A week ago we woke up to no heat. The fan would not cut in when the furnace came on. and it appeared the thermostat's battery wasn't working. So we replaced the battery and the fan cut in once and then not again. We called someone in and they replaced the thermostat and claimed the board also needed to be replaced. However we could function with the fan turned to manual. This worked for a week, now the fan still works but the furnace doesn't cut in anymore. Any suggestions? Was the compancy who assessed the situation telling us the truth?