Are all furnace problems black and white? It might be easier if your furnace either worked or didn't, but some heating issues can fall into areas that seem a little grayer. Your furnace might seem fine on most days but struggle when the outdoor temperatures drop into the teens or single-digits. Diagnosing an inefficient furnace can sometimes be more troublesome than diagnosing one that won't turn on at all.
While you should always contact a heating professional to solve these problems, it can help to understand how your furnace works and why it may be behaving inconsistently.
How Your Furnace Provides Heat
A single-stage furnace only has two modes: on and off. The furnace keeps your home warm by waiting for the thermostat to call for heating, after which the furnace turns on and runs until the thermostat is satisfied. When everything is working, the thermostat will continue to call for heat until it detects a temperature equal to your setpoint.
The three factors that influence how quickly your furnace can heat your home are the furnace's heat capacity, the difference between your set point and the outside temperature, and your home's insulation. More powerful furnaces heat more quickly, well-insulated homes retain heat for longer, and a lower set point on the thermostat means your furnace doesn't need to work as hard.
Assuming you don't adjust your thermostat, your furnace will need to work harder on colder days since it will be fighting against a lower outdoor temperature. If your home isn't well-insulated or your furnace is undersized, it may struggle to reach your set point quickly. However, if neither of these factors has changed, there may be another issue.
The Reasons Your Furnace May Struggle
Your furnace may find it harder to keep up on cold days due to poor airflow or an inefficient flame. Common causes of the former case include failing blower motors, clogged air filters, damaged ductwork, or obstructed vents. If you notice your vents aren't moving much air, you may want to check your filter and ensure you didn't place furniture or other items in front of your return vents.
An inefficient flame can be a more severe problem. If your furnace has a sight glass, you can use it to check the condition of the flame. A bright, steady blue flame means your furnace is working correctly, while a wavering red or orange flame can mean trouble. A dirty flame may also produce excessive exhaust fumes, so you'll want to have an HVAC technician investigate as soon as possible.
In either case, your furnace may produce enough heat and airflow to keep your home warm on days when the temperature isn't quite so chilly. A correctly sized furnace should be able to keep up with any typical temperature in your area, however, so you shouldn't have to live with a heating system that struggles on cold days.
If you notice any of these issues or think your furnace is having trouble, contact a heating repair professional to come inspect and repair it.