Systematic Troubleshooting Part 1

I decided to break this post up into parts, not because I am an atomist at heart, but because if history is any guide, there will be more to this topic than I originally give it credit for. So to start, lets pick up where we last left off.

In my last post, I delved into how the wisdom of knowing how a system should be operating based on the ambient, load and other conditions present, can become the basis for recognizing what went or is wrong with the system and some likely places to start investigating. Once again, let me emphasize that this is not the same as guessing what could have happened based on little to no information. This is an educated hypothesis based on the maximum amount of information that you can obtain from all of the factors mentioned above. The former will lead to replacing components or making adjustments without any basis to do so and likely will not eliminate the real reason the system failed in the first place. The latter will allow you to initiate the process of systematic elimination of possible suspects until the correct one/s are identified and will eliminate the cause/s of the failure or poor performance from further affecting the system down the road. Big difference.

So what are some of the factors that we will be taking into consideration when ascertaining how the system should be operating? Here I list a few but keep in mind that there are a lot more and there may be a combination of several working together to ruin your weekend so keep this in mind. Note how none of these require using your manifold gauges. These will come into play later. For now, this will be an inspection that only requires your human senses to accomplish.

• Low/high temperature ambient conditions if the condenser is located out side or in an unconditioned space. This is one of my favorites because one, you can't change the weather, and 2 this is something that most techs do not take into consideration. It can however, deeply impact how the system will be operating.
• What is the expected evaporator loading? Would the system be classified as being in "pull down" mode?
• Is there ice on the evaporator or any other crud that could interfere with heat transfer?
• Is the system equipped with a TXV or metering device? This is another biggie as the system will respond completely different to changes in condensing temperature (Think ambient temperature swings.) depending on which type of metering device is being utilized.
• Are there fan controls present on the condenser?
• Does the system have a receiver? Does it have a suction line accumulator?
• Are there any pressure regulators installed in the system?
• Do you hear any squealing/hissing noise along the liquid line and it's associated components? Any frost formation on any of these components?
• Is there anything near the condenser that could interfere with airflow?
• Is there a large difference in height between the compressor and condenser/evaporator?
• Is the system a compact package system or is it a split type system with long piping runs?

You may wonder what these things may have to do with how the system should be operating, but trust me, they all do.

• A split system may only take a fraction of the refrigerant and oil charge that a split system of the same capacity will require. This can determine if a suction line accumulator, crank case heater or additional oil is required or not.
• Subcooling will increase during high ambient conditions with a TXV while it will do the exact opposite with a piston metering device.
• If the evaporator is under high load, expect high superheat levels and high evaporator pressures with a TXV. This is also when an undersized metering device or a slight under charge of refrigerant will be most systematically apparent.
• A receiver will make the refrigerant charge less critical as long as there is sufficient refrigerant charge to meet the expected operating requirements such as high load. As well, it's refrigerant holding capacity is critical if there is a condenser pressure regulator installed in the system

Once you have made note of these things in your initial inspection, you will now have an inventory of factors that can or will be affecting the system operating characteristics. In other words, you have established a baseline system performance expectation that you will use for the next part which is the actual topic of this post. We have also established why I made this a 2 parter post.

To better demonstrate the concept of systematic troubleshooting, lets wrap up this first part with an example from a presentation that I did a while back of what I would term effective and non effective troubleshooting. These are based on actual situations that I have run across over the years and will hopefully demonstrate the different approaches to a problem. I will start of the second part with a few more examples then delve deeper into the subject.

Here is the introduction from that presentation:

• How well a technician troubleshoots a problem in a system is dependent on their level of technical knowledge and their approach to solving challenges.
• Even technicians with a good depth of technical knowledge can miss or misdiagnose issues if they do not take a systematic approach to understanding how things were/are operating.
• The main issue here is focusing only on a particular component or symptom and missing or not understanding the relevance of other factors that are present
• Now for the example: Mediocre Approach (Bare Minimum)
• Example 1: System not cooling
• Technician arrives. Notes compressor not starting. Ohms single phase AC compressor, windings are open between C&R and C&S. Recovers refrigerant and replaces compressor. Evacuates using manifold gauges to measure pressure.
• Stops vacuum pump when gauge states 30” Hg. Breaks vacuum with refrigerant. Installs new compressor and charges according to superheat
• Compressor sent in for warranty inspection and is found to have seized due to overheating. Warranty refused.
• Replacement compressor fails due to same causes 2 years later. Home owner furious and calls different contractor

Proper Approach (Second contractor’s technician)

• Technician arrives. Notes compressor not starting. Ohms single phase AC compressor, windings are open between C&R and C&S. Tech checks between R&S to verify internal overload is not open.
• Recovers refrigerant, weighing the charge removed and notes the system was substantially undercharged as compared to manufacturers nameplate requirements. Suspects leak.
•  Evacuates using a micron gauge to measure pressure. System will not pull down below 10000 microns and pressure rises when pump is turned off verifying a leak.
• Pressurizes with nitrogen and finds leak at TXV inlet and fixes leak.

• Replaces compressor and evacuates system to 500 microns, noting no rise in pressure when pump is turned off. Charges system according to manufacturer’s recommendation.

In addition (Remember we are talking about a great technician here.)

• Explains to home owner that the compressors failed due to the system being under charged due to a leak and that he took the necessary steps to ensure the leak was fixed. This helps alleviate the greatest concern of the customer which is "How can I be sure it will not happen again?" and builds faith in the technician.
• Tech informs homeowner that a yearly preventative maintenance inspection would have discovered the leak in the spring before the compressor failed and could have saved them a lot of money.
• Homeowner may not happy with the cost of replacing a second compressor, but now understands why it failed and trusts the tech and his employer to have done it right. 5 years later, system is still running fine with no drop in performance and contractor has the yearly PM contract with the customer.
• When it is time to replace the system with a new one, guess who the homeowner will call?