Ferdinand

Heat loss through Continuous Running Fans

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A quick question - how much heat is lost through a fan running continually at n litres per second (currently 6) with temperature differential delta-t (currently prob. 15-17C)?

 

I have replaced my fans in both bathrooms with a pair of Vent-Axia Lo Carbon Centra-T devices, which run at either 6 l/s or 9 l/s continuous setting. Before one was a bog standard thing installed by the (self) builder, and the other one downstairs was an Icon Airflow I have had in for a few years. Unfortunately for unavoidable reasons, the downstairs one is directly above a radiator, and it is the warmest room.

 

I also have a PIV device fitted, which will run at a  low setting in the winter (and switches itself off at low outside temperatures anyway) to give me some modest throughput.

 

The issue is that in the last few days with outside temps at perhaps 2-3C the heating response had been sluggish, and I have someone coming back from hospital to a downstairs room and I need to run at perhaps 23C internal temp, which is either very slow to reach or out of reach without eg a fan heater on.

 

I am wondering if my new throughput fans are part of this issue - the plan was that they would be marginal, but I am looking for causes.

 

This morning I will switch the d/s bathroom off completely to stop the fan to see, and will take some thermal piccies, but possible solutions include putting the Airflow Icon back, or replacing the d/s bathroom fan with a Heat Recovery Version.

 

Any comments and insights are welcome.

 

Thanks

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A lot...

 

As an aside I went into my new bathroom this morning at 4am. The newly fitted extractor vents to the soffit outside and has no back draught flappy thing. Fan not running but it was a balmy 13degC in there...no heat of any sort in  there yet mind.

 

Put your hand up by the ceiling and you can feel the cold draught. Don't know how long I can live with this "interim" solution! 😂

 

Can I get a back draught vent that goes in a soffit?

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As a rule of thumb, cubic metre of air has a mass of 1.2 kg.

The energy needed to heat 1 kg of air by 1 K is 1 kJ.

There are 1000 litres in 1 m3.

There are 3600 seconds in an hour.

 

 6 [lt.s-1] / 1000 = 0.006 m3.s-1

 

0.006 [m3.s-1] / 1.2 = 0.0072 kg.s-1

 

0.0072 [kg.s-1 ] x 3600 = 25.92 kg.hour-1

 

To convert from kJ to kWh multiply by 0.0002778

 

25.92 x 0.0002778 = 0.0075 kWh

 

I have skipped the conversion to kJ as air has a SHC of 1 (near enough), though there will be some energy in the moisture)

 

This gives you a loss of 0.0075 kWh for every K, or °C, temperature rise, for every hour the fan is running.

 

So taking the 20°C temperature difference in your example, to run that fan for 24 hours will be:

 

0.0075 [kWh] x 20 [K] x 24 [h] = 3.46 kWh

 

 

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1 minute ago, Onoff said:

A lot...

 

As an aside I went into my new bathroom this morning at 4am. The newly fitted extractor vents to the soffit outside and has no back draught flappy thing. Fan not running but it was a balmy 13degC in there...no heat of any sort in  there yet mind.

 

Put your hand up by the ceiling and you can feel the cold draught. Don't know how long I can live with this "interim" solution! 😂

 

Can I get a back draught vent that goes in a soffit?

 

As it is continuous there is no opportunity for backdraft.

 

The q is about how much heat will be carried by a continuous flow of air.

 

For vents, you can get all sorts of things. I used one of these in a gable, which has both a wind cowle and a backdraft shutter

https://www.amazon.co.uk/gp/product/B01G3YNLJ8/, but may not fit.

 

Or in line ones are available. Or you could change your fan.

 

Go a hunting on Amazon.

 

Ferdinand

 

 

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Just now, Ferdinand said:

The q is about how much heat will be carried by a continuous flow of air.

This is a confusing question, 'Q' is often used for heat flux.

if 3.5 kWh a day are lost though the fan, the power is:

3.5 kWh / 24 h = 144 W

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1 minute ago, SteamyTea said:

@Home Farm

What is confusing (though I often get confused too).

 

 


As part of our master bathroom refit, we were going to replace our extractor/vent fan with a better unit to assist in getting steam build out as the one in there now is a cheapo from the previous owners. Never appreciated it would lead to significant heat loss if they’re too good.

 

My reaction to your calculation was mixture of awe and confusion.

 

 

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8 minutes ago, SteamyTea said:

So taking the 20°C temperature difference in your example, to run that fan for 24 hours will be:

 

0.0075 [kWh] x 20 [K] x 24 [h] = 3.46 kWh

 

Thanks. So - a detectable amount (albeit not yuuuuge on a Trumpian scale) or 1250 kWh per annum ish = £100 or so of electric or £25 of gas.

 

My total energy usage is something like 12000 kWh gas and 4-5000 kWh electric for a 4-5 bed house (assuming the numbers I gave Cheap Energy Club are still right), for a bill of around £900-£1000 pa.

 

The effect on responsiveness and perception is more interesting, even though the power to compensate is theoretically only 150W, which should be a rounding error on a properly sized gas boiler capacity.

 

So a heat recovery fan there would reduce that by 75% in theory (may have one in stock) but give a slightly cooler than ambient draft in the bathroom by definition, but I suspect I am better reverting to the Airflow Icon.

 

Suspect a bigger issue may be 1 - that the ufh is temperamental, and slow to adjust as they always are, 2 -  that we are relatively poorly insulated for ufh, 3 - that the central programmer is immediately outside that bathroom door and so will switch off too quickly is not offset-programmed, and 4 - that I tend to use the upstairs heating relatively little, which means that I may lose a lot up the stairs.

 

But every little helps, marginal gains and all that. Back to Airflow Icon I think when I get to it.

 

F

 

 

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18 minutes ago, SteamyTea said:

This is a confusing question, 'Q' is often used for heat flux.

if 3.5 kWh a day are lost though the fan, the power is:

3.5 kWh / 24 h = 144 W

 

Fair cop. Full "question" in future.

 

Must not make people think I am in a submarine.

 

"Flood Q, Number One - Crash Dive".

 

(Damn - just drowned the spy gadget specialist. No more exploding rat-poo for me.)


F

 

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33 minutes ago, Home Farm said:


As part of our master bathroom refit, we were going to replace our extractor/vent fan with a better unit to assist in getting steam build out as the one in there now is a cheapo from the previous owners. Never appreciated it would lead to significant heat loss if they’re too good.

 

My reaction to your calculation was mixture of awe and confusion.

 

 

 

Don't forget - I use PIV fans in all my rented houses to provide condensation / mould resistance and resilience against non-ventilating or washing-on-radiator tenants, as I have not accepted trickle vents for years.

 

I often add a low volume trickle fan (used to use single room HR fans) in bathroom and kitchen to provide a "poor man's MVHR". I have always used fans with backdraft shutters and recent PIV loft fans, but have taken to the continuous ones more recently to give a low level forced airflow.

 

That is all together with high quality renovations to provide a resiliently cheaper to run / humidity managed enviroment for Ts, which aims to keep them comfortable and staying for longer, partly by giving them energy bills 1/4 to 1/2 less than otherwise.

 

The heat cost of having continuous low volume fans installed is in that context. And an extra £25-30 on your gas bill may or may not be significant in exchange for whatever difference it makes to your environment.

 

This year I have done the same at home, and I do not think I have it quite right yet.

 

In my case I have an increasingly frail mum coming back from hospital so need a warm and responsive environment. That she has moved into a downstairs south facing bedroom that is cold in winter and overheats in summer (runs up to 35C on sunny days) is the next thing I need to manage, exasperated by ufh not being *that* responsive, especially in a well-insulated but not superinsulated house.

 

At this moment, my downstairs bathroom is at about 21C,  my kitchen is at about 19C, and mums new bedroom is at about 16C. Clearly I need to rebalance the ufh again for the winter, and perhaps fit one of those carry around wireless controllers. I really want the whole downstairs at 22-23C.

 

F

 

Edited by Ferdinand

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22 minutes ago, Ferdinand said:

So - a detectable amount (albeit not yuuuuge on a Trumpian scale) or 1250 kWh per annum ish = £100 or so of electric or £25 of gas

That is the losses at 20°C temperature difference.

The real difference may well be only 3 or 4 K for a fair slice of the year and 10 K for the rest.

And the losses only count when you actually put on supplementary heating.  I turned mine on last night (as 16° in the house, 4°C outside).  But it will probably go off again after the cold spell.

 

Edited by SteamyTea

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2 minutes ago, SteamyTea said:

That is the losses at 20°C temperature difference.

The real difference may well be only 3 or 4 K for a fair slice of the year and 10 K for the rest.

And the losses only count when you actually put on supplementary heating.  I turned mine on last night.  But it will probably go off again after the cold spell.

 

 

All true.

 

Though in the Oct-Mar period I tend to have my ufh on for very long periods sometimes up to 2/3 of the day, and the house cools down relatively (to newbuilds here) quickly, and the balance is between 1 - Warm enough, 2 - Low water temp for efficiency, 3 - Higher water temp or supplementary heating for responsiveness.

 

F

Edited by Ferdinand

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Does seem your UFH is not optimal, have you got an infra-red thermometer, then you can point it at floor and see what the temperatures are.

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37 minutes ago, Home Farm said:

My reaction to your calculation was mixture of awe and confusion.

 

IF you work through it with the units in English rather than SI eg "m3.s-1" as "cubic metres per second", it may help.


Also it is perhaps not intuitive for many of us to think of air in kilograms.

 

F

 

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2 minutes ago, SteamyTea said:

Does seem your UFH is not optimal, have you got an infra-red thermometer, then you can point it at floor and see what the temperatures are.

 

I do indeed, and I will have to go around again.


And the bedroom is carpeted 🤔.

 

F

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Get the temperatures sorted, then obsess about the flow temperatures.

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