I purchased a home in April of this year a brand new furnace and air conditioner was installed prior to closing. The air seemed to work fine, but the heat will come on only when prompted by turning the thermostat off then back on. It runs for about 20 minutes or so, then has to be tripped again. The led light is indicating the furnace is working properly. We put in a new thermostat and the problem still exists. Any suggestions where to start? The papers left here state it is a Goodman GMS9 or GCS9.