In the video above, our team walks through a real furnace that was experiencing severe airflow problems. While this particular system had multiple issues, we typically see one or two of these problems in most homes. Let’s dive deeper into what causes low airflow and what you can do about it.

The Warning Sign: When Your Limit Switch Keeps Cycling

When we arrived at this job, we immediately noticed the telltale sign of an airflow problem: the limit switch was cycling on and off repeatedly, and the duct pressures were over twice what they should be. If you’re not familiar with this component, the limit switch is one of the most important safety devices in your gas furnace. Its job is to prevent your furnace from overheating, which could cause a fire or serious damage to the heat exchanger.

Here’s what happened: the gas would turn on, and the furnace would start heating up. But because there wasn’t enough airflow through the system, the temperature inside the furnace would climb too high. The limit switch would sense this dangerous temperature and open, shutting off the gas supply. The fan would continue running, cooling the system back down. Once the temperature dropped to a safe level, the limit switch would close again, the gas would turn back on, and the whole cycle would repeat.

This constant cycling is your furnace crying out for help. It’s telling you that something is preventing it from doing its job properly, and that something is usually insufficient airflow.

Why Airflow Matters So Much

Let’s talk about how your furnace is designed to work. Every furnace is engineered to produce a specific amount of heat, measured in BTUs (British Thermal Units). For example, a typical residential furnace might produce 60,000 BTUs of heat. To safely and efficiently transfer that heat into your home, the furnace needs a specific amount of air flowing across the heat exchanger, about 13-15 CFM per 1000 BTU’s. In the case of a 60,000 BTU furnace, that’s approximately 800-900 cubic feet per minute (CFM) of airflow. 

Think of it like this: your furnace is generating all this heat, and the air flowing through the system is the delivery truck that releases that heat from the furnace and delivers it via the ductwork throughout your home. If you don’t have enough “delivery trucks” (airflow), all that heat just sits there with nowhere to go. The furnace keeps getting hotter and hotter because it can’t release the heat it’s producing. That’s when your limit switch steps in to prevent damage. However, the limit switch is not designed to be tripped over and over again, so if your air flow is too low and tripping the limit switch on a regular basis, the limit switch will go bad and start to open at a lower temperature. This will cause your furnace to turn off more often as the limit deteriorates. Some furnaces are designed to stop heating altogether if the limit opens 3 or 4 times. This is called a hard lockout.

Problem #1: Undersized or Improperly Installed Ductwork

One of the first things we noticed on this particular system was the ductwork installation. The evaporator coil was attached to a plenum (the main distribution box that feeds all your ductwork) that was simply too small. There wasn’t enough transition space between the coil and the plenum, which immediately restricted airflow right at the source.

When we counted up all the duct runs going to different rooms in the house, we realized there weren’t enough of them. Even if each individual duct was the right size, the system as a whole couldn’t move enough air to match what the furnace was designed to produce. The furnace was trying to push out all this heated air, but the ductwork was like a highway that suddenly narrowed from four lanes down to two. Traffic jams are inevitable.

Proper duct sizing and installation are critical for system performance. Unfortunately, this is something we see far too often. One of the ways we check airflow and determine where the restricted areas are is by checking duct pressure in different areas of the ductwork. Imagine taking a 2-liter plastic soda bottle with a small hole in the bottom and trying to blow air through it. Not much air would flow, but you would drastically increase the pressure inside the bottle. Now do the same thing with the full bottom cut out. Lots of air would go through, and you would have very little back pressure. The same is true if we were to suck air in through the bottle. Although this would cause a negative pressure, the same principle would apply. The smaller the hole, the higher the pressure, and the lower the airflow. 

A typical duct system should have about .2” of water column pressure in the supply ductwork (the ductwork going from your furnace into the different rooms of the home) and about the same amount of negative pressure in the return air duct (the air duct going from your house to the furnace). The air filter should add about .1” water column pressure. For this reason, the return air will often read .3 if checking after the filter, return duct + filter = .3. This supply and return pressure gets added together for a total duct pressure. This is designed to be about .5 total. This particular furnace had a reading of 1.2, more than twice the recommended pressure.

Problem #2: Air Filters (The Most Common Culprit)

Now, let’s talk about what is hands-down the most common cause of low airflow we see: restricted air filters. This could simply be a dirty air filter. As an air filter collects dirt, the free space available for the air to get through goes down. This is why it is imperative to change your air filter regularly. A good rule of thumb is every 3 months for a 1” thick filter and every 6 months for a 4” thick media filter. If you hold the filter up to the light and you can’t see much light through it, change it. High-efficiency air filters are another cause of restriction. And we completely understand why homeowners choose them. Who wouldn’t want cleaner air in their home? The higher the MERV rating on a filter, the denser it is and the smaller the particles it can capture. That sounds like exactly what you’d want protecting your family’s health and your HVAC system.

However, here’s the problem: the denser the filter, the harder it is for air to pass through it. In a standard system with a 1-inch thick filter slot, using a high-efficiency filter creates significant resistance to airflow. Your furnace fan has to work much harder to pull air through that dense filter, and often it simply can’t pull enough air to meet the system’s requirements.

So what’s the solution? We recommend upgrading to a 4-inch thick media filter. Honeywell is probably the most popular brand, but there are several good options available. These thicker filters have much more surface area than a standard 1-inch filter. Because of this increased surface area, they can be denser and capture smaller particles without blocking airflow the way a 1-inch high-efficiency filter does. It’s the best of both worlds: better air quality and proper airflow.

But here’s our most important piece of advice: if your system only has a 1-inch filter slot and you can’t upgrade to a media filter, do not use a high-efficiency filter. Stick with a standard filter. Yes, it won’t capture the smallest particles, but it also won’t damage your furnace or create the safety issues that come with restricted airflow. A furnace that operates properly with a standard filter is far better than a furnace that’s constantly overheating and cycling on and off with a high-efficiency filter. If air quality is a big concern, consider a full home air purifying system such as the Reme Halo LED or the I-Wave. These products make the dust particles floating in your air clump together, making them larger and allowing a standard air filter to pick them up. These products also neutralize things like mold, viruses, and allergens. 

Problem #3: Closed Supply Registers

Here’s another issue that surprises many homeowners: closing supply registers in unused rooms actually hurts your heating system rather than helping it. We totally understand the logic. You’re thinking, “Why heat a room we’re not using? Let’s close that register and save some energy.” Restricted return air registers from furniture or other items have the same effect. 

Unfortunately, most residential HVAC systems are designed with just enough ductwork to handle the furnace’s full output. And because most furnaces are single-speed and always run on high speed, the furnace needs its full capacity of ductwork to remove that heat from the furnace. There’s very little excess capacity built in. When you close registers, you’re not reducing the amount of air the furnace is trying to move. You’re just redirecting that air or, more often, creating back-pressure in the system that reduces overall airflow.

This problem is especially pronounced with single-stage furnaces, which are the most common type in residential homes. These furnaces are either on at full capacity or off completely. There’s no “low” setting. That means whenever the furnace is running, it needs to move its full designed airflow. If you’ve closed off several registers, the system simply can’t do that.

The Complete Solution: What Your System Needs

Based on everything we’ve covered, here’s your checklist for ensuring proper airflow in your heating system:

Keep all supply registers open. Even in rooms you don’t use frequently, leave those registers open to maintain proper system airflow.

Maintain clean air filters. Check your filter monthly and replace it when it’s dirty. Even a standard filter will restrict airflow if it’s clogged with dust and debris.

Use the appropriate filter density for your system. If you have a 1-inch filter slot, use standard filters. If you want high-efficiency filtration, invest in upgrading to a 4-inch media filter system and possibly a full home air and surface purification system.

Ensure your ductwork is properly sized and installed. This often requires a professional evaluation, but it’s crucial for system performance. One thing a homeowner can do is turn on the heat and test the supply air temperature, then subtract it from the temperature in the room, often shown on the thermostat. If you have a gas furnace and the difference is 50 ° or higher, there is likely an airflow problem.

Turn off power to the furnace for 30 seconds: Some furnaces will go into what’s called “hard lockout” mode after a limit has tripped three to four times. This means your furnace will stop running altogether. First, look for a clear sight glass and a blinking light inside, and count the number of flashes. The back of the door should have the code readings based on the number of flashes. This will tell you what the furnace is seeing as the problem. By turning the breaker to the furnace, or the furnace switch if equipped, off for 30 seconds and turning it back on, you will reset the board and may restart the furnace. This is only a temporary fix unless you solve the root cause, so a call to your local HVAC contractor is still recommended. 

Schedule regular maintenance. Professional tune-ups allow us to catch airflow issues before they cause damage to your system and find permanent solutions to this problem. 

Let SureTech Heating & Cooling Help

Low airflow might seem like a minor inconvenience, but as we’ve shown, it can lead to serious problems, including reduced efficiency, higher energy bills, shortened equipment lifespan, and even serious safety hazards such as a cracked heat exchanger. The good news is that most airflow issues can be identified and corrected.

At SureTech Heating & Cooling, our certified technicians have the knowledge and experience to evaluate your system, identify airflow problems, and provide solutions that work for your home and your budget.

Whether you need a seasonal tune-up, emergency heat repair, or want to discuss upgrading your system for better performance and efficiency, we’re here to help. As a locally owned and operated company, we’re committed to serving our community with honesty and integrity. We’ll never try to sell you something you don’t need. Instead, we take the time to thoroughly assess your system, starting with a proper load calculation to determine the right size equipment for your home, and carefully evaluating your ductwork to ensure everything is sized correctly and leak-free. Our goal is to provide you with honest recommendations and reliable solutions that truly work for your family and your budget.

Don’t let low airflow problems damage your heating system or compromise your comfort and safety. If you need your furnace repaired in Vancouver WA, contact SureTech Heating & Cooling today at (360) 305-3690.

Your comfort is our top priority, and we stand behind our work. Let us show you why we’re one of the most trusted HVAC companies in Vancouver.