Can I cool my house by dumping hot air into the attic....?

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Bewildered

106 posts

Master Geek

#225652 28-Nov-2017 20:19

Hi,

A year or so ago there was a story on Stuff that talked about how you could cool your house down very efficiently by pointing a fan out of a window or door, and then opening windows/doors on the cool side of the house. The hot air gets blown out and cool air is drawn in from the other side of the house to replace it. This wasn't exactly 'news' but it did finally get me to give it a go. Surprising it worked even better than I had expected....so much so that I'd like to step it up a notch and make it a permanent feature for these hot summer months.

My theory is that I should be able to suck hot air out of my house from a central point (e.g. hallway) and then simply dump it into the attic space. From there the air can get pushed out through the myriad of gaps and holes. Like a HRV, but in reverse. To make it worthwhile the system needs to move a lot of air (so a bathroom extractor fan won't be enough) and it needs to be reasonably quiet.

My theory is I need:

A grille in the ceiling
2-3m of 250-300mm diameter acoustic ducting (to move fan away from grille to attenuate noise, provide some insulation against winter cold)
1x back draft shutter to prevent attic air from blowing back into the house (or 2x to be really sure)

The fan could discharge directly into the attic or through a short length of duct. Optional extras that are probably a good idea:

A filter on the discharge to further prevent attic air coming back into the house
A damper (manual or electric) to seal the duct for winter (when we really don't want air coming in).

Before anyone asks, we don't have moisture problems, and we are insulated (and getting more so as we renovate our way through the house). I'm simply looking for a really efficient way to dump the excess heat at the end of the day...because it is such a waste running a heat pump when a fan can achieve the same result for less.

So...thoughts? Suggestions?

Cheers!

timmmay

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#1909187 28-Nov-2017 20:27

Alternately a ventilation system pulling air from the eaves pushing it into the house. Or dumping air through a slightly longer duct outside boss the eave. Can't see any reason to use a slightly shorter duct and dump it in the ceiling cavity.

andrewNZ

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#1909192 28-Nov-2017 20:32

Pointing a fan out the window will move significantly more air (perhaps 10x ? [edit: depends on fan and window size]) than a fan into the ceiling. The fan out the window will likely be a bigger fan, and moving air moves more air with it.

Ducting will reduce the flow from a fan in the ceiling, a filter will reduce it even further (maybe halve the flow).

Insulated ducting has terrible insulation.

blakamin

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#1909193 28-Nov-2017 20:36

You'd need a seriously big fan. And seriously big fans aren't quiet.

IMO, you'd be better sucking air from somewhere cool, and just letting the hot air go out windows and slightly-open doors.

We have them here in Oz... Evaporative Air Con. Sucks air from outside through a damp screen, blows into the house, hot air goes out windows. :D

We don't have one because they're useless in a heatwave. Split system for the win! It's currently 37.3 outside my house in Adelaide at 6pm, 25 inside.

tdgeek

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#1909197 28-Nov-2017 20:47

The air may be cool outside on the lee side but how cool? Do what you are doing as it seems to help, and shut or install drapes on sun side. Shut off solar. In winter do the reverse, open all sun facing drapes to get free solar

richms

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#1909198 28-Nov-2017 20:48

Evap cooling is useless in humid heat. Makes it worse. Friend got one a few years back because the Bunnings guy said it was like a portable Aircon but didn't need a duct. Just made it even less pleasant to be inside.

Richard rich.ms

Bewildered

106 posts

Master Geek

#1909199 28-Nov-2017 20:50

So I was thinking that dumping into the ceiling would allow me to avoid having to cut a hole in the soffit. It's a 1960s house and that means cutting holes should be avoided if possible to minimise to messing with the asbestos in the old sheet rock. Using a fan out the window will likely move more air....but it is such a pain and not that quiet either. :-(

I guess I was just hoping for something a little more elegant....and automatic...

blakamin

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#1909201 28-Nov-2017 20:52

richms: Evap cooling is useless in humid heat. Makes it worse. Friend got one a few years back because the Bunnings guy said it was like a portable Aircon but didn't need a duct. Just made it even less pleasant to be inside.

That too... That's why they're only in southern states.

I think they're totally crap anyway and would never have one. If I bought a house with one, it would be coming out (lucky the house I want next has 2 split systems)

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timmmay

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#1909212 28-Nov-2017 21:16

Bewildered:

So I was thinking that dumping into the ceiling would allow me to avoid having to cut a hole in the soffit. It's a 1960s house and that means cutting holes should be avoided if possible to minimise to messing with the asbestos in the old sheet rock. Using a fan out the window will likely move more air....but it is such a pain and not that quiet either. :-(

I guess I was just hoping for something a little more elegant....and automatic...

You could give your method a go, but just not sure how effective it will be. I just turn on the air conditioner. I also have a ventilation system that pulls cool air in from the outside, but I have no idea how effective it is... just ticks over in the background.

snowfly

543 posts

Ultimate Geek

#1909214 28-Nov-2017 21:21

Jumping in here because I might be interested in doing something similar, but instead of dumping hot air to ceiling, pulling in cool air from the outside.

I installed a 4 bedroom heat transfer system in a holiday house (has a fire), works great in winter.
But now getting into summer, house heats up a lot during the day, and say at 6-7pm its hotter inside than out, so want to pull in cool air from outside.

The system is 'HeatTrans' simx system, apparently there is an 'expensive' summer add on kit, but wonder if just adding a soffit vent, some ducting (to fan), and a manual or auto damper to switch modes, could be done? (so fan pulls air from outside intake, instead of intake in lounge by fire).

timmmay

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#1909234 28-Nov-2017 22:24

My small old ventilation system brings fresh outside air into kitchen and hallway.

At 9:50pm it was 23.0 degrees inside and 16.5 outside. At 10:20pm, 30 minutes later, the temperature had gone up to 23.1 degrees. At 5am it was down to 22.5 degrees. That shows that a relatively old, cheap ventilation system bringing outside air isn't all that effective.

I guess that's because the house gets up to 23 or 24 during the day, and it's not just the air that heats up - it's the walls, the furniture, the books, etc. We had the air conditioner on to bring the air temp down, but even though that's cool the warmer books and such heated the air back up. So I guess the key is to stop it getting too hot in the first place - closing curtains, doors, etc, as appropriate.

Air conditioner when turned on is very effective, even if the room is 29 degrees or higher (our kitchen gets a lot of sun).

Fred99

13684 posts

Uber Geek

#1909310 29-Nov-2017 08:40

Bewildered:

So I was thinking that dumping into the ceiling would allow me to avoid having to cut a hole in the soffit. It's a 1960s house and that means cutting holes should be avoided if possible to minimise to messing with the asbestos in the old sheet rock. Using a fan out the window will likely move more air....but it is such a pain and not that quiet either. :-(

I guess I was just hoping for something a little more elegant....and automatic...

B.

Are you assuming that there might be asbestos in the old drywall material - or have you had it tested?

It's not obvious by looking - post EQ my FIL's house built in the early 70s looked like a prime candidate for asbestos in textured ceiling - but it was tested before removal - and was clear. My neighbour OTOH had textured ceiling dating from the 1980s which tested positive, much to his delight as he'd done a DIY removal of some of the textured coating a few years previously - without taking any particular precautions.

Bewildered

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Master Geek

#1909314 29-Nov-2017 08:50

Untested. But the specification for the house (from the council) quite clearly state that soffits will be made from high quality asbestos sheetings, and the exterior of the house shall be painted in not less than three coats of high quality lead paint. So care is required....

Fred99

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Uber Geek

#1909327 29-Nov-2017 09:19

Oh - ok. I'd assumed you meant the internal drywall linings. Yes - asbestos in "fibrolite" type sheets in soffits is almost certain, if it's an older pre mid '80s house. That said, I'd be happy enough to cut a hole in it for a vent, wet cutting, using protective gear/precautions.
As a one-off anyway - if you were doing it routinely for a living, then nope.

MikeAqua

7773 posts

Uber Geek

#1909533 29-Nov-2017 12:55

Investigate the passive cooling system in a 'Queenslander' house. They have a central roof vent.

Hot air rises and escapes and this draws in cooler air from beneath the house via floor grills. Could be few lessons there

Disclaimer: This is based on my memory of a TV house and garden type show.

Mike