Fan Forced Roof Ventilation.

[petercsea]

Could anyone here give me a clue on how you would calculate the size of the aperture for outside air coming into the roof for a specific fan forced exhaust rate? I intend to place the fan at one end of the house roof space wall and the air intake at the other.

[oldgit]

Could anyone here give me a clue on how you would calculate the size of the aperture for outside air coming into the roof for a specific fan forced exhaust rate? I intend to place the fan at one end of the house roof space wall and the air intake at the other.

That depends on the air volume required and the type of louvre being installed i.e. the calculated free area of the louvre.For example,if you wanted 2000 cubic feet/minute of air at a louvre face air speed velocity of say 600 feet/minute,the the free area required would be 2000 divided by 600=3.33 square feet.Assuming the louvre has a free area of 70%,then the size of the louvre would be 3.33 multiplied by 10=33.3 divided by 7= 4.76 square feet or a 2.4 feet x 2 feet louvre( 720mm x 600mm louvre).The fan should also be sized to overcome the distance between the louvre and the fan outlet.

[petercsea]

Could anyone here give me a clue on how you would calculate the size of the aperture for outside air coming into the roof for a specific fan forced exhaust rate? I intend to place the fan at one end of the house roof space wall and the air intake at the other.

That depends on the air volume required and the type of louvre being installed i.e. the calculated free area of the louvre.For example,if you wanted 2000 cubic feet/minute of air at a louvre face air speed velocity of say 600 feet/minute,the the free area required would be 2000 divided by 600=3.33 square feet.Assuming the louvre has a free area of 70%,then the size of the louvre would be 3.33 multiplied by 10=33.3 divided by 7= 4.76 square feet or a 2.4 feet x 2 feet louvre( 720mm x 600mm louvre).The fan should also be sized to overcome the distance between the louvre and the fan outlet.

Thank you for a great answer.

This of course leads me to follow-up questions

Is the louvre face air speed velocity important? By this I mean should I also factor in a fan size to give an appropriate air velocity?

Oversizing the fan: What sort of oversizing should I be looking at? Calculating this on 2700 Cubic feet/minute and the distance between the louvre and the fan is 10 metres

[oldgit]

Could anyone here give me a clue on how you would calculate the size of the aperture for outside air coming into the roof for a specific fan forced exhaust rate? I intend to place the fan at one end of the house roof space wall and the air intake at the other.

That depends on the air volume required and the type of louvre being installed i.e. the calculated free area of the louvre.For example,if you wanted 2000 cubic feet/minute of air at a louvre face air speed velocity of say 600 feet/minute,the the free area required would be 2000 divided by 600=3.33 square feet.Assuming the louvre has a free area of 70%,then the size of the louvre would be 3.33 multiplied by 10=33.3 divided by 7= 4.76 square feet or a 2.4 feet x 2 feet louvre( 720mm x 600mm louvre).The fan should also be sized to overcome the distance between the louvre and the fan outlet.

Thank you for a great answer.

This of course leads me to follow-up questions

Is the louvre face air speed velocity important? By this I mean should I also factor in a fan size to give an appropriate air velocity?

Oversizing the fan: What sort of oversizing should I be looking at? Calculating this on 2700 Cubic feet/minute and the distance between the louvre and the fan is 10 metres

Sorry for the late reply.The louvre face velocity is important as there will be air noise if the louvre is too small.Based on your figures,you would want a fan capable of handling 3000 cubic ft./min @ a resistance of 0.5 inches w.g.This resistance would overcome 1-no. louvre and 10 metres of straight ductwork.Is there any other obstructions between the fan and louvre i.e. volume damper,bends?I recommend that you have a remote fan speed controller wired into the fan so you can regulate the air volume.A louvre size of 7 sq. ft. should suffice.

[petercsea]

Could anyone here give me a clue on how you would calculate the size of the aperture for outside air coming into the roof for a specific fan forced exhaust rate? I intend to place the fan at one end of the house roof space wall and the air intake at the other.

That depends on the air volume required and the type of louvre being installed i.e. the calculated free area of the louvre.For example,if you wanted 2000 cubic feet/minute of air at a louvre face air speed velocity of say 600 feet/minute,the the free area required would be 2000 divided by 600=3.33 square feet.Assuming the louvre has a free area of 70%,then the size of the louvre would be 3.33 multiplied by 10=33.3 divided by 7= 4.76 square feet or a 2.4 feet x 2 feet louvre( 720mm x 600mm louvre).The fan should also be sized to overcome the distance between the louvre and the fan outlet.

Thank you for a great answer.

This of course leads me to follow-up questions

Is the louvre face air speed velocity important? By this I mean should I also factor in a fan size to give an appropriate air velocity?

Oversizing the fan: What sort of oversizing should I be looking at? Calculating this on 2700 Cubic feet/minute and the distance between the louvre and the fan is 10 metres

Sorry for the late reply.The louvre face velocity is important as there will be air noise if the louvre is too small.Based on your figures,you would want a fan capable of handling 3000 cubic ft./min @ a resistance of 0.5 inches w.g.This resistance would overcome 1-no. louvre and 10 metres of straight ductwork.Is there any other obstructions between the fan and louvre i.e. volume damper,bends?I recommend that you have a remote fan speed controller wired into the fan so you can regulate the air volume.A louvre size of 7 sq. ft. should suffice.

Thank you once again for the effort and your reply.

There would be no obstructions as I envisage the installation. Simply the fan mounted on one wall and the air intake on the opposite (10 meters away).

"0.5 inches w.g." Apologies, but don't know what that means . I understand your recommendation of a speed controller but I'm thinking I'll go with on or off simply becasue I won't be monitoring the heat in the roof.

[Naam]

"0.5 inches w.g." Apologies, but don't know what that means..

a measurement for air pressure.

[IsaanAussie]

Air volume movements would be where I would look. How many times per hour do you want to change the air inside the roof? How large is the roof cavity? Multiply the two and that gives you the volume of air to move per hour, minute or sec. The relative sectional area of inlet vs outlet from the fan will determine the air speed at the inlet.

Have you considering using the whirly bird vertical extractors in the roof instead? They are efficient, cheap and cost nothing to run.

[Naam]

Have you considering using the whirly bird vertical extractors in the roof instead? They are efficient, cheap and cost nothing to run.

efficient only if there is a certain outside windspeed driving them. otherwise they do nothing but blocking airflow considerably.

[petercsea]

Air volume movements would be where I would look. How many times per hour do you want to change the air inside the roof? How large is the roof cavity? Multiply the two and that gives you the volume of air to move per hour, minute or sec. The relative sectional area of inlet vs outlet from the fan will determine the air speed at the inlet.

Have you considering using the whirly bird vertical extractors in the roof instead? They are efficient, cheap and cost nothing to run.

Thanks for your thoughts on this Aussie,

There are a lot of opinions on this topic and a section devoted to it in this forum. The whirlybird extractors seem to be the most controversial means of ventilating a roof. The consensus (?) is that they aren't effective and this is certainly born out by other sources. Interestingly there are anecdotal opinions to the contrary. I'm thinking I'll go with a higher likelihood that being the fan forced method.

I have 2700 cu feet roof volume and the recommendation is 1 change per minute. The whirly birds even if they are running max with all the caveats of wind speed etc can hardly reach this unless you go for a couple and then the wind is still a limitation. Their objective is to create a 10 degree differential from original to final. My requirement is higher.

[Naam]

I have 2700 cu feet roof volume and the recommendation is 1 change per minute.

don't forget that for an efficient change of hot air volume the suction of the fan should be at the highest possible point. personally i consider a complete volume change every minute as absolute overkill and a waste of energy as the air in the attic will not heat up above outside air temperature within a minute.

my roof space is ~900m³ equivalent to >24,000 cubic feet and my roof fan has a capacity at full RPM of 120m³/m. but as i run the fan with a frequency converter @25Hz capacity is reduced by >50%.

[oldgit]

I have 2700 cu feet roof volume and the recommendation is 1 change per minute.

don't forget that for an efficient change of hot air volume the suction of the fan should be at the highest possible point. personally i consider a complete volume change every minute as absolute overkill and a waste of energy as the air in the attic will not heat up above outside air temperature within a minute.

my roof space is ~900m³ equivalent to >24,000 cubic feet and my roof fan has a capacity at full RPM of 120m³/m. but as i run the fan with a frequency converter @25Hz capacity is reduced by >50%.

I agree.I think 1 air change per minute is too high.As an example,boilerhouses in the U.K. are required to have an air change rate of 15 per hour to remove heat and flue gas build up.In that respect,I reckon the same should suffice for the removal of heat within the roof space.

[petercsea]

I have 2700 cu feet roof volume and the recommendation is 1 change per minute.

don't forget that for an efficient change of hot air volume the suction of the fan should be at the highest possible point. personally i consider a complete volume change every minute as absolute overkill and a waste of energy as the air in the attic will not heat up above outside air temperature within a minute.

my roof space is ~900m³ equivalent to >24,000 cubic feet and my roof fan has a capacity at full RPM of 120m³/m. but as i run the fan with a frequency converter @25Hz capacity is reduced by >50%.

I agree.I think 1 air change per minute is too high.As an example,boilerhouses in the U.K. are required to have an air change rate of 15 per hour to remove heat and flue gas build up.In that respect,I reckon the same should suffice for the removal of heat within the roof space.

That really is good news.

Makes me wonder if I got the bull by the horns somehow wrt rate of air exchange. Anyhow I will go with the 100 Watt fan and see what it does.

Thanks to both of you.

[Naam]

Makes me wonder if I got the bull by the horns somehow wrt rate of air exchange. Anyhow I will go with the 100 Watt fan and see what it does.

don't forget an easily accessible timer with an on/off switch. no need to waste energy on days with continous rain and the timer comes in handy too.

[Baht Man]

Have you considering using the whirly bird vertical extractors in the roof instead?

They are efficient, cheap and cost nothing to run.

efficient only if there is a certain outside windspeed driving them. otherwise they do nothing but blocking airflow considerably.

Driven or not, they are more effective than nothing at allowing hot air to escape, as hot air rises. Just have an intake.

[xerostar]

Has anyone considerd using these:

Solar powered roof ventilation fans made in Perth OZ.

Price $750 each.

My roof space is 18M x 18M - they recommended I use two fans and ceiling vents in all rooms.

Seem a bit expensive but the electricity savings would add up.

Also they don't have a distributor in Thailand - problems paying import duty?

[xerostar]

Woops!

Check out that price again in the photo !!

$795 each Plus 10% GST ..!!

Add import duty ...

Err ..

[lannarebirth]

^^Do they make one that installs inside the gable end vent with a remote solar panel to power it? I don't think I'd like a through roof ventilator. Not to mention it's a bit unsightly.

[Naam]

^^Do they make one that installs inside the gable end vent with a remote solar panel to power it? I don't think I'd like a through roof ventilator. Not to mention it's a bit unsightly.

neither do i although i have one because my house has four hip roofs and therefore no gable. it took the chaps several months of rigging to make it tight. fortunately the fan is located over the pool. unsightly it is not because... i can't see it

[ataloss]

Last week at Global House CMai they had on display a full house fan extractor/air cooler - large with thick honey-comb panels surrounding the inside walls- I take it, for water retention. The sales folks as usual had no idea what they were selling nor could they find a brochure. I suspect it is a left-over from the new cooling-ventilation system recently installed there, as I asked whether it was anything like the system above, pointing to the large intake piping vents coming down from the roof. They sort of, well, giggled and nodded. If so their system makes a lot of noise, and as it forces cool air into the building it may not do the extraction job you are intending. Possibly it may well extract too, but as there was no literature to peruse, I don't know.

[Naam]

Last week at Global House CMai they had on display a full house fan extractor/air cooler - large with thick honey-comb panels surrounding the inside walls- I take it, for water retention. The sales folks as usual had no idea what they were selling nor could they find a brochure. I suspect it is a left-over from the new cooling-ventilation system recently installed there, as I asked whether it was anything like the system above, pointing to the large intake piping vents coming down from the roof. They sort of, well, giggled and nodded. If so their system makes a lot of noise, and as it forces cool air into the building it may not do the extraction job you are intending. Possibly it may well extract too, but as there was no literature to peruse, I don't know.

the cool air is taken from thin air?

[lannarebirth]

It's a swamp cooler. Don't know how it ended up in a country as humid as this one. Works great in the desert though.