Underfloor Extractor fan size?

[Tomfh]

Hi, structural engineer here.

I want to install a ventilation fan under our house to freshen the stale air, remove odours (our cats kitty litter is under the house), etc. It’s fairly dry under there, so not looking to remove moisture.

The subfloor space is 11m x 9m, around 700m high, except for a central corridor around 1400 high. Total volume around 80m3. Space is open except for brick piers.

Im going to buy a ducted exhaust fan. I’m wondering whether to buy a big one and run it periodically, or have a small one running all the time. I’d like it as quiet as possible. Would it make sense to have a small one sucking from the middle, running for extended period? Is there a lot of efficiency difference between small fan running continuously vs big fan run intermittently?

Any tips would be appreciated.

 

[itsmoked]

If you're going to go with a mechanically ventilated space I'd go all-in. Otherwise the vulgarities of the wind will cause erratic venting quality and often the paid-for power will not be adequately working for you.

I'd put the fan at the downwind-est vent spot.

I'd block all the side vents and any other vents on the fan side.

I'd use a smaller fan and run it something like 16hrs a day not when anyone is sleeping.

I'd aim for something like three turnovers/hour.


Alternatively, do nothing else but put the fan intake directly above the smell source.

Or even better yet don't have the kitty litter under the house. Maybe use a dog house to house it outside.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Tomfh]

Our cat is kept inside because there is a lot of wildlife around. Outside would be ideal however to make outside work would require an elaborate cat run, which is an option down the track, particularly if ventilation fails.

I think the ventilation approach could work. I'd like a fan anyway (or much better ventilation) as the air just isn't turning over. The vents are poor. See this diagram. The 300x100 vents were added by the previous owner. They had little solar powered fans inside them like this: R

http://www.ferret.com.au/c/Envirofa...voltage-sub-floor-ventilation-systems-n666561

Those fans and solar panels are long dead.

 

[DBronson]

Tomfh:

Be careful - depending on your climate conditions and construction details ventilating can cause severe mold problems. You may want to look at this paper by Joe Lstiburek. Link There are several other papers on crawl space conditioning there as well.

These papers will give you a better understanding of what can go wrong - eyes wide open.

Regards,

Db

 

[Tomfh]

Be careful - depending on your climate conditions and construction details ventilating can cause severe mold problems.

I'm in Sydney Australia. Temperate. Average Summer temp is 23C, average winter temp 13C. Coldest you ever see is around 4C on winter mornings.

 

[itsmoked]

Oh wow. Strange venting indeed!

I'd leave all vents as is and put the fan in the... top vent hole. That should sweep all 'cat' out the best.

You might seek out a variable speed vent fan that speeds up during the day and runs slowly at night.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Tomfh]

Would a normal wall extractor fan work ok?

There seems to be two types readily available - the wall/ceiling ones which just fit into a hole - and inline ducted fans, which cost 5x as much. I don’t understand the cost difference. Are they completely different beasts?

 

[itsmoked]

They are different.. I find typical wall/ceiling fans to be cheap and absurdly noisy. In-lines tend to be quieter. T'wer me I actually be shopping for them based primarily on their noise rating, completely ignoring any that don't have a noise rating, since they WILL be loud.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Tomfh]

What about one like this:

https://www.bunnings.com.au/blauberg-150mm-white-low-profile-exhaust-fan_p4420301

These seem different to the cheap laundry/bathroom extractors

 

[itsmoked]

That actually looks pretty good!

"Fan Sound Level dB 39" fits the bill.

305 cubic mtrs/hr / cu70 cubic meters => 4 turnovers per hour. That should do it.

Unless we take your original post numbers as correct, then it take 227 hours (9-1/2 days) for one turnover.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Tomfh]

The other thing I’m concerned about if installing a fan is heat loss. The house insulation isn’t great. The subfloor ceiling is a plain uninsulated timber floor (with carpet and tiles in some areas). If I’m constantly turning over the subfloor air I’ll be likely wasting a lot of heat. That subfloor space has presumably been acting as a decent insulation layer.

 

[willard3]

In general, residential propeller fans will not do much air with any pressure drop (larger openings required)
They are not very efficient.
They are also cheap and noisy.

 

[Tomfh]

Is the fan I linked above a “residential propellor fan”?

 

[willard3]

That's one of them. They are legion