Flow temperature and efficiency – ‘It’s a kind of magic’
This winter may be the ‘new normal’ as the climate warms; it certainly highlights how quickly our weather patterns, and particularly the outside temperature, fluctuate. Last month there was a 20 degrees shift in just 12 hours, which presents a challenge for our heating systems.
How much heat does a room or house need? Firstly it will depend on the temperature which you hope to achieve. Secondly it depends on how fast heat is lost to the outside world; as it gets colder and windier outside more heat is required to replace the increased heat being lost.
If your rooms are heating up too slowly or never reaching the desired temperature, the following may help:
- More insulation in the house will reduce heat loss (roof, walls, floor, and windows and doors)
- Reducing draughts – sealing gaps so less cold air gets in and warm air escapes less quickly
- Check the radiators are working well:
- Bleed the radiators if necessary so that the whole surface gets hot
- The pump or pipes might be partially blocked and reducing circulation through the radiators:
- A plumber could advise on this – it is possible to flush a system
- The flow temperature may be too low to compensate for the heat losses in colder conditions:
- Adjust the flow temperature
The following advice about flow temperature is relevant for households with gas, oil, electric or biomass boilers, but not heat pumps:
What is flow temperature? The flow temperature is the temperature of the fluid leaving the boiler. Obviously hotter fluid arriving in a room will release heat more quickly from the radiator.
If the flow temperature is cooler than necessary:
- Rooms will warm up slowly and may not even reach the desired temperature
- The pump and boiler will run continuously
If the flow temperature is hotter than necessary: - More heat is lost from the pipes
- Heating the fluid 1 degree extra costs disproportionally more as the temperature rises
- The pump might cycle on and off frequently which is less efficient and increases wear and tear
- The condensing function will not reclaim as much energy, reducing its 92% efficiency to 10-25%
What is most cost effective? Both too low and too high flow temperatures are less efficient than the sweet spot between. In simple terms – as high as is needed but no hotter.
Can’t I do the same with the room thermostat or the radiator thermostats? You can control the temperature which you are aiming to achieve in a room with the thermostats, and of course turning the thermostat down will save money. Turning the thermostat up won’t change how fast a room heats up – just the temperature it is trying to reach. Flow temperature affects the speed a room heats up and boiler efficiency, room and radiator thermostats affect the target temperature that you are hoping to achieve.
How to adjust the flow temperature – there is commonly a dial with numbers or a scale – try to avoid this being at maximum. Some boilers have an eco setting but turn it up if the house is not warming up. Some systems show the actual temperature and even a second lower figure for the temperature of fluid returning to the boiler.
Return temperature and efficiency – the return temperature has a significant effect on efficiency, particularly with condensing boilers. Condensing boilers capture more of the energy in the escaping ‘exhaust’ gases from burning gas. The heat is exchanged into the returning flow from the radiators, but if the return temperature is above 60 degrees the condensing function is less effective or may not even function. So, if you can see the return flow temperature aim to keep it below 60 degrees; turning down the flow temperature will cause the return flow temperature to drop.
What about my hot water? For combi boilers, a similar thing is also true for the temperature of your hot water. In colder weather it may not be hot enough by the time it reaches your taps, whilst in the summer it may be scalding hot. Adjust the flow temperature – many boilers have separate controls for the heating and hot water flow temperatures. If you have a hot water cylinder the temperature is controlled in a different way.
How often should I adjust the boiler? It used to be beneficial to adjust these settings as the season changed, but with the wild fluctuations in temperature we have seen this winter it makes sense to expect to adjust the flow temperature whenever the outside temperature makes a big shift.
This article has some additional information and is clear and easy to read. https://energyguide.org.uk/boiler-flow-temperature/
What about storage heaters? Modern storage heaters automatically take account of the outside temperature when it charges,and will occasionally struggle if the temperature drops very quickly. If you have older storage heaters where the input level is set manually, keeping a good eye on the forecast will help you set the input (the amount of heat stored). There is grant funding available to almost everyone who has older storage heaters for their replacement. Please contact us to find out more.