# best material for air duct ?



## fernando (Jan 11, 2005)

I plan to construct a high-efficiency solar heat collector to heat air to be transferred into an underground gravel heat storage bin. Hopefully, the air temp will get into the 160 degree range. The ducting will be buried about seven feet deep in a heavy clay soil. 

Naturally, the best choice for ducting would be metal. However, in trying to keep cost down, i'd prefer to use some sort of plastic pipe (four to six inch) instead.

My question: Can anyone recommend an appropriate plastic that would stand up to both the heat carried and the burial pressure ?

I do thank you for any advice you might offer !


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## Harry Chickpea (Dec 19, 2008)

Here is the issue you are missing. One side of the pipe is in contact with your 160 degree air. The other side of the pipe is in contact with 55 degree soil. The pipe will be cooling the hot air all the way to your storage bin.

You need to consider insulation and the most economical method of heat transfer. Air is not a terribly good medium compared to heated water. It is less dense and requires more volume to transport the same amount of heat.

If your gravel storage is underground, that also needs to be insulated.


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## Bearfootfarm (Jul 13, 2006)

> Naturally, the best choice for ducting would be metal. However, in trying to keep cost down, i'd prefer to use some sort of plastic pipe (four to six inch) instead.



Metal would have to be insulated heavily, and would soon rust.

I'd use plastic pipe large enough to have a foil covered fiberglass "flex" duct inside



> Can anyone recommend an appropriate plastic that would stand up to both the heat carried and the burial pressure ?


I'd go with something like this:

https://keithspecialty.com/dwv.flex_and_culvert.htm


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## Hobbes (Apr 1, 2008)

with the heat, the plastic with probably outgas fumes


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## Jim-mi (May 15, 2002)

4 to 6 inch pipe will Not give you enough air volume . . .too small.
You would need more "fans" (energy) to push enough air 

Do some more home work on rock/gravel storage..........they will soon mold . . . .not good


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## fernando (Jan 11, 2005)

Thanks for the input. Mold and fumes are of no concern in this case since it will be a closed loop design. My intent is not to heat a space with air but rather to heat the space with heated sub-soil. My predicament is determining just what material to use for ducting that won't be degraded by heat and pressure and that is the advice i was hoping to get.


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## fordson major (Jul 12, 2003)

So you want to move hot air into a subsoil heat sink then pick it up again with a different method to heat some where else? I'm no expert on any of that but I do know when moving hot water underground you have to protect the pipes from water or it will wick away a lot of the heat. If your heat sink isn't insulated and protected from moving water (water proof) in the subsoil you will lose heat rapidly. I do know something about moving hot air and Jim is right you'll need much bigger ducting to move any substantial btu to the subsoil and then some kind of heat exchanger to extract the heat once it gets there. Remembering heat moves in all directions (not up) that exchanger would need to be in the center of the mass. (why gravel instead of sand which has less air spaces to limit heat transfer?) I have no idea how you would build the heat exchanger and the amount of air you'd have to push through would be a lot unless you were using an AC heat pump or something. Never mind the size of the fans needed to shift that air. I've heard of similar systems that use water or glycol that would be much more efficient if I'm understanding your project. 

No matter I'm glad you posted and hope you keep us updated!


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## TnAndy (Sep 15, 2005)

Just curious about the whole system.....how about describing it....what type collector, where located, etc.


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## fernando (Jan 11, 2005)

TnAndy said:


> Just curious about the whole system.....how about describing it....what type collector, where located, etc.


in south-central ky - the goal is to heat a yet to be constructed barn by heating sub-soil in the summer and allowing natural transfer to keep the floor above freezing in the winter - at MINIMAL cost - not looking for comfort but rather survival - the same solar panels that heat the gravel can be used for occasional space heating in the winter - one aspect that does cause me concern is that there is a slight possibility of over-heating the barn during the summer but that's a good sort of problem to manage - temps from a solar panel using two-sided reflectors can easily exceed 150 degrees, thus my concern over a heat-resistant plastic for ducting - 

gravel bed will be approx 4' x 6' x 25' - fully enclosed in salvaged barn metal with a poly moisture shield on sides and top - located in the center of the barn with a minimum of 3' of dirt on top - since dry soil is a poor conductor of heat there will be a network of drip tubes extending nearly wall to wall at about a 2' depth - i anticipate the moisture will draw heat from the gravel bed and extend it to most of the floor area - 

my collectors will be the usual glazed insulated boxes with the exception of using salvaged aluminum printing plates painted black as the heat surfaces - the plates will be formed into an accordion-pleat profile that will be at least five times as deep in the pleat as is the distance between pleats - this should allow multiple reflection points within the pleat thus allowing for maximum opportunity for heating - i plan to attempt moving the air with solar-powered fans - 

i don't expect this design to be nearly as efficient as might be possible - but that peak efficiency must be traded off for a workable budget -


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