Benjamin Franklin was one of the most creative minds in American history. To aid him in his creative problem solving, he liked to use a piece of paper with two columns drawn on it. Franklin would write the problem he was trying to solve at the top of the page. Then he would name the left-hand column “Pros” and the right-hand column “Cons.”
Next, he would write the benefits of the problem under the “Pros” column and the drawbacks under the “Cons” column. Once Franklin had all his ideas on paper, he would look at the results, make a “Decision,” and start to solve the problem. Let’s use Ben Franklin’s method as a guide to answer a common static pressure question — how many hoses should you use with your manometer?
One Hose or Two?
If you do any research on static pressure, you’re bound to come across a lot of photos that show different hose (tubing) configurations for pressure measurement. One example will show a single hose measuring pressure while other examples show two hoses. So, should you use a single hose or two? How do you know which method works best?
When I first learned static pressure measurement, the illustration I saw the most was an inclined manometer with two hoses. I was getting started, so I bought a used kit from a local tech rep.
Once I got my kit, I couldn’t find an explanation about what the hoses should read, which port I should attach each to, and why that was important. If you ever used an inclined manometer and got covered in red gauge fluid, you understand my frustration. I didn’t know what to do with the hoses, so I made a mess. Thank goodness for today’s manometers.
Test Instruments You Need
To start you out on the right path, make sure you have a pressure testing kit that includes:
- Manometer — digital or analog (Magnehelic™)
- Static pressure tips and hoses — neoprene or silicone
- 3/8-inch test port plugs
- Small drill/impact gun with a step bit and 3/8-inch drill bit with a sheath/stop
- Thin screwdriver — for cleaning out internal duct liner/insulation.
You can assemble a complete kit with all these items for less than $200. If you choose a higher-end manometer, expect to invest more.
When you perform a single-hose static pressure test, measure and record individual readings from the system one at a time. One hose allows you to see the pressure at specific locations in the system. On a typical system, you need to install four test ports to gather the measurements.
Once you document the readings, you need to do some math. Add for total external static pressure, subtract for pressure drops, or directly read duct pressures. Then compare the readings to manufacturer specifications, industry standards, or NCI Static Pressure Budgets (email me for your copy) to diagnose potential airflow problems.
Based on this information, let’s apply the Franklin method to decide whether it’s better to use one hose or two. First, get yourself a piece of paper and write down the benefits and drawbacks of using one hose. Maybe you come up with something like the following:
- Need one hose and static pressure tip — less cost
- Can see pressure at each test location — pressure profiling
- Can find the most restrictive point in the system
- Effective airflow diagnostics.
- Can measure in the wrong location and not know it
- Need to do the math
- Could perform the calculations incorrectly
- Might get inaccurate results.
Decision: Use one hose when you diagnose airflow issues. You can measure individual readings to identify the highest pressures and use them to diagnose system restrictions.
When you perform a two-hose static pressure test, you take two measurements simultaneously across a component in the airstream. On a typical system, you install test ports before and after each component (filter, coil, equipment, duct fitting) to gather the pressure readings.
You can directly read static pressures on your manometer display (total external static pressure, coil, and filter pressure drops) and then compare these numbers to manufacturer specifications or industry standards. Let’s apply the Franklin method again.
- The manometer does the math
- Less risk of calculation errors unless
- Will make me pay attention to correct test locations.
- Can’t see where the highest pressure in the system
- Will have a harder time discovering airflow restrictions.
Decision: Use two hoses if you’re taking direct measurements such as:
- Total external static pressure (TESP)
- Filter pressure drop (∆p)
- Coil pressure drop (∆p).
As you can see, there are benefits to using both hose configurations — no single method works best for all situations. If you stick with one method, you will overlook hidden airflow and pressure interactions. You must look at both methods, weigh the “Pros” and “Cons” for each situation, and make a “Decision” on the best choice.
Keep Your Readings in Context
With any static pressure test, be aware of taking measurements in the wrong test location. If you’re in doubt about the correct location, record multiple pressures on a system diagram. Document multiple pressures so you can review your readings. You may find you measured in the wrong spot, and those extra readings will give you a chance to redeem yourself.
At a minimum, measure and record at least four pressures on each system you test. You’ll find that about one out of twenty readings are a little goofy. If a reading doesn’t make sense, dig a little deeper to understand why.
On gas furnace systems, take your four measurements before the filter, after the filter, before the coil, and after the coil. On air handlers and package units, you typically take measurements before the filter, after the filter, after the coil, and in the supply duct.
Make Ben Franklin Proud
Now that you’ve seen the Ben Franklin method applied to hose setup, you can use it to make other decisions regarding static pressure measurement.
If you don’t currently measure static pressure during your installations and service calls, perhaps you should think about making a Ben Franklin chart. List the reasons why you should and shouldn’t add this practice to your service offerings. You can use a piece of paper, a dry erase whiteboard, or a digital option like Microsoft Word. All of them will work. They key is to start capturing your ideas.
You can also use this approach for other situations. The trick is to not overthink the process — keep it simple. Get your ideas out so you can examine the pros and cons in a rational, unemotional way and then decide on the next course of action.
David Richardson serves the HVAC industry as a curriculum developer and trainer at National Comfort Institute, Inc. (NCI). NCI specializes in training that focuses on improving, measuring, and verifying HVAC and Building Performance.
If you’re an HVAC contractor or technician interested in learning more about static pressure measurement, contact me at firstname.lastname@example.org or call 800-633-7058. NCI’s website www.nationalcomfortinstitute.com is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.