This project has been in the works for awhile as I had just been relying on about 1000' of 1/2' black poly tubing strung in the GH rafters and water pumped through it for an additional heat source. The solar heat worked ok (barely) last winter to heat the 9000 gallons or so of water in the GH. On a cold night I could lose 5 degrees in the water and the solar was only capable of a 4 degree rise per day. It worked out but if I would have had a long stretch of cold nights and cloudy days I would have been in trouble as the fish can only take to about 45 degrees and even that is very bad. I really wanted to make it so the water did not get below 60 degrees
The design is pretty simple, all it is is a 300 gallon tote with a wood fired hot tub heater inside it. These heaters are designed to be submerged in the water with just a few inches of them above the water line. The heaters are very efficient due to being submerged much more of the heat is transferred to the water. The tote is located outside of the GH about 40 feet away. There is a 1" run of PVC pipe coming from the additional pump that feeds the water filled tote. The second line which is 2" is encased in 4" tubing and that tubing was then filled with perlite cement to insulate it. So the 1" line pumps water to the tote and the 2" line takes it to the main pumps sump barrel simply by gravity as the new tote is exactly the same level as the totes in the GH.

I built an enclosure around the tote and filled it with 16 cubic feet of perlite for insulation. After the fact I do not believe that insulation was needed at all. The 2" line that takes the hot water out of the tote is located about 3" below the surface of the water and when you light the heater while the water deep in the tote is still very cold the surface water gets hot fast. In 20 minutes after lighting the fire the surface water temperature will be over 100 degrees and even once the pump is turned on the water never dips below 79 degrees. After an hour or so of burning the temperature rises to 86 and tends to stay there. Running the heater for 12 hours my goal was to be able to raise the 9000 gallons of water by 8 degrees and has far exceeded my goal. In 8 hours of run time I gain 10 degrees+ and that is without the help of the solar tubing. Being so small it does not burn much wood at all, it is basically a small rocket stove.

This picture was before I put the top on the tank which is also twinwall or filled the side walls with the perlite.
The design is pretty simple, all it is is a 300 gallon tote with a wood fired hot tub heater inside it. These heaters are designed to be submerged in the water with just a few inches of them above the water line. The heaters are very efficient due to being submerged much more of the heat is transferred to the water. The tote is located outside of the GH about 40 feet away. There is a 1" run of PVC pipe coming from the additional pump that feeds the water filled tote. The second line which is 2" is encased in 4" tubing and that tubing was then filled with perlite cement to insulate it. So the 1" line pumps water to the tote and the 2" line takes it to the main pumps sump barrel simply by gravity as the new tote is exactly the same level as the totes in the GH.
I built an enclosure around the tote and filled it with 16 cubic feet of perlite for insulation. After the fact I do not believe that insulation was needed at all. The 2" line that takes the hot water out of the tote is located about 3" below the surface of the water and when you light the heater while the water deep in the tote is still very cold the surface water gets hot fast. In 20 minutes after lighting the fire the surface water temperature will be over 100 degrees and even once the pump is turned on the water never dips below 79 degrees. After an hour or so of burning the temperature rises to 86 and tends to stay there. Running the heater for 12 hours my goal was to be able to raise the 9000 gallons of water by 8 degrees and has far exceeded my goal. In 8 hours of run time I gain 10 degrees+ and that is without the help of the solar tubing. Being so small it does not burn much wood at all, it is basically a small rocket stove.
This picture was before I put the top on the tank which is also twinwall or filled the side walls with the perlite.
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