Gas furnaces are quite effective at heating a home quickly. Under normal conditions, your furnace should run two to three heating cycles each hour, with each cycle lasting for 10 to 15 minutes. It may run longer or more frequently during extremely cold weather when the outdoor temperature is below zero, but it still should only run sometimes.

If you do find that your furnace is seemingly always running, it often indicates that there’s an issue that’s limiting how much heat it produces. This may be something you can easily fix on your own, but it will usually require having a furnace technician inspect your heating system to pinpoint the specific cause. Let’s take a look at some of the different issues that can lead to a furnace running constantly and how they can be fixed.

1. Clogged Air Filter

An air filter is an essential part of any central heating system, as it traps dust and debris to prevent them from clogging up or damaging any furnace or blower fan components. As your heater runs, the air filter will continue to collect more and more debris to the point that it starts to become clogged.

A clogged filter will result in very little cold air being drawn into the furnace and much less hot air blowing out of your vents. This will cause your unit to run for much longer since it isn’t producing nearly as much heat.

For this reason, the first thing you should do if your furnace starts running more is to make sure that the air filter isn’t too dirty. If the filter looks black or you can’t see through it when you hold it up to the light, then you should replace it as soon as possible. You can also easily avoid the filter becoming clogged by making a note to check it every few weeks and replacing it at least every one to three months.

2. Dirty Gas Burners

Whenever your furnace sits idle, dust and debris will collect on its gas burners and can start to clog the burner ports. The ports can also slowly become coated in soot, and you may even have small spiders that make webs in the burner ports over the summer. These issues are why you should always have your furnace professionally maintained every fall. A technician will inspect and fully clean the burners to prevent any issues.

Dirty burners will typically lead to the gas that flows out of them not fully combusting. If the gas doesn’t fully combust, much less heat will be produced, and your furnace will run longer each time. One easy way to check for this issue is to inspect the burner flames when the furnace is lit. If the gas is combusting fully, you’ll see strong, steady, blue flames coming from all the burners. Orange, yellow, or weak flames indicate incomplete combustion.

3. Ductwork Issues

Your furnace can also run more often if your ductwork is damaged and leaking air or not fully insulated. Air leaks will allow hot air to escape and limit the effectiveness of your furnace. A lack of insulation around the ductwork in your attic or crawlspace is also a major issue, as it will lead to the hot air cooling down quite a bit as it travels through the ducts.

4. Malfunctioning Thermostat or Incorrect Thermostat Setting

Your furnace could be running constantly because either your thermostat isn’t set correctly or is malfunctioning and not properly signaling the furnace when to turn on and off. In terms of settings, it could be that you have the temperature set too high or that you have the fan set to “On” instead of “Auto,” which means the blower won’t ever shut off. While adjusting the thermostat settings is obviously something you can easily do, you’ll need to have a technician inspect your thermostat to see if it’s causing your furnace to run constantly.

5. Old or Undersized Furnace

It may also be that your furnace is too old or too small for your home, both of which can lead to it not producing enough heat to keep your house fully warm. If your furnace is more than 10 to 15 years old and suddenly struggling to keep up, we recommend replacing it as soon as possible. If a technician determines your furnace is undersized, your only real option will also be to have it replaced with a larger unit.

6. Faulty Limit Switch

The limit switch plays several important roles in a furnace, and one of these is signaling the unit to start and stop. Whenever the thermostat senses that your heater needs to run, it sends a signal to the furnace’s control board. The control board then activates the limit switch, which closes the circuit so that the furnace can start. Once the thermostat signals the furnace to turn off, the limit switch opens and the furnace shuts down. The limit switch will also open and shut down the furnace immediately if it senses that the equpiment has started to overheat.

There are times when the limit switch can break and become stuck closed. When this happens, the furnace and blower won’t ever turn off unless you flip the circuit breaker so that no more electricity flows to the system. Should you ever have this happen, you’ll need to have a technician replace the limit switch before your furnace will again work properly.

7. Cracked Heat Exchanger

A cracked heat exchanger is an extremely serious issue since it will allow carbon monoxide to leak out into your ductwork. This issue can also cause the furnace to not produce much heat, as the hot combustion fumes will leak out and prevent the heat exchanger from ever getting fully hot. Unfortunately, a cracked heat exchanger often means you’ll need to have your furnace replaced.

8. Malfunctioning Blower Fan

A malfunctioning blower can also limit the amount of heat a furnace produces. Specifically, if the blower fan isn’t running at normal speed, less cold air will come into the furnace and less hot air will blow out of your vents. This most commonly happens when the bearings that drive the blower motor start to wear out. This can usually be fixed by replacing the bearings, but if you wait too long, then you may need to replace the entire blower motor.

9. Lack of Sufficient Insulation in Attic or Walls

A lack of insulation will also make it so that your furnace has to run more often. If there isn’t enough insulation in your walls, lots of air will seep inside and your house will always stay colder. You need to ensure that your attic floor is well-insulated since this will help prevent the heat from your furnace from escaping.

If your furnace is running constantly or has any other issues, the technicians at Apollo Plumbing, Heating & Air Conditioning can quickly diagnose and repair whatever the problem is. We specialize in all types of heating and cooling repairs as well as maintenance and installation. We also offer a range of plumbing and indoor air quality services. Give us a call today to schedule a furnace inspection or any other HVAC service in the Vancouver, Portland, or Boise areas.

Meet the Author
Brandon Bird
Brandon Bird

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