wind driven sub floor ventilation

Greetings,

I have been talking to a guy (I'm in Melbourne) re installing sub floor ventilation under my unit and I'm trying to understand is he is misleading me or not.

The scenario - fully detached brick veneer 12 year old unit. Bottom of a sloping block, clay soil, concrete stumps, timber floor. Vents every 2 meters or so but with the vents along the west side blocked by a concrete footpath and a few others blocked by joists.

The end result is dampness and mould spores under the house, and a musty smell in the house.

He suggests installing a wind powered turbo ventilator.

Whilst I understand the basic principle, the following three points concern me.

i/- He mentioned that the pipe connecting the ventilator to the under the floor should be positioned to in direct sunlight so that the air within it heats up and aids the ventilation by causing it's own thermal updraft.

ii/- He also says that the pipe should go straight through the eaves and roof tiles (as opposed to out around the gutter) as each bend reduces the efficiency by 10%.

iii/- Further, he suggests using a 200 mm to 100 mm reducer (where I'm not sure - i.e. at the base near the elbow that enters the sub floor, before the pipe passes through the eaves or at the top where the ventilator is fitted) in order to increase suction via a Vortex effect.


If you have time, I'd appreciate you opinion/experience on these three points?

Thanking you in anticipation of your time.

Toyo

Hi
Why dont you just put some vents further up the wall where they are covered over.
The vents dont actualy have to be below the floor to work,as long as the cavity is clear.
I think your biggest problem will be dampness, caused by the concrete path breaching the damp proof course.
I'm no expert or nothing, only been laying bricks for 32 years :D
Cheers, Allan :)

Sounds like a very expensive solution to what may be a simple problem. I would first knock out a few bricks near where your vents are blocked by joists and see if that gives the cross ventilation needed to dry out the ground and wait a couple months.


Peter.

My understanding of it is that it is just a plastic fan in a housing with a flue that runs up the side of your house and the idea is that a convection current aids in the movement of the air up and out the flue. Electric fans are available for this purpose but if you have a prevailing wind, your guy might have the right idea.

A friend of mine put in a solar powered fan, which seemed to work for him but some people say these can be counter productive because they tend to work most when the conditions are warm and can actually increase the level of humidity under your house.

Another approach is to look at sub-soil drainage in combination with standard brick vents.

One thing to be aware of is that if the clay under your foundations is reactive, it could start moving if it dries out too much. My friend noticed a couple of humps in the floor after a few months. His sub floor moisture problem was fairly severe though.

Toyo,

I'm with Peter and Allan on this one.

Trying to restore the original cross flow ventilation should be relatively easy and inexpensive.

Allan's suggestion to put new vents higher up the wall should work, but first check that the turkey who did the concrete work did not manage to block the cavity. Knock out a brick as Peter suggests and use a mirror and torch to check out the cavity.

As for the claims of the guy you spoke to, most of them make sense except for the 200-100 reducer. If the reduction is from 200 to 100 and not vice-versa there will definitely be a reduction in efficiency, just as there is with any bends.

Vortex effect eh ? methinks somthing stinketh of b.s.

It's not really that expensive.

One quote is $400 and the other is $950.
I know the $950 is a ripoff.

As far as materials are concerned -
1 x ventilator - $140.
4 m PVC pipe
1 x PVC elbow
1 x pipe standoff

Labour -
Knock out a couple of bricks, drill a hole in the eave lining, drill a hole in the roof tiles (??), fit pipe, fit the ventilator, fit the elbow, fit the pipe standoff half way up the wall, fill the hole around the elbow where the bricks were knocked out, flash the hole in the roof tiles. 3 - 4 hours max.

The ventilators come in 100mm, 150mm, 200mm throats.

If I use a 150mm ventilator and 150mm pipe I can use a terracotta tile-tite (http://www.abey.com.au/roofflashings.html) to flash the hole in the roof tiles and I can use a 6 inch (152mm) hole saw to cut a hole in the eave lining.

If I use a 200mm pipe/ventilator then I can't use a tile-tile and I have never seen a 200mm hole saw. So this concerns me.


The only other bits I'm concerned about are -

i/- How do I cut a 150mm or 200mm hole in terracottoa roof tiles?

ii/- How do I knock bricks out of a wall (never done it before)?


As for the Vortex bit, I smelled BS as well hence why I sought other opinions.

Thanks for the input.

I'm sure there are various methods but you can cut a hole in a terracotta tile by drilling around the perimeter of the hole with a masonary bit and then tapping the centre out. It's best if you can orchestrate things so that the hole is near the centre of the tile, clear of the lap top and bottom. Alternatively, you can remove the tile altogether and replace it with a flashing that spans at least the two tiles on either side.

You can use the same approach with bricks, or just go at it with a cold chisel. Depends on how hard your bricks are. The idea is to fracture the brick and then work it out in pieces.

Don't be too quick to write off the 'vortex' idea. Hot air rises and a reduction from large to small diameter pipe will create a swirling motion in the air current, as is seen when water runs down a drain. It is also the same concept that is behind the cyclone dust extractors that people rave about on this site. Whether the effect is strong enough to make any difference in this situation is where the BS might creep in.

You would probably make your life easier if the reduction fitting was below your eave.

heres a silly question.
Wouldn't it be simpler to install an electric fan?

I seems people have a disproportionate view of their electricity bill and the real costs of running electrical items.

A small to medium sized electric fan will produce continuous air movement at a very low real cost.
it will not require major building butchery.
And can be controlled in a variety of ways to ways to manage noise humidity & tempriature if you get keen.
You do need an electrician though.

cheers

Not a silly question at all and is infact one I asked along the way.

One of the common ones is the Ecofan from Edmonds - http://www.edmonds.com.au/floor.htm

People put them in on timers etc. because they can be heard at night.

They claim a running cost of $6 per year.

They are 12v so if you can reach an existing powerpoint you don't need an electrician.

Install cost wasn't much cheaper than wind ventilator.

Several things I didn't like about them -

i/- Warranty - only 12 months. Have seen some claims of 24 months. The wind ventiators have 20 years.

ii/- Fire risk. Although they are 12v, I've worked in the electrical industry long enough to have seen multiple fires caused by fan seizures etc.


If major building butchery is a concern, the pipe can simply be routed via 45 degree elbows out around the eave/gutter. Will reduce efficiency though.

Toyo

The point I was trying to make is that you only vent air above the roof if it is from the sewage system, i.e. small amount of highly offensive gas, same for smoke from a fire or cooking fumes. What you need under the house is general ventilation i.e. largish amounts of air moving gently to and fro with the prevailing breeze keeping the sub floor humidity down to ambient levels. so out with the grinder, masonery blade, gently cut mortar out around one brick every two metres or so half a metre above ground level. Finish the job off with an old hacksaw blade, remove the bricks, replace them with equivalent ventilator bricks which you the mortar in place. Job done. No moving parts, no fan. no pipe or roof perforation and no venturi effect. Then tell your handyman to go and get a licence before he starts putting holes through peoples tile roofs.

I can only speculate ast to why you would bother going 12 volt for such a fan. There are heaps of good quality extraction fans available in the electrical market
I'd go 240 volt induction motor fan from one of the commercial manufacturers.

The fires you have seen are probably due to the overload protection being engineered around protecting the wiring rather than the fan motor.
It is standard practise in this country to select breakers & fuses bassed on the cabling rather than the load connected.
It is equaly common to not see overload protection on individual components.

Appliances & stand alone machinery are a different case.

Check out fantec amongs others & install an overload device for the fan motor.