You depend upon your furnace to keep your home warm and your family comfortable, which means it can be extremely frustrating if the furnace begins blowing lukewarm or even cold air. Depending on how cold it is outside, a furnace failing to heat could even be dangerous. The following tips can help you troubleshoot the problem as well as helping you know when it is time to schedule a repair.

Check the thermostat

There are a couple of thermostat issues that could affect the air coming from your furnace. First and foremost, verify that the fan setting switch is turned to "auto." If it is just set to "on," the fan will stay on at all times. This means air will be coming from your vents even if the furnace isn't currently running, which means there will be periods when it is just blowing room temperature air. If the thermostat is correctly set, you need to have a technician check that it is working correctly and properly calibrated. Sometimes all that needs replaced is a faulty thermostat, which is a quick and relatively inexpensive repair.

Use the right filter

The filter inside the furnace can also affect air flow, resulting in a furnace that seems to be heating poorly. When air isn't properly passing through a filter, the furnace may shut off prematurely while the blower remains on, which means unheated air comes from the vents. Clogged filters are common, but this is easy to avoid by simply changing the filter at the beginning of every month. Another common but overlooked issues is using the wrong type of furnace filter or installing it improperly. Make sure the filter is facing the right way in the filter housing. Also, use the recommended filters. Some of the double pleated filters don't allow enough air through, especially if you have an older furnace.

Inspect the ignition system

The ignition system on a gas or oil furnace is what fires the burner so the air can be heated. If it isn't igniting, then your furnace comes on but no air is heated. Start by checking the pilot light—it should burn with an unwavering blue flame. If it's out, use the push button ignition starter on the furnace to turn it back on. If this fails, you will need to schedule a repair. Sometimes a pilot light valve becomes blocked or even fails completely, which may necessitate a replacement.

In most cases, if the furnace is still coming on and blowing air, only a minor repair is needed to get it working again. If the above simple fixes don't solve the problem, contact a local furnace repair service for further assistance.