"Edwin Pawlowski" wrote in message
Does the blower cause a negative pressure in that area? Very POSSIBLE
Would a negative pressure cause a downdraft in the chimney? YES

Check that the return air ducts are functioning properly or if air is
being lost elsewhere to create the negative pressure in the AC area and
thus, the downdraft.


I just had another thought on this. Previously, someone mentioned watching
the pilot light when you have somebody turn on the blower. Good idea, but
carry this a step further. I mentioned negative pressure in the area of the
AC causing the down draft.

Watch the pilot light while the AC is turned on. After the blower gets up
to speed, have that person open a door upstairs. Opening the door will
allow air to leave the house while more is needed to feed the blower, thus
it may pull air down the water heater chimney. If there is a door to the
basement, close that while the upstairs entry door is opened.