We had an issue where we would turn on our heater and the gas seemed to ignite initially, but the air never got very hot and it wouldn’t re-ignite on the subsequent cycles. Turned out to be a broken igniter. Very easy to fix but it was hard for me to diagnose on my own because it seemed to be igniting at least once every time we turned on the furnace. The repair guy pointed out that one of the pieces of metal that connects to the electrical wire and enters the furnace was loose and should not be. Figured we would share as I was having some trouble finding a solution to this issue in my own research.