In this series of articles, I will introduce you to Jeff and Mike. Jeff is a NATE certified technician and a student of his profession. Mike has just graduated from HVAC tech school and is riding along with Jeff as an apprentice. We will follow them along as they run their service calls together.
Their first call of the day is Mrs. Thomas. The service call dispatch information notes that, "Her system is continuously running and does not keep the house cool during the hottest part of the day."
Mike: "What do you think the problem is?"
Jeff : "You know Mike, I never try to diagnose a problem before I get there, so we'll have to get there and perform a proper diagnosis."
As they arrive, Mrs. Thomas is already opening the garage door and is happy to see them.
Mrs. Thomas: "I have my thermostat set to 76 degrees and it seems fine in the morning, but in the afternoon my thermostat reads 80 degrees. It still reads 80 degrees even if I set my thermostat 69!"
Jeff can see Mike is excited to be on his first service call
Mike: "I bet the system is undercharged, let's go check."
Jeff: "Okay, I'll let you tackle this one. Grab your tools and gauges and let's see what's going on."
After they check the filter, air handler, and evaporator coil cleanliness, they are satisfied that the system has adequate airflow and appears to be properly maintained.
Jeff: "Always remember before you hook-up your gauges or begin to diagnose, to always check the system to see if it is clean and properly maintained. Let's go out and look at the condenser now."
Jeff tells Mike to hook up his manifold gauges and temperature clamp. They note on their paperwork that it is a 3 ton R-22 system.
As Jeff observes, Mike hooks up his gauges.
Mike: "All set! Now I am going to check the high side pressure, the low side pressure, and superheat, just like I learned in class!"
Mike waits 10-15 minutes for the system pressures to stabilize and notes on the invoice, "High side pressure 195. Low side pressure 55. Superheat 35F."
Mike is excited because he knows EXACTLY what is wrong.
Jeff: "I know what's wrong! The high and low side pressures are low, the superheat is high. This system is undercharged! Let's go tell Mrs. Thomas and charge it up."
Mike: "So you think it is undercharged, and it is a valid observation but, what kind of metering device does this system have?"
Mike: "Why does it matter? High superheat means the system is undercharged."
Jeff: "Do you remember, from your training, the difference between a fixed metering device and a TXV?"
Mike: "Not really, TXVs are confusing to me."
Jeff: "Let me explain. Fixed metering devices meter the refrigerant based on the high side pressure. They are not very efficient under varying load conditions and don't have much control over the amount of refrigerant they meter. On the other hand, a TXV has only one function... precisely metering the flow of liquid refrigerant to the evaporator to match the heat load of the system."
Mike: "I remember that from class but how is a TXV different?"
Jeff: "TXVs respond to the temperature of the refrigerant gas leaving the evaporator and the pressure of the evaporator coil to maintain a predetermined superheat. Why don't you go check to see what type of metering device we're dealing with."
Next month we'll find what Mike and Jeff discover.
What did we learn from Mike and Jeff today?
- Do not diagnose a problem before you get to the call. NATE Soft Skills
- The customer can give you information to help you in your diagnosis. NATE Soft Skills
- To perform an overall check for proper maintenance of a system before diagnosing. NATE Core
- High superheat can mean an undercharged system with a fixed metering device. NATE Air Conditioning/Heat Pump
- The difference between a fixed metering device and a TXV. NATE Core, NATE Air Conditioning/Heat Pump