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HVAC Diagnostics are Like a Jigsaw Puzzle

Jan. 22, 2021
Inspecting the entire HVAC system allows you to see how the individual diagnostic puzzle pieces fit together, to produce the solution you seek.

When you diagnose an HVAC system, each diagnostic test should fit together, like a jigsaw puzzle, revealing the whole picture. Single diagnostic tests often lead to partial, ineffective, and unneeded repairs. Let's take a look at what happens when you focus all your attention on only one test. It's like trying to understand an entire jigsaw puzzle by looking at only one piece.

One Piece of the Puzzle
An example of relying on a single piece of the diagnostic puzzle may include testing only the refrigerant circuit's performance. Other singular tests include measuring voltage, taking a single temperature or combustion reading, testing static pressure or airflow. Incomplete diagnostic results may also occur when you focus only on duct leakage testing, load calculations, or duct design.

An example of relying on a single piece of the diagnostic puzzle may include testing only the refrigerant circuit's performance.

Staring at just one piece of a diagnostic puzzle may mislead you and your diagnostics, and that could leave the current problem unresolved. Nobody wins when you make this mistake. 

The Answer is on the Puzzle Box
The official name for a person who solves jigsaw puzzles is Enigmatologist. Every puzzle solver agrees the way to begin is to form a firm picture of the end-result in your mind. Enigmatologists study the completed picture on the front of the puzzle box. You can mirror this step by inspecting the system to discover those defects that need correction before your puzzle is complete.

Inspecting the entire system allows you to see how the individual diagnostic puzzle pieces fit together to produce the solution you seek.

Equipment manufacturer specifications and engineering data provide additional clues to guide you. Never assume you're so good at diagnostics that you're above taking time to learn from this step. Inspection and reviewing available equipment data is as essential to you as a puzzle picture on the box is to a puzzler. 

Simple, Challenging, and Difficult Pieces to Connect
When you solve a jigsaw puzzle, you might start with the easy-to-find pieces and connect them. Most gather all the obvious puzzle pieces that have straight edges on one side. These are the puzzle's perimeter or edge pieces. 

The next diagnostic step requires you to look a little broader and deeper as you discover how the loops and sockets (official jigsaw puzzle terminology) of the puzzle pieces connect. Remember, one puzzle piece alone tells little. The more pieces you connect together, the more complete the picture and solution becomes.

Equipment temperature charge testing and diagnostics is probably the most misdiagnosed HVAC diagnostic test.

Let's delve more into one of the most repeated diagnostic tests service and maintenance techs worldwide use -- equipment temperature change testing and diagnostics. Usually, the prescribed solution from this test result is incorrect.

 While this diagnostic test may seem routine and straightforward, it is probably the most misdiagnosed HVAC diagnostic test. Remember, not every test needs to go to the extreme described in this scenario. But your awareness and ability to test and use more in-depth diagnostics will help you diagnose like a master. 

Equipment Temperature Change Scenario
This scenario begins with the singular test and calculation.

1. Our technician, Phil, measures and subtracts equipment entering and exiting temperatures to find a 20-degree temperature change in cooling mode. Like most techs, he smiles, walks away, confident the equipment works correctly and moves on to the next job. Well, as you'll soon see, maybe the equipment works, or maybe it doesn't.

Phil only looked at one piece of the puzzle and thought he saw the whole picture. Do you do that too? 

2. As Phil goes back to the truck, he looks at the service request on his phone. The reason for the call was the system wasn't cooling enough. He decides to diagnose deeper as he remembers a recent article about proper temperature probe location. 

3. Phil returns to the system and installs a test port downstream from the coil at a point where the probe won't be affected by the cold coil surface. He finds the air temperature is four degrees less than his first measurement! He is shocked as he recalculates his equipment temperature change from 20 to 16 degrees.

He checks the service record and finds that his company added two pounds of refrigerant two months ago – another piece of the puzzle.

This added piece to his diagnostic puzzle opens his eyes to a much bigger picture. Was it time to hook up the refrigerant gauges? He checks the service record and finds that his company added two pounds of refrigerant two months ago – another piece of the puzzle.

His mind is invigorated by this discovery. He remembers learning that airflow through the equipment influences temperature change and refrigerant charge.

4. Next, he decides to check fan airflow by plotting it on the fan table. He measures total external static pressure at .94-in. w.c. (inches of water column). On the nameplate, he reads the fan is rated at a maximum total external static pressure .50-in. w.c. 

5. Wow! it is almost double what it should be. Next, Phil checks the fan speed setting and finds the setting is on medium speed. He plots fan airflow and discovers 265 cfm per ton. This was a challenging puzzle piece, but it fits perfectly with the others.

Phil begins to wonder why his company added refrigerant on their last visit – they stopped diagnosing the system after only one test, or did the system really need additional refrigerant? He wonders how many times he has made that same mistake because he failed to see the complete picture.

6. Another check of the service history shows the previous technician found no filter in the system the year before. This clue leads him to a problematic diagnostic piece of the puzzle. He wonders if the coil is plugged due to operating without a filter. He finds the specified coil pressure drop by "Googling" the coil model number on his phone. It carries a rating of .28-in. w.c. so he decides to measure the coil pressure drop and compare it to the coil specifications.

7. Phil installs test ports before and after the coil. He measures and subtracts to find the coil pressure is .62-in. w.c. With this additional puzzle piece connected, he sees that the plugged coil is the problem. The blockage both restricts airflow and severely limits its cooling capacity. He inspects the coil and confirms it is plugged with gunk and ice is forming.

8. Finally, it's time to check the refrigerant charge. Pressures confirm the charge is low, and Phil finds a slow leak.

9. With this many puzzle pieces connected, Phil now sees a much bigger picture and is ready to prescribe repairs. He describes the issues to his customer, using care not to get technical. He recommends coil cleaning, setting airflow to the required amount, repairing the leak, adjusting the charge, and testing to verify restored equipment capacity.

The repair cost was more cost than Phil's customer anticipated. Still, they now have increased confidence in Phil because he explains his diagnostics and findings, so they understand problems and the results. 

10. Phil's customer approves the repairs, so he cleans the coil and adjusts the fan speed. When he tested out, Phil found the coil pressure drop was within 10% of the specified pressure drop, and airflow was at 390 cfm per ton. After replacing the lost refrigerant, Phil's refrigerant pressure and temperature measurements confirm the system is properly. With measured values all within the target, his customer is delighted as he confidently reports the equipment is operating within its specifications.

11. He concludes by describing that other system defects they may now notice and opens the door to duct and airflow diagnostics, should they feel the need for more system improvement in the future.

One Piece or the Whole Puzzle?
Great enigmatologists and technicians are well-aware that with a little extra effort and skill, the puzzle pieces fit together. The more puzzle pieces you connect, the easier it gets to see the whole picture.

Hopefully, this will prompt you to combine multiple pieces of your diagnostics puzzles in the future to find better solutions and improve your skills. Your effort will be rewarded with career advancement and the satisfaction of serving your customers with excellence.

Rob "Doc" Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC professional interested in a free procedure to measure HVAC system temperature changes, contact Doc at ncilink.com/ContactMe or call him at 800-633-7058. Go to NCI's website at  nationalcomfortinstitute.com for free information, articles, downloads, and current training opportunities

About the Author

Rob 'Doc' Falke | President

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC-based training company and membership organization. If you're an HVAC contractor or technician  interested in a building pressure measurement procedure, contact Doc at [email protected]  or call him at 800-633-7058. Go to NCI’s website at NationalComfortInstitute.com for free information, articles and downloads.