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Service Clinic: Anyone Can Measure Static Pressure Part 2

July 14, 2008
by Rob 'Doc' Falke

Last month, we began a review of how to measure static pressure in an HVAC system. We discussed the required instruments and why measuring static pressure is such a good idea and a critical service we should be offering our customers. This month, Let’s take a look and the steps required to measure total external static pressure.

Static Pressure Testing, Step By Step
It typically should take less than five minutes to measure a system’s static pressure. The following are sample instructions for a furnace with a remote coil.

When drilling test holes with the 3/8-in. bit, make sure the drill bit sheath is in place to prevent drilling into the coil.

1: Locate the appropriate places to drill the test holes on the supply side (+) between the furnace and the coil, and on the return side (-) between the filter and the furnace. Center the holes for neat appearance. Stay away from any coils, cap tubes, condensate pans, or circuit boards to avoid damage. Always look before you drill.

2: Drill test holes using a 3/8-in. drill bit with a metal piercing tip. A bullet tip bit makes a clean round hole. Make sure to use your drill bit sheath to prevent from drilling into the coil. If there’s duct liner inside, be sure to penetrate it to assure a good reading.

3: Push one end of the tubing onto the static pressure tip. Place the other end of the hose on the HIGH (+) pressure port of the gauge. If required, make sure the gauge is level and zero the gauge by adjusting the screw on the face with the small screwdriver. (Digital gauges each zero differently, so check owner’s manual.)

4: Read the supply or positive (+) static pressure by inserting the static pressure tip in the test hole into the air stream with the tip facing into the airflow. The magnet on the tip will hold it in place while the value is read and recorded. This measurement is the pressure the fan is “seeing” on the supply side of the system.

Insert the static pressure tip into the air stream with the tip facing into the airflow.

5: Read the return or negative (-) static pressure by moving the tube from the HIGH to the LOWpressure port on the gauge. Insert the static pressure tip in the test hole on the return side with the tip facing the airflow. Read and record the negative static pressure. Don’t forget to insert hole plugs in the test holes when you’re done testing.

6: Calculate the system’s total external static pressure by adding the two values. Since the negative and positive signs identify the type of pressure measured, you can ignore them when adding the two values together.

For example:

  • The supply static pressure reading is (+) .26-in. w.c.
  • The return static pressure reading is (-) .21-in. w.c.
  • The total system static pressure is .47-in. w.c. (.26-in. + .21-in. = .47-in. w.c.)

Compare the measured static pressure to the Maximum Rated Static Pressure found on the equipment nameplate. If measured static pressure exceeds rated static pressure, additional airflow testing is usually required to verify the airflow problem and diagnose system deficiencies.

When you read the static pressures in the systems your company designs and installs, you’ll begin to gain tremendous insight into the performance of your systems. Doesn’t it seem strange that the duct system is excluded from 98% of our industry’s service agreements? It’s clear that the pendulum has swung too far in the direction of the equipment and away from comfort.

Measuring static pressure gives the HVAC contractor a valid basis on which to recommend duct renovation work. You also begin to see that the duct system is what controls equipment capacity and efficiency.

If static pressure is too high, you have evidence of problems with the system causing low airflow. Check for blockage in ducts, closed dampers, improper transitions, offsets or kinked flex duct. The problems can also be from the equipment and system accessories like a “high efficiency” cooling coil or restrictive filters.

The Opportunity
The answer to improving system performance is often found outside of the box (the equipment). Static pressure allows you to “see” the system in an entirely new light — airflow nearly becomes visible. Airflow is directly connected to Btus. When airflow is brought into the equation, your ability to diagnose and solve problems multiplies, as you’re able to “pull your head out of the box” and see the system as a whole.

The result of measuring static pressure is the ability to prescribe duct renovation work. You begin to see that the duct system is what controls equipment capacity and efficiency. The efficiencies and capacities of the equipment are separated from the building by the air distribution system. Only by getting the ducts operating properly can you assure the system as a whole is operating properly. The equipment is only a component of your system.

Everyone sells high efficiency equipment thinking they are distinguishing themselves from their competition. That’s been around for over 30 years! The new frontier is the duct system. When it comes to value, a properly installed, tested, and balance duct system is worth far more than a new furnace. It is our ability to design, install and balance a duct system that can set us apart from our competition. Over the last decade the equipment has become a commodity. It has a fixed price and is available from every contractor in the yellow pages.

Static pressure measurement unlocks the door to assessing the HVAC system as a whole. Using these measurements, you can gain access to an amazing amount of duct repair and renovation work from the systems you service every day. The margins are extremely high and the skills required to perform duct renovation are minimal.

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 one page procedure detailing just how to measure total external static pressure, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.