Manometers or pressure gauges have been used to measure air pressures for more than 200 years. The fathers of our industry used these gauges to learn about the mysteries of fans and duct systems. We use these same instruments (Although a little updated) to reveal similar mysteries about the systems we install and service today. Measuring pressure is just as critical in our time.
Without testing and using the right tools, many technicians make incorrect assumptions about air capacities and performance the HVAC systems on which they work.
As you become proficient at measuring static pressure, you'll be able to tap into a huge inventory of HVAC systems that operate far below the equipment capacity.
When you measure system static pressure, you'll discover a long list of duct defects and other problems that cause poor system performance. By measuring actual operating conditions, you'll be able to offer real solutions to many existing comfort problems that are overlooked by your competitors.
Having the right tools is essential. Fortunately, pressure measurement tools are fairly inexpensive. A complete static pressure kit including the necessary accessories can be purchased for less than $200.
A good quality manometer is inexpensive and is a reliable static pressure testing instrument. For residential and light commercial system testing, analog instruments should have a range from 0-1-in. w.c. Most digital gauges should be rated from 0- to 5-in. Typical resolution should be .01-in. w.c. and have an accuracy of + or - 2%. Low quality instruments or ones with too broad of a test range, or ones with a single pressure port will make testing difficult and you may end up discouraged.
National Comfort Institute recommends the Dwyer Magnehelic gauge, model 2001 AV PORT, or any good quality digital manometer plus the accessories listed below:
- The manometer of your choice
- Carrying case
- Two – 3-ft. lengths of 3/16-in. I.D. rubber or neoprene tubing
- At least one Dwyer Model A-303 Static Pressure Tip
- 3/8-in. bullet tip drill bit
- A 1-in. O.D. x ½-in. I.D. x 4 1/2-in. long sheath to use over the drill bit
- And don't forget 3/8-in. plastic hole plugs
System Airflow Capacities
We measure static pressure to interpret airflow in a system. It works similar to blood pressure, the higher the pressure on the heart, the less blood it can move. With fans, the higher the static pressure in the system, the less air it can move.
Most manufacturers’ engineering manuals rate airflow in furnaces, fan coils, and air handlers based on Total External Static Pressure (TESP). A typical furnace is designed to operate at a maximum of .50-in. w.c. If there is more than .50-in. of Total External Static Pressure, the fan is unable to move the required 400 CFM per ton.
Check the fan capacity of the furnaces and air handlers you select carefully, the pressure capacities will range from .05- to 1.0-in. That is from 1/10th to two times the capacity of the typical fan usually rated at .50-in., so select carefully.
Bottom line: Some furnaces have pressure ratings so low they can’t even move 400 CFM per ton with no ductwork and only a coil attached! The static pressure ratings are so low, there is not enough available pressure left to install a duct system. While others have enough pressure to easily handle high-efficiency filters or coils.
Review your engineering data carefully. Proper equipment selection is now your responsibility. Many rated equipment combinations have extremely high efficiency ratings but do not have fan capacity. To meet the government demands for higher efficiency, many manufacturers have lowered fan sizes to decrease power consumption, and increased the rows of fins per inch in coils to get the appearance of increased efficiency, but the net effect is equipment that has very low airflow capacity. If you’re not careful, you’ll be loser in this game of politics.
Static Pressure Testing, An overview
To determine Total External Static Pressure of a furnace you first measure the supply (+) pressure near the discharge of the unit, between the furnace and the coil. Then you measure the return (-) pressure near the inlet of the unit, often between the filter and the furnace.
Finally you add the two pressures together to determine the system’s TESP. When adding the positive and negative pressures together. Disregard the + and – signs. These only indicate the type of pressure measured.
Compare the measured TESP to the rated pressure capacity of the unit (normally found on the nameplate data). The rule is if the TESP is less than the nameplate data, the airflow will be at or above 400 CFM per ton. But if the measured pressure exceeds the rated pressure the airflow will be lower than 400 CFM per ton.
Read the manufacturer’s blower performance data to interpret what the fan is really doing under measured operating conditions. Remember also, that fan capacity can vary depending on the voltage at which a fan is operating, the cleanliness of a fan, or the condition of a blower motor, or duct configurations near the fan.
In the next Hotmail, we’ll discuss how to measure static pressure, step-by-step.
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 firstname.lastname@example.org or call him at 800/633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.