Some conversations with like-minded HVAC professionals have a way of taking on a life of their own, especially when they’re from another country. I recently had a discussion with an HVAC contractor from Australia about total external static pressure (TESP) and testing the systems they install and service.
As we discussed test locations and system types, I discovered all airside pressure measurements and equipment specifications our Australian friends use are in pascals (Pa). In the United States, we use inches of water column (in. w.c.), so I was scrambling to convert the numbers.
Our conversation left me thinking — is it time for our industry to consider using pascals for static pressure measurements? What would the transition from inches of water column to pascals look like? How hard would the transition be and what are the advantages and disadvantages of changing? Let’s dig a little deeper into what a pascal is and what our industry might encounter if we changed pressure units.
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Some Pascal History
The pascal is named in honor of Blaise Pascal, a French mathematician and inventor who lived in the mid-1600s. Part of Pascal’s work dealt with fluid dynamics and air pressure in the experiments he conducted.
From his work, Blaise defined many principles we still use today in our industry. We know one of these principles as Pascal’s law. It states pressure in an enclosed container exerts equally on all sides of the container. If this sounds familiar, it’s because the simplified definition of static pressure is very similar.
Pascals are not a new pressure measurement. The building performance industry has used them for decades. So, if you have any knowledge of building pressure testing, blower door testing, or duct leakage testing, you’re familiar with this unit of measurement. However, its use in the HVAC industry is minimal, so it may be new to you.
The Pascal to Inches of Water Column Relationship
To consider changing pressure units to pascals, we need to look at the relationship between the two and then connect them to familiar numbers you may have seen in the past.
Two important conversions to remember are:
- 1 in. w.c. = 250 pa
- 1 Pa = .004 in. w.c.
To convert from inches of water column to pascals, multiply your inches of water column number times 250 to calculate the value in pascals. Pay close attention to your decimal place or your result will be way off.
For example, many in our industry use .10 in. w.c. as their duct design friction rate of choice, although often incorrect. To convert this number to pascals, multiply .10 x 250 and you’ll find the result is 25 pa (.10 x 250 = 25 pa).
The 25 pascals number will look familiar to some of you. It is the standard pressure used for duct leakage testing. I’ve often wondered if the duct tightness testing standards assumed an average duct operating pressure based on the .10 recommended residential setting on most duct calculators.
Another common pressure measurement to consider is the air handling equipment’s maximum rated TESP found on the data plate. Common residential ratings are .50 and .80 in. w.c.:
- .50 would equal 125 pa (.50 x 250 = 125 pa)
- .80 would equal 200 pa (.80 x 250 = 200 pa).
If you want to convert pascals to inches of water column, multiply your pascal number x .004 to see what your inches of water column result would be.
For example, 50 pascals is the blower door target number when air leakage testing a building. To see what this reading would be in inches of water column, multiply 50 pa x .004 and you’ll find the result is .20 in. w.c. (50 x .004 = .20 in. w.c.).
Once you understand how to convert these two pressures back and forth, it’s time to weigh the pros and cons of using pascals to measure static pressure.
Advantages of Using Pascals for Static Pressure Measurements
One of the most frustrating parts of static pressure measurement is dealing with decimal places. If our industry adopted pascals, we wouldn’t have to deal with decimal places. Pascals are whole numbers, making them easier to interpret and diagnose. You don’t have to worry about misplacing a decimal place or remembering if your reading is in tenths or hundredths of an inch of water column.
Decimal places can also conceal excessive static pressure measurements. If the maximum rated TESP of an air handler is .50 in. w.c. and your measured TESP is .90 in. w.c., it doesn’t look that bad and can cause you to minimize an airflow problem.
However, if you looked at the readings above in pascals, the numbers would appear a little different and may stand out more. Converting to pascals, the maximum rated TESP of the air handler would be 125 pa and the measured TESP would be 225 pa. Notice the difference? Larger numbers grab our attention.
It’s much easier for your customers to understand and relate to whole numbers when you explain static pressure measurements. There is little room for interpretation, and it is easier to convert your static pressure measurement to a blood pressure equivalent. This comparison is one of the simplest ways to help your customers understand the health of their HVAC system. Using pascals can make the discussion easier.
Finally, pascals are also used to measure building static pressure. Many comfort problems you encounter are a combination of HVAC system and building pressure issues. If you’re using the same pressure units, the connection between the HVAC duct system and the building side of the duct system becomes much easier to understand and explain.
While these are just a few benefits of using pascals to measure static pressure, there are also drawbacks to consider.
Disadvantages of Using Pascals for Static Pressure Measurements
A considerable drawback is that most manometers aren’t accurate enough to measure pascals. Remember, one pascal equals four-thousandths of an inch of water column, and you need a test instrument that can measure that low.
Since a standard manometer isn’t sensitive enough, it will often read “0.” So, if you want to measure pascals, you must invest in a micromanometer. This is a manometer that measures extremely low pressures.
You’re in luck if you own a blower door or duct leakage testing fan. You already have one of these test instruments. Companies such as Retrotec and The Energy Conservatory (TEC) both manufacture blower doors and duct leakage testing fans that come with a micromanometer.
However, if you don’t have one, the cost of a micromanometer can be upwards of $1,400. Luckily, The Energy Conservatory makes a simple and affordable micromanometer called a DG-8. It fills the gap between a standard manometer and the advanced micromanometer that comes with a blower door or duct leakage testing fan. Some HVAC professionals will not make the investment needed to measure pascals.
The biggest obstacle to using pascals for static pressure measurements ties to air handling equipment ratings and specifications. Manufacturers in the United States typically rate their air handling equipment in inches of water column. They offer no pascal rating. So, you would have to convert all inches of water column measurements to pascals.
Many technicians compare their measured static pressures to industry specifications when troubleshooting. Salespeople also point to the same specifications when they discuss upgrade opportunities with their customers. If pascal values aren’t published in specifications, the numbers appear as if they’re pulled out of thin air instead of manufacturer design.
For a changeover to pascals to be practical, it would take an industry-wide effort with all equipment manufacturers agreeing to publish pascals and inches of water column on their data plates. Are dual pressure ratings important enough for manufacturers to help make pascals a reality?
Pondering the Practicality of Pascals
I hope this article caused you to think about other possibilities for how we measure and describe static pressure readings. In the end, you decide which pressure measurement to use based on their advantages and disadvantages.
Increased accuracy leads to better test results. However, improper testing practices will kill the accuracy of any manometer, no matter how accurate. High accuracy from bad test locations still equals bad results.
Know how and where to measure static pressures before you begin your journey. Don’t make that mistake when measuring airside pressures. If you’re new to static pressure measurements, email me for a quick start guide to help start your adventure.
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