Condensing boiler and flow temperature
I first heard the term ‘flow temperature’ on the Beta Talk podcast. It was mentioned in passing in a conversation between heating engineers, in which the host Nathan Gambling casually mentioned how condensing boilers that are supposedly efficient (reason why it was mandated in 2005) was only efficient when the flow temperature was low enough for condensing to occur, and that engineers installing boilers were still regularly setting the flow temperature too high. This was news to me.
Intrigued, I searched for more information and found this piece.
Save 9% in gas use, by turning down the ‘flow’ temperature
https://www.theheatinghub.co.uk/articles/turn-down-the-boiler-flow-temperature
(18/10/2021, by Jo Alsop, The Heating Hub)
Flow temperature is NOT the temperature you set on the room thermostat (typically around 20 °C); it is the temperature the boiler heats the water up to. The water is then pumped through the central heating plumbing to the radiators before circulating back to the boiler. The article is very detailed, but the information I was looking for was this:
“Only when the flow temperature is 60°C or lower will the boiler recover enough heat to reach its higher efficiency potential.”
But towards the end of the article, it mentions another figure:
“If you can run your heating system at 45°C then your system will work with a heat pump without modification.”
Heat pump challenge project
When you are thinking about getting a heat pump installed to decarbonise your home, one thought that inevitably comes to your mind is, “Is my house suitable for a heat pump?” It’s an expensive project and you hear a lot about the need to have good insulation, so this sounded like a good way to test. I gradually lowered the flow temperature setting over the course of a few months, aiming for the 45°C target.
While doing this, I came across a citizen science project called the “heat pump challenge”.
The idea is similar to what I had been doing – reducing the flow temperature in stages, initially to 60°C and then down to 50°C. Although 50°C is not as low as the temperature at which a heat pump typically run, it’s low enough to give you an idea of what a low-temperature heating system feels like and if your home is insulated enough to consider a heat pump.
So how did it go?
So how did my version of the heat pump challenge go? I managed to get down to 45°C and even beyond. When I got to 42°C, some of the rooms felt cold on chilly days. But not all the rooms. This gave me an idea of which radiators needed to be upsized for a heat pump, so that was useful. I don’t know how much money I saved through my experiment, but later the government started an information campaign encouraging consumers to turn down the flow temperature to 60°C to cut the sky-high energy bill.
Another thing I discovered through my experiment was that, strangely, the house felt generally warmer when the flow temperature was reduced. This is counterintuitive, but from what I learned from Beta Talk, this was probably to do with the boiler staying on longer. When the flow temperature is high, rooms heat up very quickly, and when the temperature reaches the thermostat setting, the boiler turns off to let the rooms cool. And when you are sitting and not moving, you feel this cooling effect. This happens repeatedly, and you are hot one minute and cold the next all the time. When the flow temperature is very low (<50°C), radiators don’t get very hot, and rooms don’t overheat so much so the boiler stays on. You don’t get hot, and you don’t get cold. You feel warm, and it feels more comfortable. At least that’s what I think is happening – do try and let me know what you think.