I have an old Sears Power Miser Natural gas furnace. It doesn't always come on, but when it does the thermostat clicks and a buzzing starts. You can hear the gas flame start and then 10 seconds later it shuts off. If I turn the fan on manually (before the 10 seconds is up) the flame remains burning and the furnace heats the house sufficiently until I turn the heat off and run the fan until cool air is coming out of the vents.