Ventilation fans built into the attic can pull a large volume of air through the home and in the old days were useful on summer nights in the absence of air conditioning. There are still quite a few of them around. The large fan creates a mega air exchange, displacing interior hot air through the attic while drawing cooler night air from opened windows.

Recently, a contractor was remodeling in a customer home during cold weather. He opened a window and switched on the attic fan to remove dust while he was cutting a row of tile with a grinder. It worked, but the heating system did not operate for the rest of the day. The customer called A B May to service the furnace. The service technician told her (owner) that a sensor had sent a false signal which tripped the shutoff switch of the heating unit. The next day, the contractor used the fan again and the furnace again shut itself down. While the AB May technician was in the act of resetting the unit's brainbox, the contractor switched on the attic fan. The tech ran up the stairs and seeing what had occurred, explained that the fan had created a 'negative draft' which was recorded by a vacuum sensor shutting down the system. The reverse draft, he explained, would have also sucked carbon monoxide fumes from the gas furnace and gas water tank into the whole house through the duct work.

The lesson - draft fans are powerful as well as being obsolete. Modern heating systems (gas/propane) are designed to sense negative draft and shut down. Attic fans were created to be effective in warm weather when all the windows are opened. When air conditioning is an option, it should be utilized and the ceiling fan removed.