Hi, All!

I don't know if that's true for all new furnaces now. But, I just had a new Goodman furnace installed and the installers ran two PVC pipes out my basement wall.

I don't know why they did that instead of using the existing stack.

Also, they used two pipes. Why would that be needed instead of using just one, please?

Thanks!

High efficiency furnaces remove most of the heat value of the gas being burnt so there is little heat energy left to allow the old style venting process that depended upon the residual heat left in the burnt gas to rise naturally up the pipe and out the stack. The old style furnaces also used indoor air that you already paid to heat to be used as combustion air which was also a great waste of energy.

The new units work so well at extracting the energy out of the gas that they have to use a small fan to move the burnt gases outside since there is little energy left and since there is little energy left that also means that the heat is all used up in the furnace and the exhaust temperature is so low that plastic pipe can be used to exhaust it. Also another energy saving fact is the indoor air is no longer used for combustion and the second pipe you mentioned is used to bring outside air in for the combustion process. All in all a very energy efficient system but to do all this energy saving stuff there are a few more parts added and thus they are a bit more of a problem cost wise to repair.

They should have also blocked off the old stack hole that was being used for the old furnace since that is no longer used during the install of the new furnace.

Thank you again hvac1000, for an outstanding answer!

Yes, they did block off the old stack hole. But, the one for the hot water heater was left in place.