Gas and Oil Home Heating Furnaces - central heat only works with fan

The heat wont come on when auto is set, only when the fan is turned on will the heat work. You have to manually turn the fan on and off to get heat. The a/c works fine.

You did not specify the furnace type, but assuming a gas furnace, there are two prime suspects in this case. The 1st more likely than the 2nd. #1 Most use a heat sensitive switch called a Fan/limit. On upflow models this is found by removing the furnace cover panel & looking just above the gas valve. It may have a seperate cover itself. Remove it. If there is a dial beneath you have found it. This rotates as the furnace heats to close contacts energizing fan motor with 120v.AC. Look closely most have terminals marked for fan & limit.Do not let dial face turn if you change set points or calibration will be lost. #2 Fan motor low speed winding may have an open. (high uses less winding so may still operate for cool) #3 Outside chance....Fan relay normally closed contacts may have failed opening low speed crcuit. Good luck.