I have a lux 1500 thermostat. When I turn the heat on, I hear it starting up but there isn't any heat coming through the vents. At 1st Filter was flashing on the thermostat so I replaced it. I also replaced the batteries. I t worked well for about 5 minutes then nothing. I turned it off then back on again. It started up and I hear it but the heat still isn't blowing through the vent. I don't know what to do