Hi,
I recently had repair man replace the exhaust pipes of the water heater unit and a separate heating unit, before he changed things, two separate pipes went from each unit into separate holes in the wall and into the flue, afterwards, he ran the water heater exhaust pipe into the heater exhaust pipe, and had the one exhaust pipe going into the wall, he then sealed up the other hole, where the water heater exhaust pipe was before. I was wondering if this could cause a problem of back flow of exhaust when one unit is on, and the other is off, causing exhaust to leak into the house.
Thanks very much Chris