Another one for heating engineers. Boiler flow temperature

Lots of people saying turning down the flow temperature on the boiler reduces gas usage. Martin Lewis being of of these people. Does it really make a difference?

 

What is flow temperature? Does that just mean 'temperature your boiler heats the water to' or something else?

 

Sounds like arse to me to be honest. It means it actually takes longer to heat the space and so with a modern intelligent boiler then that means that the heating will therefore come on earlier to arrive at the pre-set temperature as the boiler controls "learn" and will be therefore be modulating more frequently (ie stopping and starting) One of our professional heating engineers will be along shortly but my understanding on larger systems is that if the return temperature is too low then it may actually take more energy to maintain the flow temperature and again potentially result in increased energy usage..... but Im only an electrician!

 

it is the temperature of the water leaving the boiler into the heating system.

 

https://www.theheatinghub.co.uk/articles/turn-down-the-boiler-flow-temperature

this is interesting and whilst it is saying that you can reduce temperatures - what it is actually saying is- turn them down if they are too high.

Without actually doing some calculations then you need to monitor what you use and whether you are happy with the conditions achieved. If your house is well insulated and you don't open the windows then that always going to be different to an poorly insulated yet well ventilated house. In some houses then not allowing the temperature to reduce below a certain level might end up with generally colder maximum temps but a better constant and a feeling of actually being comfortable. Whic, if your at home most of the time might be preferable..... Its all rather subjective!

You can save a fortune by turning it off all together though cant you.

 

The higher the flow temperature, the greater the heat loss where you don't want it. Radiators are designed for maximum heat loss whereas the boiler and supply pipes are well insulated to retain heat.

As soon as you begin to heat your house above ambient temperature, you start losing heat through windows, ceilings, draughts etc so the longer it takes to bring your house to the desired temperature, the more heat you lose (and the longer you spend feeling cold). Rapid heating of your house with a high flow temperature will be more economical than slow heating at a lower flow temperature, providing that the boiler losses at the higher temperature don't exceed the house losses at the lower temperature during the warming procedure.

 

So..
Do we have a..

yes or no...

 

For a combi boiler it's worth turning down the hot water, as its not stored water you dont have to worry about legionella.

No point heating the water to add cold water, set it at the temp you like and use it direct with no cold water.

Boiler is the j less as its not.heat in my to a higher temp so will use less gas.

No idea about the central heating side though

Sent from my Pixel 6 Pro using Tapatalk

 

It’s not just about the temp in the rads it also helps to bring efficiency up as a modern boiler condenses better at a lower flow temp I have a external temp sensor on mine that adjust the flow temp based on external temp therefore more efficiency and less gas cost.

 

I read the advice, right or wrong, I've turned mine down. I feel colder but more wealthy.

 

The answer is yes and no. I will give you a proper answer when it’s not 10pm and I haven’t been clambering around in lofts all day.

 

Ok so heres the theory. Traditionally heating engineers have sized radiators not just based on the required heat loss for the room, but also on how they looked. For example if you have a lounge window that is six feet wide, its very unlikely you'd be happy with a tiny little radiator as it would look silly, even though its capable of heating the room. This has led to a lot of radiators actually being oversized for the room. So lets say that lounge has a heat loss of 3KWh, the radiator might be able to give off 4KWh at a flow temp of 75C, so its significantly oversized. By reducing the flow temp to say 55-60C it might then give off closer to 3KWh, the result is that you still heat the room to the same temperature but you use less energy doing it. Especially as modern boilers light at full rate and then modulate their output down depending on load, so the more short cycles you put it through, the more it costs to run, so its better to have them run at a lower temp for longer than short bursts of high heat on off cycles. This is how underfloor heating and heat pumps work. They trickle in just enough heat to compensate for the property heat loss, in turning down your boiler you are attempting to replicate this.

Now also factor in that a lot of systems were installed using standard boilers that have a flow temperature of say 70-75C, they traditionally give fairly high return temps, maybe only dropping 10C between flow and return. Condensing boilers dont condense the whole time they are running, they condense the most when there is a return temp below around 55C. Condensing is the process where heat from the flue gases that would otherwise be lost to the atmosphere, is cooled so much that it tries to turn back to a liquid, giving off that latent heat before it leaves the boiler. This gives a saving of around 25-30% over a std eff boiler. So obviously if you turn the flow temp down, by the time that water has flowed through all the rads it has a very cool return temp which in turn keeps the boiler condensing for longer.

To give you an idea, I have a combi boiler with a nest controller, my flow temperature is currently set to 50C. My house is warm, my lounge is about 20C, the only this that takes a while to get your head around is the slower recovery time, if you are used to coming in and whacking the temperature up and blasting the house up to temperature in thirty mins then youll find this way a bit odd at first. My heating is set to come on at about 4oclock so the house is warm for everyone at six pm, but its important to remember that my boiler isnt running for two solid hours, far from it.

If you have stored hot water that you dont fully turn over regularly then due to the risk of bacteria growth i wouldnt recommend you turn your boiler flow below about 55.

hope this helps.

 

A very comprehensive reply, thank you very much. But does it help turning down the flow temperature?

 

may depend on what sort of boiler you have.... unfortunately the question and answer have a Marmite in Marmite out relationship!

 

Yes


Or


No


Or


Both

 

Combi.

 

I lowered my heater also for the water temperature.
But to avoid legionella growth, I have an automatic weekly increase in temperature at my heater above 55°C for water storage.
Be careful with high-risk users such as small children and the sick - then it is generally better not to go below 55 °C.

 

Yes. Providing that you have radiators fitted that are a bit over sized. If so then set your flow to about 55 for heating.

 

55deg doesn’t kill it it just slows down the rate of growth so your not achieving much doing that

 

i wrote above - it's weekly 75°C