Septic tanks are large concrete tanks buried outside of homes in rural where there is no municipal sewage connection. Waste water goes down through the drain and out to the tank to be broken down by bacteria before being drained out in a drainage field. When the bacteria break down the sewage, they produce heat, which can be captured using a closed loop heat exchange system. This system can complement your standard forced-air furnace.
Things You'll Need
- Drill motor
- Concrete drill bit (same size as tubing)
- Copper tubing
- Liquid biothermal heat exchanger
- Tubular insulation
- Radiant heat exchanger
- Water circulation pump
Dig out the door to the septic tank using a shovel. It will be three to six feet under ground and on the top of the concrete tank. You can use a backhoe tractor to dig up the door, but you must be very careful not to hit the top of the tank with the bucket. Hitting the tank's top can damage it, ruining the septic tank.
Pry open the door using a crowbar. You will probably need help since the door is often a few hundred pounds in weight.
Drill 2 holes through the top of the door using the drill motor and concrete bit, then feed about a foot of copper tubing through each hole from the outside to the inside.
Set the biothermal heat exchanger into the liquid inside the septic tank, then connect the input and output to the copper tubes sticking through the door. Make sure you keep track of the input tube and the output tube, since it will be important to keep them straight later.
Replace the door then caulk around the holes where the copper tubes leave the septic tank. The caulk will prevent dirt and water from leaking into the tank and odors from escaping the tank.
Run the copper tubes to the house and inside to the furnace. These tubes will carry the cold water to the septic tank and the warm water back to the furnace. Use the tubular insulation to insulate both of these tubes.
Insert the radiant heat exchanger in front of the furnace's fan and fit the duct work around the heat exchanger's frame to form an air-tight seal.
Connect the tube going to the output of the septic tank heat exchanger to the input of the furnace heat exchanger and connect the other tube to the output of the water circulation pump.
Run a short piece of tubing from the radiant heat exchanger's output to the input of the water pump. Connect only the water pump's side of the tubing then fill the loop with water using the open tubing. Run the pump while you fill the system to make sure it circulates throughout the loop. Once the loop is full, connect the other end of the tube.
Tips & Warnings
- This system can also be used with a hot water/ steam heating system, however you will use a liquid-liquid heat exchanger at the furnace and you will need to modify the heating controls to allow the heating fluid to circulate below optimum temperature. The septic tank will never get warm enough to trigger the standard thermocouple to circulate the warm water through the house.
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