperature.

You can also see the effect of thermal mass in the minimum temperature just before heating comes on. This is highest when the flow temperature is 80°C. (That is, ignoring the case with the setback temperature which is heated all the time). The hotter the radiators the more heat is stored in the thermal mass of the house and this keeps the house warm longer. The size of this effect will vary depending on the ratio of thermal mass to heat loss in your house. However, this is not relevant to your comfort when heating is on. 

Space heat savings in the Salford house were up to 16%, but this will vary depending on radiators, boiler and thermostat capabilities.

In the Salford tests, starting at 80°C (which is higher than average but not unusual) gas savings were 5% at 70°C, 12% at 60°C and 16% at 55°C. However:

• This is for heating only: your gas bill includes hot water too so the overall percentage savings will be less.
• Part of the savings are due to temperature effects which will vary from one house to another, depending on the modulation capabilities of your boiler, the size of your radiators and how your thermostat is configured. 
• Part of the savings may be due to the lower flow temperature reducing heat loss from pipework in the underfloor void - the Salford Energy House has suspended floors. Pipework outside the heated envelope should be well lagged but that is not always the case. 
• The efficiency savings alone, in these tests, seem to be minimal at 70°C but about 5% at 60°C and 7% at 55°C
• The boiler efficiency savings depend on the return temperature rather than the flow temperature. Depending on the capabilities of your boiler, it may vary the pump speed to keep the difference between flow and return temperature the same. If it does, you can expect efficiency savings to be higher than in these tests, which had a fixed pump speed.

A modern boiler with effective modulation should keep the room temperature more constant. So then you would mainly be getting just the efficiency savings - which are still worth having. Even 5% is worth having and you can expect more if the boiler varies the pump speed to keep the return temperature as low as possible.

Other energy saving actions in the Nesta report including loft insulation and window film.

There are other energy saving ideas modelled for Nesta in the full report: Free and low-cost energy-saving actions to bring down bills, improve energy security and help the planet. The spreadsheet has the detail on energy savings. I am one of the authors - I did some of the modelling. There are some savings from reducing the temperature of your hot taps - another zero cost measure. There are good savings from putting window film on single glazed windows - and even more on double glazed windows since most homes are already mostly double glazed and only a few single glazed windows. However the payback time will be longer on double glazed windows. There are large potential savings from loft insulation if you do not have enough already.

[1] Salford Energy House Boiler Flow Temperature Testing (Nesta) Oct 2022

[2]  Free and low-cost energy-saving actions to bring down bills, improve energy security and help the planet (Nesta) Oct 2022