If a home has a gas-fired water heater or furnace, one of the things I look for during the home inspection is signs of “back-drafting.” This occurs when the hot flue gasses are not efficiently traveling up the exhaust system and out of the home. Instead, the hot exhaust is spilling out around the top of the appliance, causing dangerous conditions, including carbon monoxide entry into the home.
There can be several causes of back-drafting and a professional HVAC (Heating, Ventilation, and Air Conditioning) technician should be brought in to diagnose and correct it. Some of the most common causes include; unbalanced air pressure in the home, improperly installed vent pipes, and incomplete combustion at the appliance.
This condition is a serious safety issue and should be addressed as soon as possible. It is also typically not a DIY project as fixing the cause will require advanced diagnostic and repair capabilities. My recommendation is to leave this one to the pros and call your preferred HVAC company.