Pilot light blows out on the gas water heater when the attic fan is turned on unless the door to the furnace room is open ,it is a older house. The water heater is in the basement if that means anything

Hi, That could mean that there is a hole or a break in the burner vent pipe that is sucking large volunes of air up through the gas burner assembly into the attic. Have you traced out the vent pipe and inspected it particularly in the attic? This could be a significant health
(Carbon Monoxide) or fire issue. Just a thought... Please inspect it today... Let me know...