Static Pressure And How It Affects Your HVAC System

Static Pressure And How It Affects Your HVAC System

Static Pressure And How It Affects Your HVAC System

Static pressure can be at the root of many common heating or cooling problems in homes with ductwork. It’s the resistance your heating and cooling system faces when it’s trying to push air through the house.

In most cases, we can make a significant improvement to your comfort while reducing your energy bills when we address static pressure. And, with this article, you’ll learn everything you need to know about the topic and how it affects your HVAC system — and your comfort. 

We’ll cover: 

  • What Static Pressure Is
  • Spotting A Problem
  • Causes Of Problems
  • How To Fix Them 

If you recognize any of the problems here in your home in your Havertown, Wynnewood, or Broomall home, click below or call John Cipollone at (610) 446-7877. Starting with a free consultation, we’ll help you make your home more comfortable and efficient. 

  1. What Is Static Pressure?

1.1. How To Measure Static Pressure

1.2. Effects On Efficiency 

1.3. Effects On Comfort

        2.Signs Of Static Pressure Problems In Your Home

2.1. Temperature Difference Upstairs And Downstairs

2.2. No Airflow Coming Through The Vents

2.3. “Stale” Air In The Bedroom

  1. Causes Of Static Pressure Problems 

3.1. Ductwork Design

3.2. Lineal Feet

3.3. Pre-1950s

3.4. Post 1950s

         4.Fixing Static Pressure Problems 

4.1. Ductwork Sealing

4.2. New Fittings

4.3. Redesigning Ductwork

4.4. Going Ductless

  1. Heater Repair In Havertown, PA 

What Is Static Pressure?

In the HVAC world, static pressure measures the resistance caused by everything externally installed on our furnaces. The higher the resistance, the weaker the airflow or air pressure.

Your furnace has a blower motor that pushes and pulls air through a duct system. But the ducts themselves, along with your air filter, cooling coils and UV lights or dehumidifiers (if you have them) exert resistance as the air flows by them. 

With that in mind, static pressure (or “Total External Static Pressure” in scholarly terms) measures the total resistance all those items create. 

Here’s an example with some local flavor: Here on the Main Line, we all know the Conshohocken Curve. It’s that spot around mile marker 331 on the Schuylkill where traffic jams up because everyone slows down to get around a 90-degree turn.

The same thing happens to air in your system as it navigates your ductwork. And, the Curve analogy is even more spot-on than you may think. We’ll get to exactly how a little later. 

How To Measure Static Pressure

We measure static pressure by inserting two probes directly into your ductwork. It measures inches per water column (“in wc” or “inches h20”, which is related to pressure per square inch (PSI).

The ideal system runs at .5 inches per water column. The higher the number, the stronger the static pressure — and the weaker your airflow. 

Usually, it’s not worth any investment to reduce the static pressure if you’re below .9. We’ve run onto plenty of systems at that rating or up to 1.3. The worst we’ve seen here is 1.8 inches per water column. 

Effects On Efficiency 

The more static pressure you have, the less efficient your system will perform. If you consider the furnace or AC condenser the heart, your ductwork is the veins. If the air can’t travel through them, the whole thing doesn’t work the way it should. 

This issue is especially important if you invested in 95-percent AFUE furnaces. You paid top dollar for a high-efficiency system, but it may not perform properly. 

When the air can’t flow freely through the ductwork, the system works harder to overcome the problem. As a result, the efficiency drops — and your utility bill goes up. 

Effects On Comfort

High static pressure can result in uneven airflow throughout the house. As a result, some rooms remain uncomfortable when they get very little airflow. Rooms may be warm in the summer and cold in the winter. 

Signs Of Static Pressure Problems In Your Home

Static Pressure Can Cause Your AC To Run PoorlyStatic pressure problems are very common in residential ductwork. Signs of this issue in your home include:

  • Temperature Differences Upstairs And Downstairs

  • No Airflow Coming Through The Vents

  • “Stale” Air In The Bedroom

Temperature Difference Upstairs And Downstairs

Plenty of homes have upstairs rooms that are too hot or too cold. There can be a four- or five-degree difference because the thermostat is downstairs and doesn’t take the second story into account. 

With high static pressure, you’re not getting enough forced air up to those rooms. The temperature difference can be much greater. 

No Airflow Coming Through The Vents

Do you have vents with little or no air coming through them even when you hear the system running? There could be too much resistance due to a bad duct.

“Stale” Air In The Bedroom

The “stale” smell or feel of the air in a room comes from a lack of circulation. Remember, you have ducts that send air from the furnace and returns that suck the air from each room. With weak circulation, the air in a room doesn’t go anywhere and becomes stagnant.

Causes Of Static Pressure Problems 

The biggest cause of static pressure is your ductwork. Sometimes it’s the result of leaks that drop pressure along the way. But, more often, it’s the design and installation itself. 

Ductwork Design

Going back to the Conshohocken Curve: cars have to slow down over that sharp, 90-degree angle. Air has to do the same thing. 

Most ductwork coming out of a furnace has a fitting with a 90-degree turn. That’s the point where your vertical furnace meets the horizontal duct that runs across the basement and splits into individual lines. 

But, a 90-degree angle greatly restricts the airflow. You’re much better off with a sweeping turn or a 45-degree angle so the air can pass with less resistance. 

Lineal Feet

Ductwork is designed with 100 lineal feet (a straight line) in mind. But, most ducts aren’t straight. A 90-degree fitting may account for up to 65 feet of lineal duct because of how much velocity the air loses. 

As a result, the air flow will be weak beyond the next 45 feet of ductwork. Even if the run is the recommended 100 feet, that 90-degree angle hinders its performance. And, 100 feet isn’t as much as you may think when you consider the run usually has to go across the basement and then vertically up through the house. 


Ductwork from before the 1950s was excellent and much better for airflow. Most homes had 45-degree fittings, which is optimal for airflow. But, this required customization for each home. 


Most post-war HVAC installations have 90-degree fittings. As demand for new houses surged, builders went with stock parts that were cheap, widely available, and got the job done —  even if they didn’t do it as well. 

Fixing Static Pressure Problems 

Depending on the accessibility of your HVAC system and ducts, we can solve most static pressure problems. 

Ductwork Sealing

The easiest fix is if the pressure problem results from air leaks. According to the Department of Energy, you can lose between 15 and 35 percent of your ductwork capacity (and pay hundreds of dollars extra on your bills) through unsealed ducts. 

Sealing ductwork is actually fast, easy, and relatively inexpensive. But, HVAC salespeople often aren’t familiar with every aspect of the business. As a result, they more often push big-ticket items such as new ductwork or high-efficiency systems. And, to be honest, it’s not as profitable. 

Applying duct sealant is essentially like “painting” the ducts with a paste. It can cost up to $1,000 to seal all the visible ductwork (we can’t get between rooms without ripping down walls). And, you can save up to 10 percent on your energy bills every year. 

When you figure an average utility bill of $150 a month, that’s nearly $200 a year. If you plan to stay in your home more than six years, you’ll get back the money you invested — and then some. 

New Fittings

Another strategy is installing custom-made metal fittings. Unlike those one-size-fits-all 90-degree pieces, a custom fitting takes the width of your unit and the height of your ceiling into account. You’ll get a more gradual angle that doesn’t restrict the airflow nearly as much. 

Redesigning Ductwork

Matthias Showing Us How He Adjusted The DuctworkWe redesign the ductwork around the opening of a furnace in most of our installations. Since airflow is at its strongest at this opening, we consider it when solving static pressure problems. Also, longer duct runs require different designs to maintain the air velocity. 

Case Study: New Furnace And AC With Improved Ductwork In Newtown Square

That would increase the pressure the higher it goes. But, it can get expensive: at least $250 per run, plus around $300 just for the diagnostics and design. Sometimes, we have to rip through walls. It’s easy for a job like that to end up costing over $3,000.

Going Ductless

Another option is to supplement your system — or scrap it entirely — with a ductless mini split. As the name suggests, ducts and, by extension, static pressure, aren’t an issue. You can learn more about ductless here and here

Mini splits start around $5,500 for a single-zone system for one room. Multi-zone setups for an entire home run up to $17,000. 

But, you may end up spending close to that, if not more, for ductwork repairs plus a new high-efficiency forced-air system. 

By contrast, a split system offers better comfort, more customization, and much higher efficiency — which, once again, translates into lower bills all year. 

Heater Repair In Havertown, PA

Whether your heating or cooling problems stem from static pressure, a broken furnace, or any other factor, John Cipollone is here to help. We’ve built an excellent reputation for heater repair in Havertown and across the Main Line since 1953. If you notice any problems with your HVAC system, call us today.

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