Thread: Get a roof extractor fan!

Originally Posted by NickA

Cool, over the last week, I too have noticed a 5 degree drop in temperature in my house and I didn't even bother installing an extractor fan.

Same here. Just thinking about installing one has worked wonders....

Seriously, I've been wanting to put my remote sensor up in the attic so I can get a real-time measurement of the difference between the attic, ambient and room temperatures but it's been so cold I'd probably have to put on a jumper before venturing up there....

Doubtless the cold air outside falling through the fan has had it spinning like a top.

Just think, if you'd hada ona these thingies it might have been 10 degree cooler instead of only five.

You guys can laugh, but I believe there might be something to this idea...

Besides, I like being a trendsetter!

^ Got a scientific explanation for your beliefs?

Buy a bar in Pattaya....suddenly you (think you) become the font of all knowledge.

You will become a hero to a small group of sad lonely sex tourists.

You can sir on your barstool and preach about what you know.

but to the real world..................

Originally Posted by hillbilly

You guys can laugh, but I believe there might be something to this idea...

Besides, I like being a trendsetter!

In Australia fancy infra red footage showing the heat coming out of these fans. So i suppose it's a question as to whether the stronger winds down there make them effective.

Smithy, no one is disputing the fact that, if the inside of the house is hotter than the external ambient, its going to spill out some of the warmer air, provided that their is a vent in the room to allow it replace the spilled air.
What the discussion is with the "boyz who sat in the back of the physics class, playing with their todgers, instead of listening to the teacher" brigade is that, it is without the external application of a power source, these ventilation fans cannot extract any air from the room other than that which would spill out by natural convection.
If you had a room temperature of say 40 deg C and an external temperature of 20 Deg C the duct will spill out a lot of warm air and the fan would spin at a fast rate but it would extract no more, in fact less than if there were no fan.

Now for us with better than a 10th grade education, that so simple to understand but still the flat earthers amongst us don't seem to want to get it because they believed the salesmans pitch and brought the story.

In an area with little wind here in the tropics you would be better with one of these /\ over the top of a venting duct.

Even assuming a good situation as with a good breeze available
Say for example a stiff breeze of 10 M/s (about 36 km/hr) at temperature of 20 deg C and assume Fan area of 0.15 Sq M converting energy at 0.2 efficiency.
The amount of power generated is 18 w, enough to drive a small light bulb but certainly not a fan moving any significant quantity of air.

I understand clearly what is being argued, in terms of the discussion here nobody has presented a convincing argument as to why they could work.

All I am saying is that they are commonly used in Australia and many builders feel they are effective. The country may have been started by uneducated convicts, but it is still very hot in parts.

So are all the ppl using and recommending them in Australia stupid?

I think you're making the wrong comparison.
The comparison is NOT about whether having a fan means more air is extracted than that which would go out if there was a hole in the roof. Obviously, no-one has a hole in the roof or the rain would come in so that's not a proper comparison. The only possible comparison could be a fan versus having a high skylight with air vents to allow the hot air out but also keep the rain out. In that case, I'd expect a similar result to the fan.
The comparison is whether having a fan allows an outlet to be placed in the high part of the roof, which would otherwise be closed, and so more air can get out than if there was no hole in the top of the roof. The answer is obviously 'Yes' (a hole with a fan is better than no hole).
So, having a roof fan allows a hole in the top part of the roof which could not otherwise be there (unless there was a vented skylight), and means more hot air can escape, thus cooling the house more than if there were no hole.
Comparisons with shutters are irrelevant because the shutters are not in the same place - the top of the roof.
One of my houses in Australia had two fans over living rooms and a vented skylight in the main bathroom, so I've had experience of both. I think the fan did a better extraction job, but I don't have technical proof - its just my personal impression.
So, is that clear or am I waffling?

No argument with most of that Bruce,
(I got mate Bruce, he has a few Leonards chicken franchises, so I got nothing against Bruces even when they are from Queensland)

In a hot climate any form of venting is better that non at all and the most effective place is the peak where its at the highest temperature.
My argument is that the fan makes almost no contribution at all, as per the above calculation in even in a strong breeze it generates 3/5's of fvck all.
You can do the calculations yourself at a brilliant engineering info site I use almost daily
Engineering ToolBox

So are all the ppl using and recommending them in Australia stupid?

Not necessarily stupid, Smithy, just misinformed, I mean here they hang big Amulets around their necks to prevent bullets impacting on them, now that IS stupid.

Yeah, Bruces rule the world!

I don't remember anyone actually claiming on here that having a fan generated more air movement than just having a hole in the roof, or did I miss that? I only remember people saying its not true, which is a bit pointless if no-one said it was true.

You must admit though, those spinning fans look pretty cool up there going round and round.

On yer Bruce.

Well this topics almost done, Lucky dogs pissed off, because there are too many words with more than 5 letters in them so he doesn't understand most of it.

Oh, and I tried to keep it simple like, talking about 'holes' and so on; he shouldve unnerstood that, shouldn't he?

Those fans spinning get up a bit of momentum, like a top you know, so once its going round, wouldn't it only require a bit of push from rising air to keep it spinning? And wouldn't all that spinning make a bit of a sucking effect? I ask as a completely non-technical person seeking enlightment from those of greater wisdom...

The thing is Bruce, that the Fan has to put in significantly more power to increase the the velocity of the gas going up the duct, so say the gas going up a duct with a temperature differential, one end to the other of 20 Deg C, gives a pressure differential of 0.08 Kg/M on a 250 Dia short length duct that would give a gas velocity of about 1.5 M /second (very roughly)
A fan would have have to have about 75 w of power to get 50% more gas velocity, won't happen with that small a wind driven turbine.

On very high exhaust ducts where the duct friction losses would amount to biggish numbers we would attach exhaust suction fans to help the gases maintain velocity and they where usually driven by motors of about 2 Kw with 200mm Dia ducts.

BTW Where are you in Brissy ? I got a place in Main beach in Tinsel town, but rent it out as I can't afford to live there.

The house is in Sunnybank near Mt Gravatt.

I doubt if it would need a 50% increase to keep a fan spinning once it had some momentum. These things move really easy, just from the lightest touch.

I recall a similar agument about Hiclones, a device which fits into the air intake on a vehicle and is claimed to use fins to cause a swirling circular effect and increase the density of air which can be rammed into an engine, thus increasing the power output. There is great scepticism about whether these things actually work or are just a marketing tool. If they worked, wouldn't manufacturers do this as standard practice, is the response. I have no idea who is right, but its an interesting argument.

You could get a degree in thermal regulation from this thread.
LOL excuse the pun

^I don't think you've sat near me after I've had a few tins of bins, have you?

^Professor Fart, I presume?

As a hillbilly I am still confused.

Can we all agree that if we have a hole in the roof here in Thailand, the house would be cooler?

Originally Posted by hillbilly

As a hillbilly I am still confused.

Same here.

Originally Posted by hillbilly

Can we all agree that if we have a hole in the roof here in Thailand, the house would be cooler?

Try it now and let us know the results....

^^Yes.
But don't assume that you can put a fan in the hole and run it with the movement of the air going out through the hole.
Not without losses, in this case loss in air speed.