Our Heil gas furnace and central air were installed in 2004. Last summer the a/c quit working, I replaced the thermostat, which did not solve problem. A serviceman replaced a part on the outside unit & also the 5 amp fuse on the furnace circuit board( I know this because the old one was left on top of unit). A few weeks ago we turned on the furnace, fan runs but no heat. I replaced the igniter assembly & made sure everything was clean including vacuum hose going into motor. There is no diagnostic code on smart valve, light is solid green. Any ideas on what else I could try? Thank you!