Some assume that the wider the connecting pipe from the toilet to the composter, the less chance of streaks going down it. That’s probably true; however, this connecting pipe can act as a chimney through which odors can back up into the toilet room. In fact, the larger the diameter of the connecting pipe and toilet seat opening, the greater the chance of odor.

• However, the taller the exhaust pipe, the greater the chimney effect and effective odor exhaust.
• To reduce the toilet-as-chimney effect, make sure that there is not a negative pressure in the toilet room, so air is not pulled up the connecting pipe.
• Disconnect or reverse the bathroom exhaust fan. (The composter’s exhaust can take its place).
• Be aware that whole-house attic fans, fireplaces, wood stoves, and air conditioners can pull odor from the composter into the building.
• Fans are built into many composting toilet systems, and usually their speeds cannot be regulated. These fans will pull air into your composter from either the toilet room or wherever the air intake is. Make sure this is not cold air that will cool your composter. Yes, that might come at a cost, as the composter exhaust fan can suck in as much as 100 cu ft of air per minute (100 cfm or 2.83 cmm) from the building, increasing your heating bill.
• That’s another reason why a fan-speed controller should be installed with a powerful fan (25 to 100-plus cfm or .7 to 3.0 cmm), so that you can set the minimum amount of fan power to control odor. Again, most composters will need only 1 to 5 cfm (.03 to 14 cmm) for adequate aeration for composting, so your primary issue with fan speed is managing odor.
• If the toilet room is cooler than the composter, heat might rise from the composter into the room.