What is Static Pressure in HVAC?

What is Static Pressure in HVAC?
Luke Watson
Residential Sales Professional

I am a Residential Sales Professional for Fire & Ice. I provide customized solutions based on a customer’s home needs and desired comfort.

About This Article

Good ductwork can make a huge difference in the effectiveness of your HVAC system. In this article, we examine why static pressure in the ducts matters so much.

We at Fire & Ice use the term “static pressure” quite a bit when we’re talking about ducts, filters, and other HVAC items. The wrong static pressure can strain your system, and maybe even shut it down.

We’d like to pass on what we’ve learned about static pressure so that you can understand your system better. The more knowledge you have, the better the questions you can pose to your HVAC contractor.

It’s especially important when you consider your ductwork, which is responsible for circulating the air around your home. Properly sized and installed ductwork has multiple benefits. It helps reduce the strain on your HVAC equipment, prolonging its life, and saves you money in energy costs.

Your air will be cleaner with fewer dust particles, allergens, and germs floating in the air. The better the air circulates through a home, the more your filter will eliminate these hazards.

Your overall comfort is also part of the equation. Sufficient ductwork will provide consistent, even temperatures throughout your home. The better the air circulates, the more your system can do what it was designed to do: provide clean, comfortable air to every area of your house.

You can spend tens of thousands of dollars on your HVAC system. But even the most expensive heat pump relies on ducts to circulate conditioned air.

How Does Static Pressure Affect HVAC?

If you were to take a straw, and poke three holes in it, and blow through the end, none of the air comes out of the sides. It goes straight down the middle, the path of least resistance. But if you use your finger to block one end and you blow, the air will go out the sides. That’s static pressure in action.

It works exactly the same way in your ducts.

In HVAC, static pressure is the resistance to airflow in ductwork. Too much static pressure can cause problems. Too little, and you get different problems. If left untreated it can result in premature failure of the system.

The easiest way to picture static pressure is to consider a hose when the water is coming out unimpeded. If you put your thumb partially over the tip of it and start to change the diameter of the hole, it increases the pressure, and the water will speed up.

Air and water are similar. If you shrink the size of the hole in the ducts, it increases the velocity of the air. If you have low static pressure, you can taper down an eight-inch return air duct to six inches. That will increase the static pressure.

HVAC contractor with duct

What Causes High or Low Static Pressure?

  • Improperly sized equipment
  • Changes to the home, such as an addition
  • Restrictive or clogged air filter
  • Poor duct design

Improper static pressure comes from one of two things. There’s either not enough ductwork on the supply side for the air to get out, or not enough return coming to the furnace. If your static pressure is off, it causes your furnace to work harder, the blower motor is working harder, and other parts can malfunction prematurely.

Pressure that’s too high or low is usually caused by improperly sized equipment. An oversized furnace can be too powerful and can cause high static pressure inside your ductwork. An undersized furnace won’t be able to move enough air through the system to keep you comfortable. Air coming out of your vents won’t have much force.

Almost half of all HVAC systems are incorrectly sized. That means 50% of homes and apartments have imbalanced static pressure, and aren’t experiencing the full benefit of their HVAC.

Let’s say you have a home with a two-ton system and the ductwork is designed for two tons. An HVAC expert can do a load calculation, and it turns out you need two and a half, three tons. The ductwork can’t handle the larger size because it isn’t designed to do that. As a result, the pressure will be too high unless something is done.

A Manual D calculation is similar, except it’s designed for ductwork. Based on the cubic feet per minute (CFM) moved by your system through the ductwork, its size, the velocity of the airflow, and a few other factors, the Manual D calculation will let an HVAC contractor know what size ductwork should be.

You may want to build an addition, which opens up part of your home. Airflow to that new room might be weak because the airflow to that part of the house will be too little. That additional duct run to that room will also reduce the air going to other rooms.

Not enough return air is usually caused by a restrictive filter. If you don’t have enough return air coming, it’s like running a marathon while wearing a mask. It can be done. You can breathe through a mask. It’s just a lot more restrictive and a lot harder. That’s what’s happening to the furnace. The blower motor has to work a lot harder to move that air.

A dirty filter is super restrictive of airflow. A one-inch filter that’s supposed to be changed every 90 days ought to be replaced every 30. A filter that’s so dirty that it blocks the airflow suffocates the machine (as well as creates breathing issues for your family).

Another issue is incorrectly sized ductwork. Return airdrops can be too small, creating high static pressure and not enough air to the furnace. A lot of the estimates we provide here in Columbus include the replacement of return airdrops because past HVAC systems were undersized.

You also might have a return that’s too big. If it’s oversized, that creates low static pressure.

Problems Caused by Poor Static Pressure

  • Noisier operation
  • HVAC units may run longer
  • Parts may wear out quicker
  • Leaks in the ducts

Imperfect static pressure might create a noisier blower because the motor is working harder.

If there’s not enough airflow, you could wind up with hot and cold spots in your house. Not enough heat or cool is blowing out the vents, so your system will have to run for a longer time. It can’t heat or cool your home adequately.

You’ll also suffer from air quality issues. Your air conditioner or heat pump cools the air, but it also removes humidity. If the airflow can’t reach certain rooms, those areas of the house could be sticky during humid summer days.

Those areas of the house could also not get the full benefit if you’re using an air conditioning product, such as a humidifier or dehumidifier. Certain rooms will always be too humid or too dry,

If there’s too much static pressure on the return side of the ducts, the blower will work harder and probably wear out sooner. If the system is heating and it can’t dissipate that heat quickly enough, you’ll get other internal issues, such as premature cracks in your heat exchanger.

If air isn’t circulating as it should, it limits the effectiveness of your filter, which can capture:

  • Pet dander
  • Pollen
  • Cigarette smoke
  • Flu viruses

HVAC contractor with duct

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What’s the Best Way to Regulate Static Pressure?

Step one is choosing an HVAC company that does a complete assessment of your full system, and that includes all units that comprise your HVAC system: furnace, air conditioner or heat pump, and ductwork.

Regulating static pressure means finding out what it should be in the first place. An HVAC contractor can do a proper load calculation to see if your ductwork is adequate. They can also do a duct calculation with a device called a ductulator, which calculates airflow.

Ductwork Care

  • Replace and/or clean your air filter on its recommended schedule.
  • Survey major ductwork for any cracks or holes that might indicate leaks.
  • Keep an eye on your vents. If airflow stops or dramatically slows down from one or more vents, there’s a problem. And don’t block vents with furniture. Also, look for dust and dirt inside the vents or coming out of them. This can indicate unhealthy air and clogged ductwork.
  • Schedule routine maintenance for your HVAC equipment. This usually means twice a year; once for the furnace, once for the air conditioner or heat pump. If you have noticed any problems mentioned above, talk to your maintenance technician about it. They’ll be able to diagnose the problem.

HVAC contractor with ductwork

How Do You Fix Static Pressure Problems?

The common solution to static pressure problems is to alter the ductwork. We’ve done a lot of ductwork modifications. It doesn’t have to be super expensive. It can be as simple as changing just a portion of your ductwork. So instead of several thousand dollars of redesigning ductwork, you could be looking at a few hundred to redo a smaller section, and that could be enough to create the change that you need.

Let’s say your supply duct is 16” by 8,” and there’s so much static pressure, the air is restricted. It wouldn’t be out of the realm of possibility to increase it to 18” by 8” or 20” by 8” to get more air flowing through it.

Or, if the supply duct is too big, you can replace the first four or eight feet, then taper it down. As it tapers, it’s going to get a little more velocity. That will get you more airflow.

Another possible solution is increasing the return airdrop, or maybe increasing the return air trunk. You could add a return air duct if it’s feasible.

Last, changing your filter is an easy step, and works wonders. Your furnace will thank you.


Your HVAC System Depends on Good Ductwork

To put it into physiological terms, if the main equipment represents your organs (thermostat = brain, furnace = heart, etc.), your ductwork is the circulatory system that connects everything and allows the organs to do their job.

Without correct ductwork, nothing’s going to heat or cool your home as it should. Your comfort level can increase dramatically with some alterations. And the potential risks include health concerns, meaning that you may be inviting contaminants into your lungs by not properly fitting your ductwork to your HVAC system.

The solution is simple: Be aware, be informed, and talk to your HVAC partner when it comes time for either maintenance visits or to replace a major component of your system. You won’t be sorry that you did.

Need to get in touch with an HVAC contractor? Are you in the Greater Columbus, Ohio area? Give us a call to get things started.

Read more:

The Art and Science of Designing and Installing Home Ductwork

HVAC Ductwork: Best Practices for Cleaning, Modifying, and Care

Indoor Air Quality: Air Filtration & Air Purifying HVAC Products

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