how RS Components survives

: SNIP : :Unless you live in Tasmania, one fan type roof vent is not enough. They are :cheap, and can be easily blocked in winter, (and the blades fixed) which is :what I do. : :MrT. : :

It also depends on the volume of air which must be exhanged in a given time in order for any cooling effect to become meaningful. As you say, a single vent is far from ideal but I would have thought that a fan driven vent (Powervent) which is quoted as being capable of exchanging 250L/s would not take too long to remove and replace the air volume in my roof space. Passive "whirly bird" type vents don't move that much air and that's why you need 6 - 8 of them to provide similar proficiency to the fan driven unit. I would certainly like to try to retro-fit self closing flaps onto the fan when I get it just so the vent stays closed when it is not required during winter.

My only gotcha with this is where does the "cooler" air come from? when everything around your hose is 40 deg C, not much point in pulling in air at the same temperature.

FWIW, There are 12"(?) room vent fans made with self closing flaps.

common practise to pump heat into the ground hence returning cooler air to the house and very efficient

I'm really not looking forward to digging the trench for that. I just wish I'd had thought of it when I put in a network of stormwater drainage pipes. It would have been so easy to have just droped another run of drainage pipe into the trench higher up.

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Yes, it was that type I was referring too, but they are cheap to buy, cost nothing to run, and require no maintenance. So about four for an average house is no big problem IMO. Nor is blocking/unblocking them twice a year IMO. Your powervent will obviously work on a completely still summer day however.

Most people don't use them simply for the same reason they buy halogen downlights, they have no clue!

MrT.

Here's my take on it. Putting in under eaves vents is actually wrong. If you have cavity walls, air will be drawn up from under the floor which will definitely be cooler than the air in a non ventilated roof space.

MrT.

Thanks for that tip. Since my house is double brick - cavity construction it has numerous vent bricks low down on the external wall so I will wait to see how it performs without eave vents.