Imagine waking up or coming home to a frozen house because your heater never kicked on. Your first impulse may be to call a heating contractor for a repair. But before you do, here are a few things to check that may help you avoid a costly repair bill. Check that the thermostat is set to �heat.� This sounds obvious, but in many instances, an improperly set thermostat is the cause of your heater not working. Also, when you throw programmable thermostats into the mix, it can make things even more complicated. So, before you call a repair technician, check to make sure your thermostat is correctly set. Check the filter. If you're like most homeowners, you don't think much about your HVAC system until there's a problem. But, if you don't keep your air filters clean, you can wind up with various issues that impact the performance of your system. Dirty air filters impede airflow, which makes the system work harder and less efficiently. A thoroughly clogged filter can stop even the most reliable heater, and that's the last thing you need during winter. Check the gas. As with the thermostat, it's easy for the gas valve to get turned off and forgotten about. So, check that the gas is turned on and while you're at it, check the pilot light too. Check the exhaust flue. Another problem that can cause your furnace to malfunction is a clogged exhaust flue. It's not uncommon for birds or other critters to leave debris in the exhaust flue. Check the ducts. If it's only a few rooms in the house without heat, check the ductwork for blockages. Also, check to see if any separations have formed at the branching joints and repair them with duct tape until you can get them repaired by a professional. To avoid heater and furnace problems, it's wise to schedule furnace maintenance and tune-ups in the fall before the heating season starts.