Originally Posted by
tipsrfine
It is a definite possibility that your water heater is spilling flue gas at start up, and then vents properly after it has run for a while. The cause of this could be that your openings to your return ducts are depressurizing the basement. When the flue gas spills from the water heat flu pipe it is sucked into your air handler via the return side and then distributed to your supply vents.
Get someone out their to do some pressure testing of your basement and testing of the water heater venting.
The reason why this does not happen in winter can be due to the fact that it is colder outside, so your water heater flue gas vents quicker/easier. Also, your furnace runs in winter and that assist the venting process for the water heaters.