Static Pressure Testing 101
It’s unfortunate that many HVAC field service and installation personnel unknowingly install equipment on undersized and poorly performing duct systems every day. The result is that the average HVAC system delivers less than 60% of its rated capacity into the building. Fortunately, you can discover many of the duct-related defects leading to this condition with a single measurement—static pressure.
In less than five minutes, you can determine the overall health of an HVAC system and what factors affect it. The answers lie in knowing what to measure, where to measure, and how to make sense of the readings. Once you measure static pressure, you’ll discover more inefficient systems than you can possibly correct. Let’s look at what you need to know about this foundational HVAC airflow principle, so you can put it to use.
Static Pressure: What Is It?
Before we can discuss static pressure measurement, it’s a good idea to understand what static pressure is. In simplest terms, it’s the amount of pressure the blower in an HVAC system must push and pull against to move air through a duct system.
Inches of Water Column is the measurement unit used when discussing static pressure. This value is typically abbreviated to inches WC, inches WG, or inches H2O. This is different for those of you used to measuring pressure in pascals.
Every HVAC manufacturer rates the blowers in its equipment by maximum total external static pressure (TESP). This is the maximum static pressure the blower is intended to operate against. You find this rating on the nameplate of the air-moving portion of the HVAC system (see Figure 1). The average residential HVAC equipment has a maximum rated TESP of 0.5 inches WC. As you begin to diagnose static pressure problems, this rating is a vital reference.
How Static Pressure Works in an HVAC System
Pressures change throughout an HVAC system. This is important to understand when interpreting and diagnosing static pressure. Every component, duct fitting, and length of duct influences system pressures.
Pressure on the supply side of the duct system is highest at the air discharge of the blower. The lowest pressure on the supply side is usually found past the supply register, as air enters a room.
Pressure on the return side of the duct system is highest at the air inlet of the blower. You’ll find the lowest pressure on the return side of the duct system just before room air enters the return grille.
Static Pressure Test Instruments
To measure static pressure, you need the following test instruments and accessories to get readings inside the equipment and duct system.
- A manometer is used to measure pressure. The most common manometer is the analog Magnehelic gauge. For residential testing, the 0- to 1-inch Magnehelic gauge works well and is very visual for explaining readings to customers. The top port of this gauge measures positive pressure, while the bottom port measures negative pressure.
- The digital manometer is another popular option. Make sure it is equipped with two pressure ports. Digital manometers should also have a range from 0 to 5 inches WC and display pressure out two decimal places.
- A static pressure tip enables you to access pressure inside the ducts and cancel out any turbulence, which could result in inaccurate pressure readings. The Dwyer A-303 static pressure tip is a favorite among many technicians.
- Tubing connects the manometer to the static pressure tip. A 3/16-inch i.d. (inside diameter) neoprene tubing works well, since it handles field abuse and cold temperatures.
- A 3/8-inch drill bit with sheath is used to install test ports into the equipment and duct system, through which you’ll insert the static pressure tip.
- Finally, you’ll need 3/8-inch test port plugs to seal your test ports and allow for future access once you’ve obtained your pressure readings.
Five Steps to Measuring Static Pressure
It typically requires five steps to measure static pressure. Don’t be surprised if it takes you 30 minutes or longer the first time you practice these steps. Allow yourself that time to build confidence. As your skills increase, the time to complete these steps drops to 5 minutes or less.
Step one: Identify the locations where you plan to install your test ports.
Step two: Install your test ports in the equipment and/or duct system.
Step three: Take the necessary static pressure readings.
Step four: Calculate TESP, component pressure drops, and duct pressures.
Step five: Compare your pressure readings to the manufacturer’s nameplate pressure ratings.
While these steps have been simplified, they give you an idea of what to expect as we continue.
Static Pressure Test Locations
Most systems need four test locations to access the necessary pressures. Knowing where and how to install test ports in these locations is crucial to your success. Static pressures are often misdiagnosed because contractors chose incorrect test locations. Each equipment type is rated differently by the manufacturer. Consider this before you install test ports.
It’s always best to know what is behind your test location, so please don’t use a hammer and scratch awl to install them. If necessary, remove any accessible panels and get the right tools and accessories to prevent costly accidents.
It’s considered best practice to use a drill bit sheath or short step bit when installing static pressure test ports in equipment. This helps you avoid damage to electrical components, drain pans, indoor coils, and heat exchangers. It also helps you avoid that awful hissing sound made from a pierced refrigerant line.
Figures 2 and 3 show where various system pressures are measured on a gas furnace and an air handler respectively.
Upflow Gas Furnace
Horizontal Air Handler
Before you begin to install test ports in the field, it’s a good idea to practice. Installing a test port into a homeowner’s equipment scares most technicians to death the first time they do it. Try installing ports in old duct or scrap equipment first; then test the HVAC system in your home.
Static Pressure Tests
There are a variety of static pressure tests you can perform, and each has a different purpose. Knowing when to perform each test and apply it to the diagnosis of a system can save you lots of frustration. The most common tests are
- air filter pressure drop;
- indoor coil pressure drop; and
- duct pressures.
Let’s look at these pressures in a little more detail.
Total External Static Pressure
TESP is the first and most commonly performed test. It is the total of pressures taken where air enters the equipment (suction pressure –) and where air exits it (discharge pressure +).
You add these two pressures together to indicate the total pressure the fan must push and pull against. The + and – signs represent the type of pressures found in these two locations; they are not mathematical symbols; so disregard them and just add the two static pressure values together.
Air Filter Pressure Drop
A restrictive air filter has the potential to turn a 20 SEER air-conditioning system into a 12 SEER system. You can identify whether a filter is too restrictive by measuring the pressure drop across it. Just because an air filter is visibly clean, that doesn’t mean it allows the proper amount of air through it.
You find air filter pressure drop by measuring the pressure on each side of the filter, then subtracting the two pressures to find the pressure difference. For example, let’s say pressure before the filter measures 0.11 inches and pressure after the filter measures 0.31 inches. Subtract 0.11 from 0.31 to find filter pressure drop at 0.20 inches.
Indoor Coil Pressure Drop
You measure pressure drop across the indoor coil in the same way that you measure the pressure drop of an air filter. With two pressure measurements, you can determine if a coil is dirty or restricted without removing any panels from the equipment. Technicians fall in love with this test when they see how much easier it makes their job, and how much it increases their diagnostic ability.
Just take pressure measurements on the entering and exiting sides of the coil, then subtract them to reveal its pressure drop. Ideally, these readings should be recorded when the coil is wet. For example, let’s say pressure before the coil measures 0.41 inches and pressure after the coil measures 0.11 inches. Subtract 0.11 from 0.41 to find indoor coil pressure drop at 0.30 inches.
Taking a single pressure reading near the equipment in the supply and return duct system can help you find restrictions, or tell you whether the duct is undersized. Pressure readings before the air filter provide the return duct pressure. Pressure readings after the coil provide the supply duct pressure. The side of the duct system with the highest pressure typically has the most restriction.
Diagnosing Static Pressure Measurements
It’s best to look at pressure readings individually when you use static pressure as a diagnostic indicator. Look at readings individually so you can see where the highest resistance exists in the duct system.
One of the first tests in static pressure diagnostics is the TESP measurement. Simply compare the field-measured TESP to the equipment's maximum-rated TESP. If measured TESP is higher, the health of the HVAC system is probably not very good, and repairs are needed.
Pressure drops are best diagnosed when you compare each field-measured pressure drop reading to the equipment’s manufacturer-rated pressure drops. If the measured pressure drop is higher, it indicates a restriction. When this information is not readily accessible you can look for the component that has the highest pressure drop. It is probably the most restricted.
Duct pressures won’t have a manufacturer-rated pressure, since they’re field assembled. Look for measured pressures that are over 0.1 inches. If duct pressures exceed this value, you can be assured that side of the duct system is restricted somewhere.
To help make sense of field-measured pressure readings, National Comfort Institute, Incorporated (NCI) has developed static pressure budgets that aid you in diagnosing static pressure problems. To help you out, I’m offering the NCI Static Pressure Budgets, which includes the basic information you’ll need to start your diagnosis. Send me an e-mail to request your copy.
Discussing Static Pressure
It’s important to remember that most people just don’t understand the concept of static pressure. If you get “techy” with your explanations, you’re doomed. Keep this in mind as you discuss it with your customers. Be sure to communicate the principles in a simple yet easy-to-understand manner.
Talk about static pressure as if it were blood pressure. This simple concept is one that NCI has taught for over two decades. Everyone understands blood pressure. Just as high blood pressure is an invisible killer of people, high static pressure is an invisible killer of HVAC systems. Simply put, high static pressure is not healthy and indicates that a system needs immediate improvement.
For example, you might say, “Remember when we measured the pressure in your ducts? Your system is designed to handle a maximum pressure of 0.50, but our measurement shows yours at 0.82. This is like having a blood pressure reading of 200 over 130 in a human body. That is not good. For your system, high static pressure means it can’t deliver the air your home needs until this is corrected.”
When you communicate the results of static pressure to your customers using the blood pressure comparison, you’ll see the light go on in their eyes. This can be one of the greatest lead generation tools you ever use. By helping them discover problems with their system that have been hidden, you can provide solutions to cure defects that hinder system performance. Give your customers the opportunity to improve the health of their HVAC system.
If you're an HVAC contractor or technician interested in learning more about static pressure, and/or if you want a copy of the NCI Static Pressure Budgets, contact David at firstname.lastname@example.org or call him at (800)633-7058.
NCI’s website is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.
Testing Static Pressure Is the First Step
While this has been a brief overview of static pressure, don’t be misled into thinking that testing static pressure is the silver bullet to HVAC system performance. Other aspects of system operation, such as airflow and temperature, need to be accounted for too.
Testing static pressure is the first step, because static pressure is the foundation of airflow. You must understand both of these points as you move forward. When you fail to recognize the impact of static pressure, systems often fail to deliver their intended design results. This is one reason why duct-sealing programs haven’t reached their full potential for decades. Only when those who work on HVAC systems understand static pressure can true HVAC system performance be achieved.
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