It's a Payne Gas Pack forced air furnace and A/C unit. At first the a/c functioned properly during the summer,now in winter the heat would'nt work. It's a unit that sit's on top of the roof in a apartment complex. First I thought it would be the tstat so I replaced it with a upgraded flux 400 series mercury low voltage tstat, that did'nt do it. Then I replaced the spark ignition module and gas valve and I also replaced the contacter and step down transformer. There is 120v going in to the transformer and 24v coming out to the ignition module,24v at the tstat. Nothing functions!

Look at the electrical diagram for the unit and identify the safety devices (limit switches, draft safeguard switches, flame roll-out switches, etc.). Locate and check these for any that may require manual reset.