Naturally, we expect the furnace to cycle on and off repeatedly in line with thermostat settings. Maybe you’re used to hearing your furnace run for about 15 minutes then cycling off for about the same period before roaring back to life.
However, sometimes the ON cycles may seem too long. Or worse, the furnace may continue to run even after passing the thermostat setting.
Although it can be scary, you don’t have to press the panic button right away as it could be something very small.
Below we discuss ten reasons your furnace may seem to run nonstop, sometimes even after passing the thermostat setting, and what you can do.
How Long Can a Furnace Run Continuously?
Standard furnaces typically run for 15 minutes continuously then cycle off. However, not all furnaces will run for exactly 15 minutes before cycling off. For instance, some furnaces run for just ten minutes then cycle off for ten minutes.
Meanwhile, others can run for 20 minutes continuously before cycling off. It depends on several factors, including age, type of furnace, size, and so forth.
For instance, furnaces tend to run slightly longer as they get older and weaker. Longer ON cycles allow the appliances to compensate for the weaker output.
Is it Okay for the Furnace to Run Continuously?
No, it’s not okay for the furnace to run continuously for too long. For one, heating cycles put a massive strain on the furnace. In fact, one of the reasons manufacturers design furnaces to cycle ON and OFF throughout operation is to allow some resting time.
Otherwise, the constant running can wear down the moving parts, resulting in faster wear and tear. Keep in mind that excessively long ON cycles also expose the furnace’s internal components to extreme heat, which can cause damage. For instance, the wires within the furnace are insulated with plastic.
The insulation can melt if the conditions inside the furnace become too hot. Typically, the furnace is ON (heating) half the time it’s running and OFF (powered but not heating) for the rest of the period. If it’s ON for longer, say a third of the time it’s powered, your utility bills would jump 50%.
So, if you paid $70 last month when the furnace was cycling normally, you can expect to pay $105 when it’s running 50% longer.
Reasons Your Furnace Won’t Shut Off with Thermostat
The following are common reasons why the furnace may refuse to shut down with the thermostat and what you can do in each case.
1. Faulty/Dead Thermostat
First and foremost, you need to ask whether the thermostat is in good working condition. Does it work normally? Or is it jumping values? Can you even see readings on the screen? Or is the screen blank or black?
Solution: Check whether it has readings on the display. If it doesn’t, the thermostat is dead, or the batteries are depleted. So, you need to change the batteries o replace the thermostat. If it has readings, perform other tests to rule out an internal malfunction.
2. Wrong Thermostat Setting
If you programmed the thermostat to go off at a specified time, but it hasn’t, you may have the wrong temperature setting. Specifically, you probably set the thermostat too high, perhaps at 80°F or higher. Furnaces find it extremely difficult to achieve and maintain such a high temperature at the peak of winter.
So, your furnace will run longer, typically beyond programmed schedules.
Solution: Fortunately, it’s easy to rectify thermostat settings. All you need to do is keep the settings between 68°F and 70°F in line with the DOE recommendations.
3. Electrical Issues
The furnace can also run nonstop and effectively refuse to shut off with the thermostat due to underlying electrical issues. For instance, an internal relay (the furnace has several) can become stuck in a closed position, thus keeping the furnace running until you shut off power at the breaker.
Solution: Unfortunately, diagnosing electrical issues can be a complicated process. Moreover, fixing electrical parts requires industry-specific know-how. So, it’s best to hire an HVAC pro.
4. The Pressure Switch is Stuck Closed
The pressure switch is a unique switch located in the blower compartment. Its primary purpose is to control the pressure inducer fan, which removes exhaust gases from the furnace. When you hear the furnace humming even before the heat begins to come out, that’s the pressure inducer fan.
This pressure switch can get stuck in a closed position, thus keeping the pressure inducer motor running for as long as the furnace gets power even if heating is OFF.
Solution: You need to clean the pressure switch and then reset it. Resetting the switch is as simple as pressing the button at the back of the switch.
5. The Limit Switch is Stuck ON
The high limit switch controls the furnace blower motor, the second fan on your heater. The blower fan has the sole responsibility of pulling cold air through the supply air plenum, forcing the cold air through the exchanger, and pushing warm air out of the furnace into the ducts via the supply air plenum and ultimately into your rooms. If the limit switch is stuck ON, the fan keeps running even when heating is off.
Solution: Regular maintenance can help prevent this problem. However, if it happens anyway, clean the fan and reset the switch. If the problem persists, replace the switch.
6. The Low Limit Switch is Set Wrongly
The standard limit switch has three temperature settings, i.e., low, ON, and high. The ON setting, typically 100°F, is when it comes on. The high limit (typically 200°F) is the temperature at which the switch opens to avert overheating damage.
And, the low limit, usually 90°F, is when the switch opens because the furnace is off. Suppose you mistakenly set the low limit to say 50°F. In that case, it will keep running even after heating is off as long as the furnace is switched ON because temperatures inside the furnace rarely fall below 60°F even when heating is off.
Solution: You need to set the limit switch temperatures correctly. More importantly, make sure the switch isn’t faulty as a faulty one can jump values even when the furnace is already running.
7. Dirty Filters
If you have dirty filters, the furnace cannot bring in enough air through the clogged filter. As a result, you may have very little warm air coming out of the supply vents.
Since efficiency is a matter of how fast the furnace can pull cool air into the furnace and how much warm air it pushes out through the supply vents per minute, the furnace may need to be ON forever.
Solution: Clean your furnace filters regularly. We recommend following the manufacturer’s guidelines. If not, clean it at least once weekly. Also, make sure to replace the filters at least once per month.
8. Leaky Ducts
Leaky ducts also reduce the furnace’s effective warm air output. For one, leaky ducts allow warm air to escape out of the “supposedly” closed heating system. Additionally, cold air from the surrounding environment can enter the ducts via the leaking holes.
In both cases, you end up losing warm air. If the leaks are significant, the furnace may need to run harder and longer to compensate for the lost heat.
Solution: You need two things here. First, always schedule a seasonal tune-up to ensure that your furnace is best prepared for the heating season. Additionally, take better care of the ductwork.
9. The Ducts/Vents are Blocked
Ducts can also become compromised if something is blocking airflow. A particularly common cause for blockage is nesting birds and dead animals. Animals frequently find a way into the home’s ductwork during fair weather when you don’t use the ducts. So, you may have birds nesting there.
Sometimes mice and raccoons may even die within the ductwork.
Solution: Scheduled seasonal tune-up can help take care of this problem. The technician will ensure to clean the ducts thoroughly to remove any items blocking airflow. If it happens mid-season, contact an HVAC professional immediately.
10. Incorrect System Size
If you’ve exhausted the typical reasons, you need to begin thinking outside the box, and one possibility that comes to mind is an undersized furnace.
Furnaces are sized in BTUs, and you need about 20BTUs per square foot during the peak of winter. This means you need about 40,000BTUs for a 2,000 square-foot home for optimal heating.
Now, imagine that your furnace is 30,000BTUs. The furnace will need to run longer to satisfy your heating needs.
Solution: Make sure to buy the right size furnace. And, this doesn’t mean you should oversize your furnace. Oversizing comes with several challenges, too, including short cycling and unnecessarily high utility bills.
What to Do it Your Furnace Won’t Shut Off
We’ve already seen what to do in each case. But, just to summarize, the following are the four steps to troubleshoot and fix a furnace that won’t shut off with the thermostat;
- Schedule seasonal tune-up: Once, before the onset of the heating season, have a licensed HVAC professional inspect and tune up the furnace to ensure that it’s ready for the grueling period ahead.
- Ensure regular maintenance: Regular maintenance means ongoing cleaning and repairs to keep the furnace in top shape. Make sure to wipe down the furnace at least once weekly. Also, keep your ears, nose, and eyes out for signs of trouble and fix the issues before they become major problems.
- Change the filters regularly: Ideally, you want to follow the manufacturer’s recommendations. However, the rule of thumb is to clean the filters at least once every two weeks and change them monthly.
- Set the thermostat and limit switch correctly: The HVAC professional will set them during the scheduled tune-up. However, you need to keep an eye on the two throughout the season and adjust the settings as appropriate.
Signs it’s Time to Repair the Furnace
It’s time to repair the furnace if it refuses to start, refuses to go off, or produces strange smells and noises. It’s wise to begin with DIY troubleshooting and repairs. However, don’t hesitate to call the pros whenever you feel overwhelmed.
We also advise against handling gas leaks yourself. It’s hazardous and against the law. So, you may end up either hurt or in jail.
It can feel scary when the furnace seems to run nonstop, even after reaching the thermostat setting – and it should be. So, it’s important to diagnose the issue right away and fix the problem to avert further risk and conserve fuel. Now you know where to begin troubleshooting.