Why Does My Furnace Shut Off After 15 Minutes?

If your furnace shuts off after about 15 minutes, it’s short cycling, meaning it isn’t completing its regular heating cycles.

You may need to repair your ductwork to prevent air leaks, replace the flame sensor, or even replace the furnace to fix the problem. However, don’t despair. It could also be something as trivial as a wrong thermostat setting.

Read on to find out the common reasons why a furnace may shut down after 15 minutes and what you should do to fix the problem.

How Often Should the Furnace Cycle?

Perhaps we should begin by understanding how a regular furnace operates and the duration of a standard heating cycle. All heaters run in cycles.

Like your refrigerator or air conditioner, the heater runs for a specified duration, cycles “off,” runs for several minutes again, and then cycles off – the process goes on and on until you switch off the heater.

This mode of operation allows the furnace to rest in between operations to extend the appliance’s life. It also helps guard against potential overheating while helping conserve energy. Above all, it keeps the overall cost of heating down.

A standard furnace has three to eight on-off cycles every hour, each lasting 9-20 minutes. However, the majority have four cycles per hour, i.e., two heating cycles and two off cycles, each lasting 15 minutes.

What is Short Cycling and Why is it Bad?

If your heater cycles more frequently than it did in the past, you may be experiencing what’s known as short cycling. Technically, short cycling is the tendency for heaters (and other HVAC appliances) to turn on and off before reaching the target setting.

In-home heating refers to a situation where the furnace cycles off without reaching the set temperature on the thermostat.

So, it’s important to note that short cycling isn’t always about the number of cycles per hour but more about whether the heating cycle allows the furnace to reach the thermostat setting.

Thus, if you’ve set the thermostat at 70°F, any heating cycle that stops before the thermostat reading reaches 70 is a short cycle.

What Causes Furnace Short Cycling?

The causes of short cycling are many and vary widely. However, the seven most common causes are as follows;

1. A Bad Flame Sensor

Most people don’t know this, but a bad flame sensor is one of the most common causes of furnace short-cycling. You’re likely aware that the flame sensor keeps track of the burner flame and will turn off the furnace whenever it loses “sight” of the flame.

Unfortunately, a defective flame sensor may fail to “see” the burner flame even if one is present, causing the furnace to shut off abruptly.

However, since the furnace is designed to retry ignition automatically, your furnace will keep going on and off repeatedly until something gives.

Solution: Access the flame sensor and physically inspect it for damage. Dents and cracks are telltale signs of damage. Also, test whether it’s voltage and resistance using a multimeter. If it’s just dirty, clean it. Otherwise, replace it.  

2. Furnace Overheating 

The furnace can also short cycle if it’s overheating internally. Furnaces overheat for many reasons.

For instance, your furnace can overheat if you have a bad heat exchanger – the part that transfers heat from the combustion chamber to the supply air plenum. A cracked exchanger can cause too much heat to enter delicate parts of the furnace.

The furnace has sensors that watch out for the slightest signs of overheating. If overheating is detected, the system can shut down, wait several minutes, and try to run again.

Solution: If your furnace’s short-cycling is caused by overheating, you need to fix the underlying issue. If it’s a cracked heat exchanger, replace it. If it’s a gas leak, fix the leaks.

3. Dirty Furnace Filter 

When the filter is blocked, air intake becomes a problem, often due to the buildup of dirt and debris. You see, your furnace takes cold air from your home through return vents, heats it, and sends it back into the house via your ductwork as warm air.

However, the supply air must pass through a filter before it reaches the combustion chamber.

If the filter is blocked, very little air can pass through. Thus, the entire heating process is compromised. A common consequence of such blockage is overheating, which, as we’ve seen, can cause short cycling.

The high limit switch will force the furnace to go off. However, after it cools down, the unit will try again, only to encounter the original problem.

Solution: Keep your furnace clean and the filters cleaner. More importantly, know when to replace the filters. If you’re in doubt, consult your manual or just replace it once monthly.

4. Thermostat Issues 

The wrong thermostat settings or a faulty thermostat can also result in short cycling. For instance, if you mistakenly set the thermostat a 60°F rather than the standard 70°F, it will cycle more frequently as it will reach the low setting faster.

Alternatively, the wrong settings could be the result of a thermostat malfunction. For example, a faulty thermostat with poor memory can jump values without warning.

Solution: Double-check the thermostat reading to ensure you input the correct values. Also, diagnose it for malfunction. If it seems faulty even after testing with a fresh pair of batteries, it may be time to replace it.  

What Happens When the Furnace Short Cycles?

The effects of furnace short cycling are numerous. The short cycles can impact heating efficiency, lead to faster equipment wear, and even increase your energy bills.

  • Heating inefficiencies: A short-cycling furnace will always leave you short on warmth because it cycles off before reaching the thermostat setting. If this goes on for an extended period, your indoor temperature will be permanently below the thermostat setting.  
  • High energy bills: Another problem with short cycling is that frequent furnace restarts can lead to very high energy bills. That’s because furnaces consume a lot of energy when starting up. Secondly, the increased ON-cycles mean your heater is effectively heating longer than usual. The result is usually higher energy bills.
  • Faster equipment wear: Every time the furnace starts or stops, it puts extra strain on the moving parts and electrical circuit. Now, imagine if your furnace starts ten times per hour! You’ll need to replace the motors and other moving parts fairly often.
  • Noise/comfort issues: Finally, furnaces make the loudest noises when starting or stopping. Therefore, frequent start-stops can make the furnace very noisy and significantly compromise indoor comfort. Add to the temperature swings, and the home can quickly become unlivable.

How to Fix a Furnace that’s Short Cycling

The only way to fix a furnace that is short cycling is to address the underlying issue. If it’s a bad flame sensor, clean/replace it. If it’s dirty filters or blocked vents, clear the blockage, etc. Short-cycling will persist until you address the underlying issue.

What If your Furnace Turns Off Immediately?

If your furnace turns off immediately after you light it, you may have a faulty flame sensor, a blocked gas valve, gas supply issues, or a problem with the ignition system.

Conclusion

Now you know what to do if your furnace keeps shutting off every fifteen minutes or more frequently. You must fix the underlying issue to address the short cycling.

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