Saturday, November 7, 2009

The New 70

Many of the old timers I talk to who are still leery of R-410A want to know what the “new 70” is. They want a target number for the suction pressure of an R-410A air conditioning system. Many service technicians are in the habit of charging to a target number on the suction side. This is frequently 70 psig on R-22 systems. The saturation temperature of R-22 at 68.5 psig is 40°F and for many years the standard evaporator temperature at design conditions has been 40°F. But 70 is a nice round, easy to remember number and close enough for quickie rules of thumb. The problem is that systems seldom operate at design conditions. Another big problem is that conditions other than charge can cause low suction pressure. If a technician is only checking the suction pressure, they are not collecting enough information to recognize other system factors that can contribute to low suction pressure. A common error of inexperienced techs using the “70” method is overcharging systems that have low evaporator airflow. I have gone behind someone using the “70” method who had overcharged the system so severely the compressor stalled at startup and pegged my high side gauge past 500 psig. The customer was told that the compressor was bad. To be fair, most experienced technicians that use the “70” method understand how airflow and system operating conditions affect system pressures, they just don’t want to bother with manufacturer’s charts. They just modify the target up or down as they judge conditions affecting system pressures. These folks can usually get a system cooling; that is why they still have a job. However, the system will often not be performing optimally when they leave. Today’s customers are paying a premium for systems that are more efficient and have less environmental impact than older systems. Beginning January 2010, even the least expensive unit a customer can purchase will be non-ozone depleting and more efficient than the least expensive unit they could purchase just a few years ago. This means everybody is paying for efficiency and reduced environmental impact. However, if technicians don’t charge units properly, customers are not getting what they are paying for. Your students can be the vanguard of a new era that values professionalism. Make sure your students know how to handle new refrigerants like R-410A and know how to read and follow manufacturer’s charging charts. That will put them in a position of leadership early in their career.

Fundamentals of HVAC/R has a detailed discussion of system charging in Unit 27 Refrigerant System Evacuation and Charging. Variables affecting system pressures, common methods of determining the correct charge, and common methods of adding refrigerant are all covered. Charging is also discussed in other units including

  • Unit 35 Residential Split system Air conditioning installations
  • Unit 36 Troubleshooting Split System Air Conditioning
  • Unit 52 Heat Pump Installation
  • Unit 53 Troubleshooting Heat Pump Systems
  • Unit 83 Troubleshooting Refrigeration Systems
  • Unit 84 Installation Techniques
  • Unit 85 Planned Maintenance
  • Unit 86 Troubleshooting

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