The end of the year is often a festive time, but a furnace that doesn't work well can turn those cozy family holidays into cold and miserable affairs. Of course, not every heating problem means that your furnace isn't working at all. In many cases, you may find that furnace simply can't keep up with the heating load, resulting in long run times and a chilly house.

There are numerous reasons why your home may take too long to heat up during the winter months. This guide will help you understand how your furnace works and why you might find yourself shivering under a blanket as you wait for your interior temperatures to creep up.

The Basics of Furnace Sizing and Operation

Most residential furnaces have two modes: on or off. These straightforward designs engage the burner and blower when the thermostat requests heat, shutting off once your home's temperature reaches the desired setpoint. Two-stage furnaces and variable blower units can run at less than full blast, but these furnaces are still relatively uncommon.

Since most furnaces only operate at one speed, sizing a heater to your home's heating load is critical.  When your contractors installed your furnace, they performed an evaluation that included everything from your home's size to the insulation values of your windows. This evaluation (known as a Manual J calculation) is essential to selecting a furnace that can efficiently heat your home.

A poorly-sized furnace will behave inefficiently in several ways. A furnace that's too small will run for too long, potentially never reaching your thermostat setpoint on frigid days. On the other hand, an oversized furnace can short cycle, which can cause temperatures to fluctuate wildly between too hot and too cold.

Common Causes of Slow Heating

If your furnace has always struggled to keep up on cold days, your heater may be undersized for your home. Unfortunately, there's not much you can do to improve this situation aside from installing a new furnace. You may also want to consider an energy audit since poor insulation or drafty windows can increase the load on your furnace.

Less drastic causes include inadequate airflow and thermostat issues. Anything that restricts the airflow through your system can reduce its heating ability, forcing your furnace to work harder and run longer. If you can't feel much air moving through your vents, start by changing your filter, and then call in a professional to evaluate your blower if a new filter doesn't resolve the issue.

Finally, a failing thermostat or improper settings can also leave your home too cool. If you have your fan mode set to continuous ("on") or circulate instead of auto, your blower may be pushing cool air around your home even while the furnace isn't running. A faulty thermostat may also stop requesting heat too soon.

A correctly sized and operating furnace should heat your home relatively quickly. If yours doesn't, it may be time to schedule a visit with your local heating services contractor.