I have a Heil gas furnace. It is a high efficiency type built in 1983. When I went to change the thermostat from cool (summer mode) to heat (winter mode) the furnace came on but only blew cold air. I went to the unit and pulled the cover panel off. I saw that there was no combustion flame. The combustion air blower was on but no flame. To trouble shoot, I replaced the furnace filter, checked the discharge pipe to make sure it was not plugged, and then pulled the vacuum line off the combustion air blower. I slightly sucked on tube going to the vacuum switch and the pilot ignited and gas came on with full flame. As soon as I sopped applying suchion to the vacuum switch the flame went out.
Does this mean I have a bad vacuum switch or could it be something else. I even sucked on the line, got flame pinched the line and the unit ran through a complete cycle until the upper heat exchanger sensor told it to shut down flame.