I have a gravity furnace. On one leg of the system I' ve Placed a small 110volt blower fan with a manual switch. Is there any type of switch that can be wired into the 24 volt furnace thermostat so the blower only runs when the furnace is on.. I've added this to give a little heat to the basement. Works great just runs too much.

All you need is a common relay with a 24 volt coil. All the forced air furnaces and central air conditioners use them. Look for a relay with a 24 volt coil and contacts rated for a little more than your motor is rated for at 110 volts. Hit one of the home centers stores to avoid questions about model numbers and so forth, or go back to where you bought the blower. Radio shack and other electronics stores sell general purpose relays too.

A more sophisticated system would be a temperature limit switch installed above the furnace that closes at 140 degrees, NOC. When the furnace came on, the switch would start the blower after the air warmed up and shut it off after the air cooled off when the furnace went off.