Rad Fans, do they offer any energy savings

matelodave

It may take heat from the radiator faster but, unless you deliver more heat to the rad by increasing the water temperature or upping the flow rate then, it it wont make it hotter because the fan isn't adding anymore energy.

It'll just spread it around a bit quicker, possibly making the place feel warmer where previously the air wasn't circulating adequately but not actually saving any energy

Reed_Richards

The power you get out of a radiator is given by the equation:

P = F x (Tin - Tout) x d x c

P = Power
F = Volume flow rate
d = density of water (or whatever is circulating inside the radiator)
c = Specific heat capacity of the fluid inside the radiator
Tin, Tout are the inlet and outlet water temperatures

If, as I contend, you can get more power out of a radiator by forced convection then the consequence will be that the outlet flow temperature will drop.  This will not save you energy, it will cost you energy.  If you can make the radiator give out more heat then the boiler will have to work harder.

QrizB

Here's some illustration from the real world.

Take this radiator for example. Rated output is 7208BTU/h / 2112W per EN 442 - inlet temperature 75C, outlet 65C, room temp 20C. If you could add a fan (or fans) and get the outlet temperature down to 55C, without changing anything else, you'd double the heat output from 2112W to 4224W.

I have no idea how practical this would be.

(You'd also improve the efficiency of your condensing boiler, but that's another topic entirely!)

[Deleted User]

Reed_Richards said:

I was trying to provide a simple thought experiment to prove that fans work; I should have said that I was assuming the water was hot by comparison with the room temperature and you are right to point out that my thought experiment unravels if that isn't true.

Perhaps It is easier to assert that if a fan can cool you then you must be heating your surroundings faster than your would otherwise do.  And it you accept this then it's not too much of a leap to accept that the same thing holds true for a radiator.    

Fans in themselves do not cool people. The breeze accelerates the evaporation of body sweat which then makes people feel cooler. A stove fan will redirect the heat.

Reed_Richards

I suspect that the illustration from @QrizB is wildly optimistic about how effective the fans would be - but if you could get the outlet temperature down from 65 C to 64 C that would give you 10% more heat out.  Or perhaps it's the other way round, if you could get 10% more heat out then the outlet temperature would fall from 65 C to 64 C.

matelodave

QrizB said:

Here's some illustration from the real world.

Take this radiator for example. Rated output is 7208BTU/h / 2112W per EN 442 - inlet temperature 75C, outlet 65C, room temp 20C. If you could add a fan (or fans) and get the outlet temperature down to 55C, without changing anything else, you'd double the heat output from 2112W to 4224W.

I have no idea how practical this would be.

(You'd also improve the efficiency of your condensing boiler, but that's another topic entirely!)

However, if the the return temp to the boiler is now 55 degrees instead of 65 degrees the boiler has to work harder and use more energy (gas, oil or leccy) to raise the flow temp back up tp 75degrees.

The extra 2kwh doesn't magically materialise from nowhere, you have to get it from somewhere, either by robbing it from the rest of the system or by making the boiler work harder (the drop across the boiler is now 20 degrees rather than 10). 

Reed_Richards

[Deleted User] said:

Fans in themselves do not cool people. The breeze accelerates the evaporation of body sweat which then makes people feel cooler. A stove fan will redirect the heat.

It takes heat to evaporate body sweat.  It's this removal of this heat (the latent heat of vaporisation) from your body that makes sweating work to keep you cooler.  If the evaporation of body sweat is accelerated then you will lose heat more rapidly.  So you not only feel cooler, you actually are cooler!  

QrizB

matelodave said:

QrizB said:

Here's some illustration from the real world.

Take this radiator for example. Rated output is 7208BTU/h / 2112W per EN 442 - inlet temperature 75C, outlet 65C, room temp 20C. If you could add a fan (or fans) and get the outlet temperature down to 55C, without changing anything else, you'd double the heat output from 2112W to 4224W.

I have no idea how practical this would be.

(You'd also improve the efficiency of your condensing boiler, but that's another topic entirely!)

However, if the the return temp to the boiler is now 55 degrees instead of 65 degrees the boiler has to work harder and use more energy (gas, oil or leccy) to raise the flow temp back up tp 75degrees.
The extra 2kwh doesn't magically materialise from nowhere, you have to get it from somewhere, either by robbing it from the rest of the system or by making the boiler work harder (the drop across the boiler is now 20 degrees rather than 10).

No-one is saying otherwise? This is about getting heat into a cold room faster and maybe improving boiler efficiency by a % or two, not about breaking the First Law.

Reed_Richards

There is quite a good brochure here for a range of fan convectors, a type of radiator with a built-in fan.  Page 3 shows the heat output for maximum and minimum fan speeds and the heat output on Max is almost double that on Min.

https://www.myson.co.uk/static_files/my/media/downloads/iVector MKII.pdf

QrizB said:

The extra 2kwh doesn't magically materialise from nowhere...

No-one is saying otherwise? ...

Certainly not me.  Whilst we may disagree about whether powered radiator fans would actually work, we do all seem to agree that they do not offer any energy savings, which is what the OP asked originally.

matelodave

We got there in the end