Heat your greenhouse and home from broken glass

[Colin]

It depends on the back pressure and what flowrate you need really.
The most powerful 12v fan i`ve come across is a 3" bilge blower, it`ll do 90cfm and can handle about 1/2" wc pressure. It draws 1 amp flat out and is fairly quiet running.
Not exactly cheap but not too pricey at £25 (ebay is the cheapest place to find one, search for a 12v - 2500 LTR BOAT BILGE BLOWER WITH BRACKET.MARINE)
I bought one a while ago and they definitely blow the socks off any pc or axial type fan.

[Colin]

Hi Green Aura,
When you delve into the math and physics its just about the bare minimum needed for the task.
Its not easy to tame the sun using a cubic metre of gravel.. its like fighting a forest fire with a glass of water

[Green Aura]

Hi Colin

Don't know about the maths and physics - we just adapted an idea we saw on "It's not easy being green" and it seems to work pretty well. If you put your hand over one of the outflow hole you can feel a marked difference in temp.

I guess Mr Strawbridge did all the calculations - we just reaped the reward - best thing is it cost us the price of a small fan off e-bay, a bag of gravel and one length of 2" pipe - we had everything else so it was less than £20, no maintenance and as I said no freezing!

[Colin]

If its working for you thats the main thing.
I haven`t seen the original calculations do you have a copy?
If its a one size fits all type approach it wont be accurate as every greenhouse has a different air volume and heating load based on the glazed area, the orientation affects the solar gain.
If you use the same fan in a large greenhouse as a small one, the air turnover rate won`t be as high. The entire greenhouse air volume should ideally pass through the mass 10 times per hour, with 5 times being the minimum.

[Green Aura]

We have a 10x15 polytunnel. The box of gravel is one of those organic meat delivery boxes, it filled nicely with a full bag of b+q gravel. The fan is approx 3.5-4 inch diameter. I'm sure with bigger everything we could get higher temps, but that wasn't the point really.

We have a min/max thermometer, which we keep at soil level as that's the bit we're interested in.

As I said before, the wind speed up here drops the temp dramatically and we don't/won't heat the polytunnel, with paraffin or electricity. So we just needed something to keep it above freezing.

I should also point out that the polytunnel has a door at both ends, with mesh instead of plastic in the top sections, so the wind rattles through it.

Our olive tree survived, with all its leaves, through the last winter in there! The one outside didn't

So all in all it does a fantastic job.