Originally Posted by kazoo

The small fan that is running is called a draft inducer blower. It is the first device ordered to start by the control board upon a call for heat by the thermostat. One problem with the units of that vintage is that all the little kinks of meduim-high efficiency weren't worked out. From that draft blower is a piece of soft tubing that loops over to a diaphragm with a microswitch on it. Pull the tubing off the diaphragm (you don't need to shut anything off) and hold it down from the blower. Expect a half-teaspoon of water to come out. Reattach the hose to the diaphragm, and there should be an almost immediate 'click' as the diaphragm verifies blower operation and closes the switch. Once that switch closes, the hot surface igniter is energized, heat feedback authorizes pilot, cad cell eye verifies pilot, main flow gas is authorized, and main burner fires. Hot surface igniter is de-energized, and cad cell eye monitors main burner and will cut off gas if flame fails, rollout heat sensor activates, high limit safety activates, or blockage occurs in the flue, which would cause the diaphragm switch to open. Otherwise, once the heat exchanger is warmed, the main fan will come on and you'll have heat until the thermostat is satisfied.