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Use Static Pressure Measurement to Pinpoint Duct Deficiencies

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For more than 25 years we’ve collected HVAC system static pressure measurements from across the country. Based on those numbers, we discovered a new opportunity in duct system upgrades for HVAC contractors. We found that static pressure in HVAC systems is similar to blood pressure in humans. When we compare static pressure to the normal blood pressure of 120 over 80, the average HVAC system blood pressure equivalent is 200 over 133. Let’s take a look at how static pressure measurements quickly verify duct systems “HEALTH” via installed performance.

Verification of Duct Design and Installation

ACCA Manual D (Residential Duct Design) and ANSI/ACCA 5 (Quality Installation) require static pressure measurement to verify the field installed systems’ performance. Static pressure is compared to the maximum rated Total External Static Pressure (TESP) to assure the duct system is designed and installed properly.

If static pressure is too high, the blower cannot move the required system airflow and installed system performance takes a nose dive.

Design and installation without verification of operating static pressure is wishful thinking. Your design efforts lead you to believe the system performs perfectly. You may assume your installed system meets at all the manufacturer specifications and that fan airflow operates at 100%. Unfortunately, if you don’t measure, you’re just guessing.

Decades ago, when this all began, my family HVAC company had been in business 40 years. We were completely confident each of our systems worked perfectly. Once we began to measure static pressure and airflow, we thought we were going to die. Our world turned upside down and we began to change everything. The first changes were the way we designed and installed duct systems.

Hopefully your outcome from measuring static pressure won’t be the same. You’ll suffer far less pain than we experienced. In those days at my company, we discussed our problems with other industry professionals who had no idea what we were talking about and thought we were a little crazy.

When an HVAC contracting company begins to measure static pressure, sirens go off and red lights begin to flash top to bottom throughout the company. The moment you know your typical residential HVAC systems’ blood pressure is equivalent to 200 over 133, things change. Just like if you check into the hospital with blood pressure that high, your life immediately changes.

Static Pressure Test Instruments and Accessories

The test instrument you’ll need to measure static pressure is a dual port manometer. If using an analog manometer, or Magnehelic, use a .0” w.c. (inches of water column) to 1.0” w.c. model. If you use a digital manometer use a .0” w.c. to 5.0” w.c. model.

Accessories you’ll need include a 3/8” drill bit with a sheath, 3/8“ test port plugs, a static pressure tip, and matching pressure tubing to connect the tip to the manometer.

A kit to measure static pressure costs less than $200.00 and will serve you well, for years to come.

Measure Total External Static Pressure

The static pressure test required by ACCA Standards is total external static pressure. Here are six steps to effectively measure total external static pressure. We’ll use a residential upflow gas furnace as an example.

  • Step One. Install a test port where air enters the furnace, after the air filter.
  • Step Two. Install a test port where air exits the furnace. If it has an external coil, this is before the coil.
  • Step Three. Turn the system ON with the fan set to deliver the highest required airflow. This is cooling mode for most of the country.
  • Step Four. Connect the static pressure tip to the pressure tubing on one end of the manometer and then to the other end. Measure and record the pressures at these two test sites.
  • Step Five. Add the two pressures together to find the measured system TESP.
  • Step Six. Compare the measured TESP to the manufacturer’s rated TESP listed on the air-moving equipment nameplate.

If the measured TESP is lower than the rated TESP and your fan speed is set correctly, fan airflow is adequate. Congratulations, you are in the minority. If TESP is more than 10% to 20% higher than the maximum rated TESP, you probably have an airflow problem degrading the system’s performance.

Congratulations, you have found an opportunity — an invisible need your customers have to repair and improve the performance of their system.

Isolate Duct System Static Pressure for Fast Diagnostics

Savvy HVAC companies don’t wait until verification time to see if their systems work as designed. They know the best verification happens from the beginning of the sales process and continues until the system is commissioned.

When verification is done in advance, static pressure testing is called diagnostics. When servicing and selling, static pressure testing uncovers opportunity to redesign and upgrade the duct system. This is often done during equipment replacement when customers are hungry to receive more than a new box that does the same thing the old one used to do.

During installation, static pressure diagnostics help avoid call-backs for expensive repairs after the fact. It also allows installers to know their job was done right. Let’s ramp it up a little and move beyond verifying duct design and installation. We’ll focus on duct static pressure diagnostics.

Duct Diagnostics

A single static pressure reading can be taken to troubleshoot the design and installation of the supply duct system.

Here’s how: Install a single test port above the cooling coil (we’ll continue to use the upflow furnace example shown above).

Take a single static pressure reading in the airstream with the pressure tubing connected the positive (+) port on the manometer. Be sure the static pressure tip is facing into the airstream.

Compare this number to 20% of the maximum rated TESP. (Twenty percent is the typical pressure in the supply duct system for a well-operating HVAC system.) For example: A system fan has a maximum rated TESP of .50” inches w.c.  Multiply .50” x .20 (or 20%) to find pressure in the supply duct system. It should not exceed .10”.

If the supply duct pressure measures .20”, the duct is either undersized, poorly installed, or maybe there’s a possum taking a nap inside. Whatever the case, you need duct system upgrades for the system to operate properly. Once your customers understand this, they will recognize the symptoms they have been living with and eagerly accept the duct upgrades.

The return duct system can be diagnosed in the same way. There are a few differences you’ll need to be aware of.

  • The test port on the return side is installed just before air enters the filter (if the filter is located near the air moving equipment.)
  • The pressure tubing is attached to the manometer at the negative (-) pressure port.

The testing, percentages and math are identical to those on the supply side.

Duct Diagnostics and Repairs

Knowing the system pressure is higher than it should be is the hard part. This will naturally lead you to troubleshoot the problem. If duct pressure is high:

  • Inspect for pinched or blocked duct and correct it.
  • If the ducting is undersized, add additional ducts, especially to hot or cold rooms. If possible, from the duct or plenum, close to the fan.
  • If transitions are restrictive to airflow, replace them.
  • If duct suspension is causing flexible duct to bind, add better duct suspension.
  • If duct access is restricted, find a new route for that duct portion.

Once you can “see” the duct is restricted through static pressure measurement, you’ll find the problem area and be able to fix it. Once you understand the problem, you’ll delight in explaining to your customers how you found the problem and help them understand the benefit they will receive from the repairs.

Got five minutes? Measure static pressure. Once you do, you’ll take a large step up in your career and with the services you can offer and provide to your customers.

Rob "Doc" Falke
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Posted In: Building Performance

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