Alternative Technology Assn Forums

1. 

   Chippa2014
   Member

   Hi,

   I'm considering placing a fan in my roof either in front or behind the opening of my man hole. The man hole cover itself I've modified so that there is now mesh centre with the timber frame. Works well and air now enter the roof space but what I am looking to do is draw more air in to the roof and have almost a constant flow when the fan is switch on.... especially on hot days.

   I was speaking with a number of roof insulators who suggested placing a fan at the opening so that the back of the fan would sit near the opening and will draw air into the roof, where others have suggested placing the fan such that it blows air over the man hole which will create more of an up draft effect...

   Net result I'm after is cooler air entering the roof and being moved towards the powered whirly bird resulting in as much of the hotter air being removed as possible.

   We don't have eaves and the roof design is such that getting any sort of cross ventilation or airflow is not an option.

   Anyone done anything similar and have some words of wisdom?

   Cheers

   Chippa

   Posted Monday 23 Mar 2015 @ 12:56:11 am from IP #
2. 

   Chip
   Member

   When you remove air from inside the house it will be replaced by air from outside and, if the roof space is hot, the air outside the house will be hotter than inside the house so you'll be heating your house up to ventilate your roof space?

   What is your roof clad with?

   If you have a tile roof then there will be plenty of air getting in to match what the whirlybird removes. Metal roofs will vent plenty of air in at the edges usually.

   Do you have ceiling insulation?

   Unless you visit the roof space a lot, it may not matter if it gets hot in there if that heat can be prevented from conducting into the house and insulation will do that plus give winter benefits.

   Posted Monday 23 Mar 2015 @ 1:29:28 am from IP #
3. 

   Benny
   Member

   I agree generally with Chip. You say you want the fan to operate "on hot days". As Chip says this will mean you are drawing air in through the door seals etc and will be heating you house. When its hot outside you want the house tightly sealed up (unless you have evap cooling). Better to vent the roofspace directly from outside if you feel it really needs it. What makes you think the powered whirlybird is being throttled by lack of input air ? How hot is it in your roofspace ? If you have good insulation it doesn't really matter if its really hot up there.

   If you plan to use this ceiling ventilator at night when its cool outside and there's no natural breeze to cool the house then thats a different story.

   Posted Monday 23 Mar 2015 @ 2:02:18 am from IP #
4. 

   Morbo
   Member

   buy a pedestal fan and place it so it cools you.
   A 40W fan wont do anything reg the amount of air you need to move, more like 500W. But that is not doing anything either if its not cooling down outside. Faster for your roof to cool down by itself via -> night sky radiative cooling.

   Posted Monday 23 Mar 2015 @ 4:08:58 am from IP #
5. 

   Chippa2014
   Member

   Hi,

   Thanks for the responses. Man hole is positioned in the garage where the temp is significantly cooler than the roof temp which often reaches >65C Garage is also insulated and often one of the coolest parts of the house, hence my want to take the cooler air from there and have it circulate through the roof space. Perhaps I should've stated that in the first post... my apologies.

   Roof space is 20m x 8m x 1.8m (at the peak) and used for storage

   Corro\tin roof - poor quality sarking that really does need to be replaced. Ceiling Insulation is "Cool or Cosy".

   Benny,

   What makes you think the powered whirlybird is being throttled by lack of input air ?

   I don't, I just want better air circulation and if that results in more air out then Im happier..

   How hot is it in your roof space ?

   It has reached 74C. I know because I was getting something out of there but it was a 42C day. This has been significantly reduced with the powered whirly bird... down to mid 50's on the hottest of days last summer.

   Morbo,

   Don't need the pedestal fan .... I have aircon

   Posted Monday 23 Mar 2015 @ 9:39:13 pm from IP #
6. 

   Morbo
   Member

   ->What makes you think the powered whirlybird is being throttled by lack of input air ?

   because then there is a higher pressure drop. Try with a vacuum cleaner, close the inlet with your hand. It has to work harder then, as you can hear.

   Posted Monday 23 Mar 2015 @ 11:15:25 pm from IP #
7. 

   Benny
   Member

   I don't think its being throttled - I was asking if you think it is. As chip said - there are usually plenty of gaps in a roof for the powered whirlybird to be running flat out and not needing an extra opening - even if it has a fan in it. I can see all our comments were irrelevant though given your later statements - ie its used for storage and you're not extracting air from the house.

   Posted Tuesday 24 Mar 2015 @ 1:43:23 pm from IP #
8. 

   Chippa2014
   Member

   Just in case anyone is wondering, a 170W fan (non oscillating) placed behind the opening and positioned on a slight upward angle approx. 10 degrees, approx. 400m from the edge of the blade to the opening seems to be the most effective location for drawing air into the roof in my tests.

   The air flowing over the roof creates a stronger draw of air through the man hole than placing the fan in front of the man hole and "sucking" the air up.

   Posted Thursday 26 Mar 2015 @ 10:11:53 pm from IP #

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