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Contributed by: JonJaffer
A common alternative to venting up the chimney is venting into the sewer pipe. Depending on how your house is built, if you push air into your sewer pipe, the air may travel up through your house sewer standpipe. However, it is possible to vent directly into the actual sewer system.
Venting into the sewer is generally safe. If someone is working in the sewer nearby, they might smell the sweet pungent aroma of your crop (Not likely at night), but they have no way to tell where on the block it is coming from. Since the pipes large enough to walk in are quite far away from individual homes, the smell has lots of time to dilute.
[Editor's note: the stinky sewer will also hide your crop's odor up to a point. The sewer system itself is sealed, but you should check nearby manholes covers just incase for excessive blower noise and smell. The undeground sewer system will also remove and cool exhaust heat - excellent protection against infrared detection.]
 To start you will need to locate your sewer pipe in your basement. This is the pipe coming from your toilet(s) on upper floors.
Near the basement floor, there should be a "Y" connection at an angle, with a cap on it. Some houses have metal pipes, other houses will have ABS or PVC plastic sewer pipes.
 You will need a pipe wrench to remove the cap. Sometimes, if the house is very old, you will need to demolish the cap to get it off. You can buy a new one later at the hardware store if you need to.
This will reveal a hole 3 or 4 inches in diameter, depending on where you live, it may even be another size.
CAUTION: Fumes from the sewer can be poisonous, so make sure to use caution when removing the cap, and once a fan is installed, make sure to keep it ON all the time , so that there is no backflow into the home.
You will need to visit a hardware store, and pickup the various connectors and aluminum flex ducting to connect a fan to the hole. The most efficient fans for this purpose are centrifugal inline fans, but a squirrel cage fan will work too. The major difference is power consumption - an inline fan will use about 1/4 the electricity of the same size squirrel cage, they are VERY efficient, but a little bit noisier.
 The fan pictured here is 438 cfm, and only uses 115 watts. The inlet and outlet ports are 6 inches in diameter. I use a reducer from 6" to 4", and then I use 4" aluminum dryer ducting to the sewer hole, and of course plenty o' duct tape!
Once you are finished, you can test your work by standing outside the basement door, and holding a lighter to the crack at the bottom of the closed door. The flame should shoot inwards, because air is being drawn from the upstairs.
If you have successfully create this 'negative pressure' environment, there is no way that odorous grow room air can enter the house, hence the smell is contained!
[Editor's note: the sewer contains methane - a smelly and potentially explosive gas! Keep those gases away from your water heater pilot light!
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