Why are Combi boilers more efficient at lower flow temperatures?


My boiler was set to heat my radiators to 75 degrees. When I had my annual check done, the engineer said it should be set to 60 degrees because that is a more efficient temperature.

What I don’t understand is why is that more efficient in the long run. Yes, the water is heated to a lower temp saving fuel, but now I have to run the heating for longer to get the house warm. Does having the heating on for longer not just cancel about any benefit from lowering the flow temp?

In: 8

Maybe think of it as smoking or cooking meat… low and slow is a thing because you’re not jacking things up to scorch temps here. I have forced hot air in my home, but grew up in a house with a boiler with radiators. What I hated was that they operated in one fashion – OFF or ON and cooking everyone to death. Eventually my parents realized that lower temps kept the house at a more stable temp because it evened out the temp vs having a hotter temp that would take longer to shut off.

Granted there are many more smarter HVAC folks that will say their peace with things – it’s just a matter of what I know I dealt with in reality living in that same situation.

Boilers always waste some heat through the exhaust, because for example if the water temperature coming in is 100F, your exhaust temperature cannot be lower than 100F.

If you run your boiler at a lower temperature and the water coming in is 80F, you can cool your exhaust to a lower temperature, meaning you transfer more heat to the water. Your system is more efficient. That’s why it’s better to use a lower temperature for longer than a high temperature for a short amount of time.

This is even more true for modern boilers that try to extract the last drops of heat from the exhaust by condensing the water vapor it contains: there is a lot of energy you can extract this way (because water vapor contains a lot of energy, that’s why it takes ages to boil water away on a stove), so if your incoming water is too hot you’re wasting even more energy.

It’s because natural gas (like most fuels) is a ~~carbohydrate~~ hydrocarbon, carbon & hydrogen, CH4 in this case. Burn CH4 and you get CO2 & H2O, water vapour.

There is a lot of energy involved in turning liquid water into water vapour (or vice versa), the latent heat of vaporisation; it’s a lot*. If the boiler return temperature is kept below 46 degC, that water vapour condenses and the latent heat of vaporisation is released and warms up the flue gases. That heat can then be recovered by the boiler. You get more of the energy going into the water in the boiler’s heat exchanger.

* You probably can’t get a mental image of how much energy is involved in turning 1 litre of water into vapour or back again. It takes more energy to evaporate a kilogram of water than it does to melt a kilogram of iron. That’s a lot of energy.

The modern “condensing boiler” is more efficient when it can use not just the heat from the combustion of natural gas, but also a condensing process where vapor is converted back into water droplets, which surround the heat exchanger, which in turn extracts the heat and converts it into *additional* heating energy.

When the flow temperature is high, 80C or above, this condensing process doesn’t work so this extra energy that could be harvested is unused and lost. But with a lower flow temperature the boiler can harvest this extra energy.

So running your boiler at a lower temperature will run radiators slightly cooler but for delivering a certain total amount of heat energy into the room, the lower flow temperature will require burning less gas, even though it takes a little longer.

c.f. “If I drive my car flat out in top gear I’ll get there quicker, so how come you’re telling I’ll use less fuel if I drive slower… surely that will take longer so I’ll use more fuel”

How do I adjust the temperature on a combi boiler?