I while back our company got a request to install 2 furnaces at a remodel. At the time we were too busy and had to turn the work away. more recently we were asked to install a high efficiency water heater at the same house. While installing the water heater i saw something that concerned me
there are two rheem 90% furnaces. they each are vented with 2 inch pipe, but halfway accross the ceiling a Y fitting was used to combine the two pipes into one 3 inch pipe. this pipe has a couple elbows and about 10 feet of pipe and then exits the house. on the outside of the house it was an elbow pointing down whitch ends about 2 inches above the ground.
Does any manufacture allow common venting on 90% models? these work fine, but im concerned about exaust from one unit backing up through the other when only one is running. since these take their combustion air from the basement, any gasses that back up would escape into the house.
should we recommend to the home owner that all of the venting be redone so each furnace is vented individually? obviosly if its a problem it should be fixed by the installing company, but judging by the rest of the work they look like hacks, so i doubt they will be coming back