If your furnace turns on only to turn off soon after (think just a few minutes later), you should know that this behavior, also referred to as "short cycling," can be dangerous.
Furnace short cycling can lead not just to your home feeling uncomfortably chilly. It can also result in safety risks, such as gas leaks, and expensive repairs.
Gas leaks, in particular, can be severe furnace safety risks, as they can lead to carbon monoxide (CO) poisoning.
According to the U.S. CDC, CO can kill without warning. It claims the lives of over 400 Americans, lands about 100,000 people in emergency departments, and results in over 14,000 individuals requiring hospitalization each year.
Given those dangers, preventing furnace issues by familiarizing yourself with short cycling solutions should be one of your top priorities.
Is It Normal for a Furnace to Cycle Every 5 Minutes?
No, it's not. If your furnace completes a cycle (turns on and turns off) within just five minutes, you should take it as a sign of short cycling.
In most cases, "healthy" furnaces only cycle three to eight times per hour. Each cycle also lasts longer than 5 minutes, typically between 10 and 20 minutes.
What Is the Most Common Cause of Short Cycling?
A furnace short cycling can be a result of several factors, including airflow restrictions and oversized equipment. It can also be due to improper installation or a faulty thermostat.
Airflow Restrictions
A dirty air filter is one of, if not the most common cause, of furnace short cycling and loss of home heating efficiency. If you have a clogged air filter, it restricts airflow, causing the system to malfunction and turn on and off repeatedly without ever completing a regular cycle. It can also lead to the system overheating.
When you have blocked vents, which can be due to intentional closing or unintentional obstruction, they can also cause airflow restrictions. If the restriction is severe enough, it can lead to short cycling.
Oversized Equipment
A furnace that's too big for your home will heat your living spaces too quickly. It might seem like a good thing, but it's not, because your furnace's thermostat will reach your desired target temperature before the system completes a full cycle. If it does, you'll experience short cycling issues.
Improper Installation
Suppose you've recently had a furnace replacement. If you partnered with a highly experienced and reliable HVAC contractor, your new heating system should not just be energy-efficient but also appropriately sized and correctly installed.
Proper installation is necessary for your furnace to function as it should, so if there's a problem with its placement and setup, it will likely experience issues, including short-cycling. ENERGY STAR, the U.S. EPA and DOE's joint program, also says that improper installation can lower system efficiency by as much as 30%.
Faulty Thermostat
If your thermostat is old, malfunctioning, or miscalibrated, it can cause your furnace to short-cycle. It can do so by misreading your home's indoor ambient temperature, which can then "confuse" the system.
Let's say the actual indoor temperature of your home is 65°F during a fall day. You want to bring this up to 70°F to enhance comfort. However, because your thermostat is malfunctioning, it thinks your indoor temperature is already at 68°F.
In that example, the problem with your thermostat can make your system turn on for just a short time and turn off soon after because it thinks your home's indoor temperature has already risen from 68°F to 70°F.
How Can Furnace Short Cycling Be Dangerous?
Furnace short cycling can be dangerous if not addressed because it can wreak premature damage to your system's heat exchanger.
Your furnace's heat exchanger consists of metal coils. One of its roles is to act as a physical, airtight barrier between the air that the system circulates in your home and the dangerous combustion gases that form as a byproduct of the heating process.
If the heat exchanger sustains damage, such as a crack, it can allow gases like CO to leak into your home.
Another way furnace short cycling can pose dangers is if it's due to overheating. If left unaddressed, an overheating furnace can increase the risk of house fires.
Frequently Asked Questions
Can You Fix Short Cycling Yourself?
Yes, you can fix short-cycling yourself, provided that the cause is due to simple issues like clogged air filters or blocked vents.
If your heating system is short-cycling, take it as a sign that it's time for furnace maintenance, including replacing or washing your HVAC filters. Make this a regular habit: Inspect your filters monthly, and replace them as soon as they show signs of debris accumulation.
If the culprit behind your furnace's short cycling is an obstructed vent, you just have to open or clean the vent. Ensure no furniture stands right in front of your air vents, and that there's a clearance of at least six to twelve inches.
When Should You Call a Pro When Your Furnace Short-Cycles?
If DIY tactics don't work, consider scheduling your furnace for a professional inspection and servicing. An HVAC expert can accurately diagnose the cause behind your furnace's short cycling and address it before it worsens and becomes more dangerous.
Is It Cheaper to Fix a Furnace or Replace It?
If your furnace is under 10 years old and the cost to repair it is less than half the worth of a new furnace, then it's cheaper and more practical to have it fixed. However, if your existing system is already ten years old or older, it has seen significant repairs over the years, and now has a cracked heat exchanger, too; replacing it may be a more cost-effective option.
You'll pay more upfront to get a replacement, but it will be worth it. A brand-new, highly energy-efficient system can help you save on energy bills for many years to come.
Never Ignore a Short Cycling Furnace
Furnace short cycling can be a safety hazard, potentially leading to dangerous gas leaks and fires. At the very least, it can make your home less comfortable while also driving up energy bills. All these should be good enough reasons to address this problem ASAP.
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