What is Static Pressure in HVAC?

What is Static Pressure?

Static pressure is the resistance to airflow in ductwork. You want the right balance of static pressure in your system because too much or too little can cause problems, including premature system failure and energy inefficiency. 

The easiest way to picture static pressure is to consider a hose when the water is 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. For example, you can taper down an eight-inch return air duct to six inches if you have low static pressure. That will increase the static pressure. The ideal static pressure is 0.5 inches for the lowest noise and to properly take care of the equipment.

We at Fire & Ice use the term “static pressure” quite a bit when talking about ducts, filters, and other HVAC items. The wrong static pressure can strain your system and even shut it down. Static pressure also affects how efficiently your system runs, impacting how much you spend on your energy bills.

Knowing more about your system will also enable you to ask better questions to us or any other HVAC company or contractor.

How Does Static Pressure Affect My System?

Poor static pressure, either too high or too low, can cause the following issues:

• Noisier operation
• HVAC units may run longer
• Parts may wear out faster
• Leaking ducts

Imperfect static pressure might create a noisier blower because the motor works harder. Many of the customers we service here at Fire & Ice describe this noise as sounding like a jet engine taking off.

Additionally, 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, removing humidity. If the airflow can’t reach certain rooms, those house areas 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 can’t dissipate that heat quickly enough, you’ll get other internal issues, such as premature cracks in your heat exchanger.

How Do You Fix Static Pressure Problems?

Improper static pressure comes from one of two things. First, there’s either insufficient ductwork on the supply side for the air to get out or not enough return coming to the furnace.

You may want to build an addition that 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.

A restrictive filter usually causes not enough return air. 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 should 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 insufficient air for 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, which creates low static pressure.

High vs. Low Static Pressure

Pressure that’s too high or low is usually caused by improperly sized equipment. An oversized furnace can be too powerful and 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. Incorrect static pressure can also be caused by

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

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.

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. This causes your furnace to work harder, the blower motor is working harder, and other parts can malfunction earlier than expected.

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.

How Do I Prevent Static Pressure Problems?

The first step in preventing static pressure problems is to choose an HVAC company that does a complete assessment of your entire system, including 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 clean your air filter on its recommended schedule
• Survey central ductwork for any cracks or holes that might indicate leaks.
• Keep an eye on your vents. There's a problem if airflow stops or dramatically slows down from one or more vents. Don’t block vents with furniture. 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 and once for the air conditioner or heat pump. If you have noticed any problems mentioned above, talk to your maintenance technician. They’ll be able to diagnose the problem.

A standard solution to static pressure problems is to alter the ductwork. At Fire & Ice, We’ve done a lot of ductwork modifications. It doesn’t have to be super expensive. It can be as simple as just changing a portion of your ductwork. Instead of thousands of dollars redesigning ductwork, you could be looking at a few hundred to redo a small section, which could be enough to create the change you need.

Let’s say your supply duct is 16” by 8,” and there’s so much static pressure 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 will get more velocity and give you more airflow.

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

The speed of your fan on your HVAC system and making sure it communicates with your system can also help.

Changing your filter is an easy step and works wonders. Your furnace will thank you.

Your HVAC System Depends on Good Ductwork

Without proper ductwork, nothing’s going to heat or cool your home as it should. As a result, 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 correctly fitting your ductwork to your HVAC system.

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. The right amount of static pressure will also allow your filter to work better and do its job, resulting in fewer dust particles, allergens, and germs floating in the air. 

Of course, your overall comfort is also part of the equation. 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.

Static pressure can seem like a complex problem to solve with your HVAC system, but we can tell you that it doesn’t have to be. Talk to your HVAC partner when it comes time for maintenance visits or to replace a major system component about the issues mentioned above. If you live in the greater Central Ohio area, give us a call and schedule an appointment. We’re always here to keep your HVAC system performing the way it should so you stay comfortable in your home.