Here’s less expensive way to heat your pond using your house duct work, if you have an electric furnace.
First the pond needs to be tented to hold the heat in, next your house needs to have a heat pump and/or a forced air electric furnace. Then you can tap into the main duct work and run a branch line from the house to the pond. Now every time the house furnace comes on it will sent warm air out to the pond.
I have found this system to be considerably less expensive than running direct submersible electric heaters.
This system is not recommended if you heat with a combustible fuel like oil or gas, because they give off enough harmful exhaust fumes to be dangerous to the fish.
I tent my pond by using two layers of clear 4 mil greenhouse plastic. You can also use the clear 4 or 6 mil construction plastic, but it’s stiffer and doesn’t let as much light thru. There is a tradeoff between allowing in enough light verses holding in the heat; also two thinner layers will be less apt to tear in the wind than one thicker layer.
The size of the duct will vary based on the size of the pond, the distance from the house to the pond and what the outside temperatures are. My pond is 22 x12 ft. (8,500gal), the tent peaks about 4 ft. above the water, my run is about 12 ft. and our winter temperatures can hit the low teens. So I find that an 8” duct works well for me. I can hold the pond temperature for most of the winter in the mid to high 50s, dropping to the high 40s for the few really cold days.
The duct work needs elevated off the ground and wrapped with any standard house fiberglass insulation, then over wrapped with a water proof plastic covering. The better its insulated and kept dry, the less heat loss you will have. You can also use the flexible insulated duct, but it has more line resistance than metal duct and is not as well insulated.
The duct to the pond works best stubbed just under the tent wall and aimed horizontal across the water toward the center of the pond. Be sure that the end of the duct stays wide open and is properly supported; we don’t want it to collapse & restrict the air flow or to slip down into the water.
Another advantage to this system is that by adding clean fresh air, it forces the naturally occurring CO2 gases out from under the tent. This helps maintain a good dissolved oxygen level in the pond and I have been able to maintain a nice healthy green algae coating all winter.