This time of year...

[Radian]

...Cold nights, frequently below 5C, occasionally below zero... Sunny days but with outdoor temps still pinned back.

 

Consequently my unheated Woodworking Workshop (to left of garage, behind the entrance door in the brick alcove) hovers around 10C while the large empty roof storage space above the cars gets up to 25C. There's a lot of hot air up there.

 

Every year I think about fanning the hot air above down to a vent in the woodshop to make some use of all that wasted energy. I never get around to it though because these conditions only persist for a month or so. Recently however we have a new set of rooms extending the garage/workshop and while the inhabited half of the roof warms nicely via the Velux roof windows and glazed doors in the gable, the downstairs room would remain around 10C without a little help from (currently) a 2kW convector.

 

 

You can probably see what I'm thinking.

 

To do a practical experiment I'd need to buy around 10m of ducting, an inline fan and bore a suitably large hole in the wall between the downstairs room and the central woodshop, and another from the woodshop up to the storeroom. This would all be fine if it worked and gave me some useful free heat energy part of the year but I'd rather do it as a desktop exercise first. My suspicion is that it would only give a marginal benefit - based on the observation that people don't seem to go around discussing this sort of thing.

 

So how could I go about calculating the benefits? I know the approximate volume and temperature of the hot air available - if I was able to displace it into the target room, it would fill it about 1.5 times. I also know the loss in kW of the room per degree difference to outside. I feel that should tell me how it would pan out in a one-off air transfer but I'm hazy on how this would go on as the hot air was used up?

[tonyshouse]

There is not as much useful heat in the air as you think, temperature and heat are different. 
 

try it and see is my advice without making the holes 

[SteamyTea]

8 hours ago, Radian said:

There's a lot of hot air up there.

 

7 minutes ago, tonyshouse said:

There is not as much useful heat in the air as you think, temperature and heat are different. 

Yes, do the sums.  I did once and it was not worth pursuing. 

[Radian]

Thought as much. Same reason compensating for the recommended air change of 1/3 room volume per hour doesn't result in complete bankruptcy without heat recovery. Even though it sounds like it ought to.

 

Heating 63m3 of air from 10C to 20C in one hour:

Specific heat capacity of air ~ 1 kJ/kg/K,  density of air ~ 1.2 kg/m3.  So 63 × 10 × 1.2 × 1 /3600 = 0.21 kWh

 

Not much. But I can't visualise the dynamic nature of how it might play out in practice. The replenished outside air in the roofspace would need reheating. It may only have gotten to the high 20's after cooking all morning. The slate roof is a good re-radiator but how to quantify the energy transfer? 

 

It certainly would be easier and more accurate to try it but getting hold of a suitable fan isn't painless. I'm seeing something in the ballpark of £120 for something that would be suitable.

[SteamyTea]

1 hour ago, Radian said:

certainly would be easier and more accurate to try it but getting hold of a suitable fan isn't painless. I'm seeing something in the ballpark of £120 for something that would be suitable

A car radiator fan maybe.

[JohnMo]

dMEV fan, almost silent, could be used move air from one place to another

[tonyshouse]

A mate of mine in Wales did something similar using two  cheap bathroom fans and 50m of duct, the air does not need to move very fast