There are a few things that might cause your furnace to “ignore” a call for heat from the thermostat. I have addressed them below and suggest a few things you can try to fix the problem. However, when in doubt, consult a licensed HVAC technician to assist you.
Filter – The most common cause of furnace problems is a dirty filter. If you furnace filter is clogged, the airflow will be restricted. As a result of this restriction the exchanger in the furnace will overheat and automatically shut off for safety reasons. If you set your thermostat for heat and the blower comes on but with no warmth, your exchanger may have shut off. Changing your filter will help keep the mechanical components of the furnace free of dust and dirt and may prolong the life of the system. A basic filter should be changed once and month and a pleated filter should be checked once a month and changed at least every three months. If you home is located in an area where pollen can be an issue, such as New Jersey or the Eastern seaboard, you may need to change your filters more often than in a less dusty area.
Thermostat – If your thermostat isn’t properly functioning, your furnace won’t respond in the correct manner. First and foremost, check to be sure it is set to “heat”. This simple step is often overlooked. Make sure it is securely attached to the wall and no wires appear loose or frayed. If it is a programmable unit, be sure the time and date are correct. Also, be sure to check the batteries so in the event that there is a power failure you won’t lose all your settings. Test it by setting the temperature five degrees higher than the temperature of the room; see if it comes on (any programming can be overridden by holding the temperature up arrow and holding it for several seconds until the called for temperature rises: then hit “hold”). If you hear the fan respond you know that your thermostat is functioning as it should.
Electrical – Be certain your red emergency switch is on; It may have accidentally been switched to the off position over the summer. Next, check the circuit breaker at your electric breaker panel. Check for a blown or tripped fuse. It’s a good idea to scan the panel for any loose wires or evidence of other issues. If you find anything more complicated than a tripped fuse you should call a qualified heating repair technician.
Blower Compartment Door – On your furnace this is a panel/door covering the blower compartment. This door, when properly installed, trips a switch that signifies to the system that is sitting correctly. If the switch isn’t engaged, the system will not respond.
Pilot – Most furnaces have an automatic electronic ignition. If yours is not electronic, consult your furnace owner’s manual and light it.
Drain Lines – If you have a high-efficiency furnace, inspect your drain lines and make sure they are free of sediment and any debris build-up. Flush the hose out with a bleach solution if it appears dirty.
Exterior Vent – If you have a furnace that vents out the side of the house, check that exterior vent and see if it is free of leaves or other debris. Sometimes birds and rodents use vent openings to make a nest so this step is particularly important in the spring and summer months. Ensure this hasn’t happened. Your furnace needs to vent properly to function properly.
A few simple steps can save you time and money. There is a lot of helpful information on the web. One site I found to be particularly well-executed is the following: http://www.familyhandyman.com/DIY-Projects/Heating—Cooling/Furnace-Repair/simple-furnace-fixes/View-All. If your furnace is still not functioning properly after following my suggestions and possibly some of theirs, be sure to consult a qualified professinal. Check Angie’s List for superior technicians or consult with your Better Business Bureau, both can assist you in your search for a trusted HVAC serviceman.