Advice wanted on setting up 2 fans to cool garage roof space

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Darkhawk

4 posts

Wannabe Geek

#317721 9-Nov-2024 13:51

Hi, I'm after some advice on the best setup to extract hot summer air from my garage roof space.
At the moment I have one 14 cfm extractor fan above the mezzanine floor on a thermal relay which activates when the roof space rises above 33 degrees. I'm finding that although it makes a difference, the roof space is still reaching temps that potentially will damage things I have in storage up there. I've bought an identical fan and was wondering what the best setup option would be to have them working together.

Advice online for mounting 2 fans in a line is, don't do it as you aren't actually doubling capacity. So I was planning on mounting it on the opposite end of the shed. I'm wondering is it better to have it reversed so it is sucking outside air into the building, moving cooler outside air across the roof towards the original fan which is extracting, or leave it as two extractor fans pulling in opposite directions, fresh air flowing in from the gutter eaves and gap under pedestrian door.

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#3307274 9-Nov-2024 14:49

I would generally look at using passive ventilation down low (vents or leakage) to allow air in, then have both as exhaust.

A better option may be a roof cowl fan, exhausting air upwards.

You'll generally want a high airflow but shouldn't need much static pressure - stack effect alone will give you a little flow.

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k1w1k1d

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#3307277 9-Nov-2024 14:57

I would suggest that a lot of the heat is coming in through the plastic panels.

Could you partially cover them, especially on the sunny side, to see if this cuts down some of the heat??

SomeoneSomewhere

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#3307278 9-Nov-2024 14:58

Yeah, running a layer of kingspan or similar between the purlins would significantly reduce heat gain.

cddt

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#3307283 9-Nov-2024 15:30

Given the size of the shed and the distance between the fans, I reckon you'll get approximately the same result whether you reverse the fan or not.

But since you have to make a decision I'd be inclined to exhaust hot air.

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Bung

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#3307289 9-Nov-2024 15:52

k1w1k1d:

I would suggest that a lot of the heat is coming in through the plastic panels.

Could you partially cover them, especially on the sunny side, to see if this cuts down some of the heat??

I'm working on a Karaka green roof at the moment. Any time after 10am until about 4.30pm the roofing iron is too hot to touch or kneel directly on. I think the shed roof looks like a dark grey so the heat will be radiating from the underside of the iron.

richms

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#3307297 9-Nov-2024 16:33

Paint over the plastic panels with white paint and do the rest of the roof too will be the biggest help. Or cover it with solar.

Never got the point of those plastic panels in a garage with wiring in it for lights, since they only let light in during the day and you have no control over the light that you get.

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cddt

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#3307304 9-Nov-2024 16:47

I guess natural light could be better if you were working inside there quite a bit.

But you're right, white paint over the whole roof will do a lot to lower temperature too.

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neb

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#3307311 9-Nov-2024 18:21

SomeoneSomewhere: Yeah, running a layer of kingspan or similar between the purlins would significantly reduce heat gain.

On our light-grey roof (chosen for high albedo) the temperature difference between the roofing iron and attic air is a fairly consistent 10 degrees, so anything you can do to keep the heat in/on the metal roof rather than directed downwards will help.

neb

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#3307312 9-Nov-2024 18:28

SomeoneSomewhere: I would generally look at using passive ventilation down low (vents or leakage) to allow air in, then have both as exhaust.

In theory a ridge vent would help a bit, but as others have pointed out the primary steps would be to increase the albedo of the roof and then to get something in between the hot roof metal and the space beneath.

tweake

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#3307333 9-Nov-2024 20:28

Darkhawk:

Advice online for mounting 2 fans in a line is, don't do it as you aren't actually doubling capacity. So I was planning on mounting it on the opposite end of the shed. I'm wondering is it better to have it reversed so it is sucking outside air into the building, moving cooler outside air across the roof towards the original fan which is extracting, or leave it as two extractor fans pulling in opposite directions, fresh air flowing in from the gutter eaves and gap under pedestrian

now this gets kinda interesting.

the early advice you got given was not quite right.

to have the two fans exhausting you need a huge inlet. its actually substantially bigger than what you would think. however those walls and door are often pretty leaky. but if they have seals or a tight fit its may not be enough. it also depends on how much pressure or suction the fans can handle. some fans loose a lot of flow when they are trying to suck air through a building. so you actually need a fan blowing in as much as a fan blowing out. that can also help make a bit of circulation.

neb

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#3308129 12-Nov-2024 14:53

neb: On our light-grey roof (chosen for high albedo) the temperature difference between the roofing iron and attic air is a fairly consistent 10 degrees, so anything you can do to keep the heat in/on the metal roof rather than directed downwards will help.

Just providing some more data for a typically hot sunny day, air temperature 25 degrees, roof metal temperature 47 degrees, attic air temperature 32 degrees. So there's a 15 degree temperature difference created by the building paper and thin layer of air above it, and that's with a decent breeze hitting the roof metal to provide at least some cooling, I don't have any data yet for a wind-free scorcher of a day.

In your case you'll have the equivalent of a garage-scale oil column heater running all day.

Kickinbac

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#3309501 15-Nov-2024 23:23

Are you sure it’s 14 cfm? That’s like nothing! 6.6 L/s! A 100mm bathroom exhaust fan is around 25 L/s.
You could be on a hiding to nothing unless you can add insulation or shading. The metal roof is a huge radiant heater in the sun.
Ventilation can only achieve the ambient outside air temp at best and you need to move a lot of air to overcome the heat gain of the structure.

Darkhawk

4 posts

Wannabe Geek

#3311702 22-Nov-2024 08:26

Kickinbac: Are you sure it’s 14 cfm? That’s like nothing! 6.6 L/s! A 100mm bathroom exhaust fan is around 25 L/s.
You could be on a hiding to nothing unless you can add insulation or shading. The metal roof is a huge radiant heater in the sun.
Ventilation can only achieve the ambient outside air temp at best and you need to move a lot of air to overcome the heat gain of the structure.

My bad, it's 1256 cubic m/hr
was quoting wrong figure, the 14 comes from 1400rpm

Went for this size only based on the dB level (54) needed something that wasn't going to piss the neighbors off with the noise as their windows are only about 8m away.

Darkhawk

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Wannabe Geek

#3311709 22-Nov-2024 08:39

Thanks to all your suggestions above. There's some good ideas, though I just want to focus on what position to place the extra fan at this stage. I prefer to keep the natural light coming in from the clear panels but I might install solar on the dominant side at a later date which would help to buffer the roof from the direct rays.

Looks like setting them both to extracting is the best solution, but should I mount them side by side instead of op ends? Wondering if that would be more efficient with them both pulling in same direction instead of competing against each other.....or doesn't that matter, we are talking about 1300 cubic m of workshop space, plenty of air for them to fight over.

olivernz

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#3311712 22-Nov-2024 08:48

I have a fan venting my home roof space. What I noticed (measured with temp sensors) was that expelling heat was not really making that much of a difference but blowing in fresh air did massively. It would pay trying it both ways. So make sure you can reverse the fan.

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