Recently installed a new water heater. Water heater vents at a 45 degree angle and joins the furnace flue. Due to the new water heater I have been scrutinizing it's performance and noticed that when the furnace is running, there is a backdraft down the water heater vent and out the top. The air is so hot that it is capable of warping the plastic molding around the in/out holes on the top of the water heater. I never noticed this before, but as I stated, have just started to pay close attention to the new water heater. My assumption is that this has been happening for some time. CO detectors have never gone off, but this is very concerning. I have a wife and 2-year old that I love very much and would appreciate any recommendations.

More than likely there is debris blocking the vent pipe after they tie together,most likely right where they tie into main flue if it is a masonry chimney.