There are several reasons for the lack of heat, even if the thermostat is on. One typical culprit is a dirty filter. A blocked or clogged filter will slow down the efficiency of your heating system. If the air filter is dirty, make sure to replace it. In general, you should replace a filter every two months. More reasons for no heat include: Dirty ignitor pilot The thermostat is on the wrong setting Battery issues Your gas furnace's gas valve isn't open The heater is cut off from the power switch If your furnace isn't producing heat and you're unsure why call the professionals at Hales AC to diagnose the problem.