My furnace does not seem to put out the same a mount of heats it once did. I checked the fuses in the access panel, checked the resistance on the coils and checked for proper voltage on the sequencers. all seem to be working properly, but I don't seem to have the same blast of hot air at the vents, there is alot of air flow, just not really hot air. do coils die out ? any thoughts would be greatly appreciated.