Rating The HVAC Pressure Diagnostic Methods

Dec. 1, 2005
Recently we have experienced an increasing number of tech support calls about different pressure diagnostic methods here at National Comfort Institute

Recently we have experienced an increasing number of tech support calls about different pressure diagnostic methods here at National Comfort Institute (NCI). Let’s take a look at some methods that are being used across the industry. Some are time-proven and accepted methods of testing, while others offer questionable conclusions.

First of all, there is good in every test, and the more knowledge the better. However, as you learn to diagnose HVAC system performance, you’ll soon understand that rarely can a complete system diagnostic be made on a single test. It’s like those Christmas letters we all received last year with photos of family and friends, according to the pictures, all is well. But life and HVAC systems can’t be evaluated by a snapshot. We’re looking for video.
With that in mind, let’s start with the best pressure test methods, and work our way downhill from there.

Total External Static Pressure

This is the granddaddy of them all. Manufacturers include static pressure ratings for every air handler made. Pressure is the foundation of airflow. It’s impossible to evaluate fan performance without it, but surprisingly few of us are really good at measuring it and interpreting what it means. Some say it cannot be effectively read or relied upon. Sorry to say, these folks simply haven’t paid the price to learn how.

Total external static pressure (TESP) is a combination of two pressure readings. The first is the air pressure entering the equipment. The second is the air pressure leaving the equipment. Simply add the two pressures together to find the total external static pressure of the system. There are 20 or so rules (each simple and common sense) that govern good static pressure readings. But that’s another article.

This TESP can be plotted on the manufacturer’s fan charts and, when intersected with the fan speed setting, will reveal the airflow the fan is moving. Once you know fan airflow, it’s easy to figure duct leakage and to pinpoint many invisible system defects.

Pressure Drop

This test involves taking the air pressure on either side of a component of a duct system. To find pressure drop, simply subtract the two pressures. The difference is the pressure drop of the filter, coil, fitting, or duct run.
When the manufacturer rates a fan, at say 0.50-in. TESP, and the system measures 0.83-in. TESP, airflow will typically be at 70% to 75% of capacity. At that airflow, heat transfer suffers significantly and the system performs poorly. Systems do not operate well outside of the manufacturer’s specifications. Another interesting fact is that the higher the AFUE or SEER, the worse the equipment will operate at low airflow.

It’s our job to fix the system by reducing static pressure when needed. We do this by measuring pressure drop over each of the suspected restrictions in the system. If the “high efficiency filter” has a pressure drop of 0.52-in. that fan can’t afford that filter, and the filter must be changed.

This same test applies to many newer coils. A percent of new coils are too restrictive, and the fans used to get the high efficiency numbers can’t move sufficient air through them. This is a serious flaw with current efficiency rating methods.

Pressure drop pinpoints system defects. When we can find these hidden flaws in our systems, they’re normally easy to repair.

Building Pressure Diagnostics

These are valid diagnostic measurements, but you must get the big pressure picture before you can effectively apply this test method to diagnostics. There are endless factors that can affect building pressure besides the fans we are testing. The windward side of the building we test on, stack effect, simple heat flow, an open window, or a dog wagging its tail can affect building pressure readings.

To measure building pressure, attach a long hose to your manometer. Stand inside the building and run the end of the hose to the outside of the building. The difference between the two pressures appears on the manometer. Of course, there are another dozen little rules to learn that can make a significant difference in your reading.

Good readings are tough to get, but until you learn to consider the entire building and the constantly changing indoor and outdoor effects, be cautious with your recommendations. Ideally, most buildings should be under slight positive pressure; about .02-in. .03-in. is recommended.

There are extensive test protocols using over 100 building and system pressure test to guess airflow loss the outside of the building and, supposedly, other fabulous revelations. Study these test methods with the big pressure picture in mind and you’ll find they really don’t make much sense in the field.

Duct Blowers

Duct blowers have crept into our industry and have found acceptance as a decisive HVAC performance test method by government and utility programs. These are small, calibrated fans that are used to pressurize a system once all the registers and grilles have been blocked off. Any airflow that can be forced into the system, at typically 0.1-in. of static pressure, is deemed to be duct leakage. These tools are supported by ASHRAE Standard 152, but have never been implemented as the standard specifies.

The idea that gave birth to duct blowers came form a SMACNA test standard that was established decades ago. The SMACNA standards call for ducts to be pressurized with to up to 8-in. of pressure and produces very reliable results, unlike duct blower testing at one-eightieth of that pressure.

Even if they could accurately identify live duct leakage, these devices only test one element of performance: duct tightness. The belief that tight ducts are right ducts is a fallacy. True duct tightness, like correct total external static pressure and airflow, is only one of many critical aspects of a well-performing system.

But duct blowers are here to stay and have been written into laws and codes across the country. Many of you are required to use this partial system performance evaluation test method and abide by its results. But defaults are being written into the energy saving programs to compensate for what they are learning about their shortcomings.
Be extremely cautious when using these tools to assure consumers their systems work properly. No program or institution will back you up if the system fails to perform. Ask them.

Blower Doors

The blower door is another test method that has found its way into the HVAC industry. However, it is the conclusive test for building envelope tightness and has been for decades. This is the daddy of the duct blowers and it operates on the same principles, but it is used to test building tightness — not ducts.

A large fan, a frame that fills the door space and a multi-function manometer make up this tool. The building is pressurized or depressurized to typically 0.20-in., and then airflow is interpreted through the fan. This airflow is deemed to be building leakage.

This test is essential to determine envelope leakage as required by ACCA Manual J. The building leakage is poured into the heat loss or gain calculation. But so are window thermal efficiencies and insulation values. It’s estimated that less than 1% of homes are tested for envelope leakage before a load calculation is completed.

Aside from envelope leakage, the tool has nothing to do with HVAC system performance measurement. It does a couple of neat tricks with duct leakage, but the legitimacy of blower door tests on HVAC systems was rejected by ASHRAE years ago.

There are many local building scientists and weatherization program participants who own these tools, and most of them work at very reasonable rates. You can hire them for far less money than it would ever take to buy the tools and pull your technicians away from income producing work to do this building diagnostic test for you manual J calculations. If you suspect a blower door test is needed, you can always bring them in after the sale is made, if the customer wants to pay for the test.

Pressure Pans

These lay on the bottom of the pressure testing barrel. They follow no known physics or formula that can be reproduced. At best they can start a serious rumor about the performance of an HVAC system, yet they survive in pockets around the country in utility programs.

Pressure pans look like a cookie sheet on a broom stick. A pressure hose is inserted into the cookie sheet and connected to a manometer. The device is placed over a supply register and, amazingly, airflow and duct leakage is calculated.

I searched the Internet and found a couple government sponsored articles form the 90s. So although pressure pans haven’t died yet, the species is dwindling.

System Operating Pressures

Pressure is a critical diagnostic measurement. But it takes knowledge and practice to effectively use it as one of the tests required to measure system performance.

Learn to read and interpret manufacturer’s fan performance data to identify fan airflow. Study how to measure pressure drop over system components and evaluate the effect on airflow and Btu delivery. These are “foundational” tests that pinpoint system defects and allow you to identify exact duct repairs and dramatically increase HVAC system performance.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a no cost Static Pressure Diagnostic Procedures contact Doc at [email protected] or call him at 800/633-7058. NCI’s website is found at www.nationalcomfortinstitute.com.

About the Author

Rob 'Doc' Falke | President

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC-based training company and membership organization. If you're an HVAC contractor or technician  interested in a building pressure measurement procedure, contact Doc at [email protected]  or call him at 800-633-7058. Go to NCI’s website at NationalComfortInstitute.com for free information, articles and downloads.