Whirlybirds vs Roof Ventilation vs Solar Cooling

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Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby james22442 » Mon Nov 25, 2013 10:18 am

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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby Benny » Tue Nov 26, 2013 10:42 pm

Ventilate what for ? Just fresh air ? Open a window ;)
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby james22442 » Wed Nov 27, 2013 10:57 am

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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby Benny » Wed Nov 27, 2013 4:57 pm

How hot is it up there ? Is the ceiling hot ?
I'd suggest increasing the insulation rather than ventilating.
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby Smurf1976 » Wed Nov 27, 2013 10:14 pm

Suppose that you have a 100m2 roof area (a small house) and it's a medium colour. You're going to get about 50kW of heat on that roof when the sun is directly overhead.

Now consider the amount of air blown by the fan on, say, a 2kW electric fan heater or a 6kW reverse cycle air-conditioner. If you're going to get rid of 50kW, and do so at an air discharge temperature that isn't too much higher than the incoming air temp, then you need to move a massive amount of air to do it.

It can be done to some extent either through natural convection or fans, but you do need to be moving a lot of air to make a real difference to the temperature. That's not so say you shouldn't do it, but be realistic about the extent of temperature drop that's actually going to be achieved.

Simply painting the roof a light colour would achieve a bigger temperature reduction than you're likely to achieve with fans etc.
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby bpratt » Thu Nov 28, 2013 9:36 am

Smurf1976 wrote:Simply painting the roof a light colour would achieve a bigger temperature reduction than you're likely to achieve with fans etc.


+1 to that.

Our old house had a zincalume roof which we painted with Dulux Weathershield in Thredbo White... for some reason we missed a small section with the paint, and on a sunny day you would burn your hand on that unpainted section, but could easily lay down on the painted roof right next to it all day.

Our next house that'll be getting built in the new year will have probably Surfmist colourbond roof, and will also have solar powered fans/vents on the southern side of the roof.

Why southern side ? ... 25degree roof should at least partially shade the fan panel in winter (I might even make more of a sunshade for it), so it is not drawing out as much air in winter, but come spring/summer, the sun will be higher in the sky, and therefore powering the fan just fine.
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby davidg » Thu Nov 28, 2013 11:09 am

bpratt wrote:Why southern side ? ... 25degree roof should at least partially shade the fan panel in winter (I might even make more of a sunshade for it), so it is not drawing out as much air in winter, but come spring/summer, the sun will be higher in the sky, and therefore powering the fan just fine.

Add a temp sensor switch and 365 day time clock to the system so if it's a certain time period of the year and if its over a certain temp outside then run otherwise don't, fairly simple. :mrgreen:
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby bpratt » Thu Nov 28, 2013 5:44 pm

Trying to achieve my goals with it without adding too much complexity at the moment. :)

Might get a bit fancier if I need to later.
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby davidg » Thu Nov 28, 2013 5:58 pm

bpratt wrote:Trying to achieve my goals with it without adding too much complexity at the moment. :)

Just use a bimetal temp sensor stud then they are simple, use no power they are either on or off rated around 5-10A with 2 spade connectors, can't get much simpler.
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Re: Whirlybirds vs Roof Ventilation vs Solar Cooling

Postby bpratt » Thu Nov 28, 2013 6:36 pm

davidg wrote:Just use a bimetal temp sensor stud then they are simple, use no power they are either on or off rated around 5-10A with 2 spade connectors, can't get much simpler.


Just got to work out what temperature rating I need to get to put it in.... I doubt getting one rated at say 25 degrees would work, as in winter you really want it to get up as hot as possible.


All thoughts to mull over. :)
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