I have a 2.5 ton 2 stage Goodman furnace. It's about 5 years old. Problem is sometimes when I start it I can hear the blower start up but no heat just cold air. This is a intermittent problem sometimes it heats but most the time it doesn't. I've maintained it according to the manual, filter is clean no obstructions anywhere. I changed the batteries in the thermostat. I pretty much just turn it on and off the same way since it was installed.