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Unfortunately the colder temperatures that have recently come have left me with an excruciatingly high gas bill to heat my greenhouse hot enough to keep my tilapia nice and productive. I have had to lower the thermostat to 55F, and the water temp is hovering just above 60F, and my fish's appetite has declined. I have a design brewing for a RMH but that is still down the line, and my conventional (expensive) heater will have to suffice for now. So I have been thinking of other ways to increase my water temperature. I have heard people talk about how if you're not careful with a NFT system the water can really heat up, this gave me an idea. My idea is to create an extra ten foot loop of 4" PVC in the gravity fed line to my flood and drain GBs. This loop would be lower than the outlet into my grow beds so it would stay filled with water. I will paint it black and place it in the direct sun. My questions...

Would this increase the temperature in my 500gallon system enough to justify the effort, despite the heat loss from my flood and drain GBs?

How big of variant in air/water temp can you have before evaporation and condensation become a problem?

Does anyone have any other suggestions?

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I think you're definitely on the right track as far as looking into a possible solar option for heating your system. Insulating the fish tank would also help with this being the largest water storage in your system. Laying compost around the outer edges of the greenhouse during the winter would also help with heating. I do have one idea using the 55 gallon plastic drum painted black. Tie in a three-quarter inch line from the top of the barrel to the top of the fish tank and you tie another line of the same size or smaller from the bottom of the barrel to the bottom of the fish tank. If you know how a engine block heater works, in theory that should do the same thing. As the sun heats the barrel during the day it should drive fresh cooler water from the fish tank in through the bottom of the barrel and heated water out of the top. Put screens were your pipes are inside the fish tank prevent fish waste getting inside there. Also the upper pipe would still have to be below the fishtanks constant water level. You will want to install valves to control the heater. If your electrical savvy an in-line thermostat controlling two solenoid valves would be the best option for automated control of the heater. I have not had a chance to test this design myself but I believe in theory this could work very well with no pumps required and I have seen these plastic barrels filled with water left out in the sun reach temperatures of 140+ degrees. If you feel you do need a pump to circulate it better I would add a very low flow pump at the bottom pipe inside the tank and control it with that same in-line thermostat. The only way to know for sure what will work best for you however is to just test your ideas good luck!
You can also disable your siphons and run constant flood. Siphons in gravel beds are very effective heat exchangers and if the air is cold they will chill your water.

I'm not sure if your pump is running constant or not but if it isn't and you change it to running constant you will gain some heat from the water pump.

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