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Posts: 1,068
Reply with quote  #21 
You could be right John. The fellow in the article is a retired 'radiant' heat installer so his experience would have made a hydronic system more familiar to work with.

A general question for you. What type of control system is used when warming the soil? Is there a concern about overheating and cooking the roots?

Both temperature rise and airflow are integral to comparing hot air collectors


Posts: 27
Reply with quote  #22 
I use differential thermostats which runs the fan based on the temperature difference between the air and the soil. Its not easy to overheat the ground using naturally heated greenhouse air, the highest mine ever gets is 80F.
If you put black plastic plant pots in a sunny spot, the soil and roots on the sunny side of the pots can be subject to 140F or more, the plants wont be too happy but they`ll usually survive it..
A solar air root heater would run at much lower temps than needed for home/space heating. The air entering the collector would be at the soil temperature, perhaps 40-50F worst case. In a closed circuit, the air only needs to be accelerated once giving you the option of using cheaper axial fans. I imagine the ideal layout being a raised bed running east/west with a full length solar air heater on the back (facing south) which puts the inlet and outlet right at the ends of the bed.
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