# GH/hot air/soil heating system



## bluefish (Jan 27, 2006)

I'm building a greenhouse this spring and have been looking at one of those systems that reclaims otherwise wasted hot air that would be expelled from the GH and sends it through the soil to help heat the soil. As far as I can tell, they all use fans to move the air. Where the greenhouse is, there is no power. Is there a way to get a system like this to work passively? Or should I just set it up and save for a solar fan down the road?

I'm trying to make this as passive as possible but think the extra heat in the soil would be very helpful in the winter. It would probably be worth the fan, but it would have to wait a bit.


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## wy_white_wolf (Oct 14, 2004)

I don't see how it could be done passively.

Another idea would be digging a pit in the center of the GH that could be filled with compostable materials in the fall. As it composts it would release heat into the soil/GH.Grow in pots/containers over the pit or put your potting bench there.

WWW


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## CesumPec (May 20, 2011)

fair warming: I've never tried this, just an idea. I borrow this idea from old ship steam engine design, the "D" engine where pipes loop in a roughly D shape into and out of the fire chamber until the pressure builds to the desired level. 

As heat rises, it creates a vacuum behind it and will draw air as in a fireplace/chimney. This is why many fireplaces will actually cool your house because they suck heated air out of your house and cold air filters in thru the cracks. 

If your hot exhaust air from the GH goes from the top of the GH into a duct that goes down to the area you want to warm and then up and out, well higher than the warm area inside the GH, you should get enough draw to suck the hot air down. The downside being you might be setting up a siphon that will suck all the warm air out of the GH. 

You probably need to put a butterfly damper inside the duct to turn off the chimney when appropriate. Without electricity to automate the damper you might need to get someone really creative who could probably modify one of those temp sensitive pistons to shut and close the valve. 

This is the sort of thing you could try with the largest scrap PVC pipe you have on hand to see how it works before spending money to build a production model. The dimensions of your exhaust stack will of course vary with the volume of the GH. You might need to build it with large duct work and put in baffles to limit the air flow till you get just the right flow.


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## Ky-Jeeper (Sep 5, 2010)

Look up earth tubes. Mold and fungus could be a problem though in a humit gh.

Me I would just use clear or maybe black plastic film mulch. It will heat your soil with out a lot of trouble. Head over to the garden section for more help on this.


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## jwal10 (Jun 5, 2010)

In my off grid cabin I collect the heat at the tray ceiling in the sun porch. I use 4" insulated flex tube to run this heated air to a register in the wall of a closet. There is a 12 volt fan that circulates this heat to the livingroom and bedroom. I use dampers to route it where I need it....James


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## SolarGary (Sep 8, 2005)

Hi,
I'm not by any means a greenhouse expert -- we are just in the process of doing a first cut at a 4 season GH for MT, so I'm thinking about the same idea of taking hot air at the peak of the GH and using it to heat the beds.

I'm thinking about placing an air to water heater exchanger in the peak of the GH. Circulate water or non-toxic antifreeze through it and then through the growing beds. 
This sounds kind of complicated and not very passive, but it could be quite simple. The pump could be something like the TopsFlo (link below) -- this can run on a single 12 volt PV panel, and the PV panel also basically becomes the controller in that it only runs with sun on it. A simple $10 thermal snap switch could be put in the pump power line so that the pump would be prevented from turning on until it go up to the temperature the snap switch was set for.
So, all you have is the heat exchanger, the pump, a small PV panel, and some tubing.

One place that carries the TopsFlo pump is at: www.sun-pump.com/pumps.htm

Example of kind of heat exchanger I'm talking about: http://www.brazetek.com/finned-coil-water-air-heat-exchangers

Here is one similar example of using this scheme -- http://www.builditsolar.com/Projects/Sunspace/LowCostHtStorageNathan.pdf
Nathan is using the GH air to heat fish tank water, but same idea could be used for heating soil.

Edit: forgot to mention that a fan to push air through the air side of the heat exchanger would also be needed. It could be solar powered as well. Getting more complicated 

Gary


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