[
3) Just occurred to me that we recently installed a radon mitigation
system. The system sucks air beneath the basement floor. This reduces
the air pressure in the basement. Could this be the cause, i.e., the
exhaust from the furnace actually come back to the basement because of
the low pressure.[/quote]

YES YES YES

Gas Engineers UK / Gas Technicians USA

Test the flue pull or chimney draw by closing all doors and windows enclosing the gas appliance and turning all air extracting devises ON...A smoke test or spillage test is performed on the gas appliance to ensure that the flue functions at its worst conditions...

In the USA folks have furnaces in basements...Something like a wood stove installed in the basement is powerfull enough to create a suction on the gas appliance...any suction or negative pressure around an open flued/conventional flued appliance is VERY VERY BAD....It can cause appliance failure leading to C/O poisening...

Even a door way to the basement left open can cause a flue pull through the house leading to a suction on the gas appliance vent pipe...


Solution ......every time you add air moving devises to the room with an open flued appliance get a gas engineer to safety check the applaince and possibly upgrade the VENTILATION REQUIREMENTS ..

p.s. buy a carbon monoxide detector for your house...its like a smoke alarm and cheap too....