Optimum Boiler Temperature

[warey4life]

I live in an old house (1890ish) with high ceilings, it has fairly good loft insulation and has cavity wall insulation so I was told, but it's not the warmest house in the world. Our boiler temperature was set to 80c, I dropped it down to 70c and the wife didn't notice it, so this morning I've dropped it to 65c. I don't think I'll go lower than this because the boiler heats the HWS cylinder via a heat exchanger.
Interested to know what people have there boiler temperature set to.

 

[denso13]

Interested to know what people have there boiler temperature set to.

As low as is comfortable, while heating the cylinder hot enough. I wouldn't overthink the theory.

 

[Stuart981]

To quickly heat the house from cold I have the flow temp set to it's max of around 85 deg C although the downside of that is that the condensing capability of the boiler becomes ineffective.

 

[MNW67]

How old is the boiler. Is it a condensing boiler.

 

[Mister Banks]

To quickly heat the house from cold I have the flow temp set to it's max of around 85 deg C although the downside of that is that the condensing capability of the boiler becomes ineffective.

That’s not the only downside, it’s also emitting more CO2, costing more in gas, adding extra wear to components and shortening their life.

 

[Stuart981]

That’s not the only downside, it’s also emitting more CO2, costing more in gas, adding extra wear to components and shortening their life.

It costs a bit more because you don't recover the latent heat that goes up the chimney but you benefit from a more responsive system.

 

[warey4life]

How old is the boiler. Is it a condensing boiler.

It's about 10 years old, and it is condensing yes.

 

[MNW67]

It's about 10 years old, and it is condensing yes.

I was just asking because (very) old non-condensing gas boilers (like mine) need to be run hot to stop them corroding.

 

[warey4life]

It costs a bit more because you don't recover the latent heat that goes up the chimney but you benefit from a more responsive system.

I think I'd rather shiver for a bit and save the money.

 

[Stuart981]

I was just asking because (very) old non-condensing gas boilers (like mine) need to be run hot to stop them corroding.

The flue gas temp needs to be high enough to prevent condensation from running down into the boiler but not so high it wastes fuel. It will be set during boiler annual service and is not something a home owner should mess with.

 

[DP]

A condensing boiler can be run at lower temperature
A condensing boiler will be max efficiency at startup
A modern condensing boiler pump runs at different speeds to maintain efficiency
Return water temperature needs to be below 55 degrees.
My boiler is set 75 for radiators but water in the underfloor pipes is circulating at 35 degrees and extension where the UF is installed is at 20 degree ambient temperature

 

[MNW67]

The flue gas temp needs to be high enough to prevent condensation from running down into the boiler but not so high it wastes fuel. It will be set during boiler annual service and is not something a home owner should mess with.

Yes, that is correct. It's because the flue gas is naturally acidic. If it condenses on a cast iron heat exchanger then it will corrode. The modern heat exchangers found in condensing boilers are made from materials like stainless steel and aluminium which don't corrode. So, the advice on setting the temperature is totally different depending on the type of boiler. That is why, whenever an OP asks about the optimum temperature, I always check which sort of boiler they have.

 

[SandyLyon]

The problem with turning your boiler temperature down is the radiator sizes (if done correctly) would have been calculated by doing heat loss calculations for each room then sized accordingly, years ago this would have been based on a boiler flow temperature of 80 degrees.

So if a required room temperature is 20 degrees and the boiler flow temperature is 80 degrees, the radiator output should be based on Delta T60 (80 - 20 = 60) by reducing the boiler flow temperature you are reducing the radiator output.

This is why Heat Pump systems require the existing radiators replacing with larger output radiators because the flow temperature is much lower (some as low as 35 degrees) which would give you a Delta T15.

Modern gas boiler systems are based on a Delta T50, Older systems Delta T60, ASHP system could be as low as Delta T15, so this affects the output of the radiator as below.

At Delta T60 the output is 1000 watts
At Delta T50 the output is 787 watts
At Delta T15 the output is 185 watts

So turning the flow temperature down will affect your radiator output