R410A Charging & Metering
One of the bright sides of the change from the HCFC R22 to the environmentally friendly HFC R410A refrigerant and the higher minimum efficiencies of air conditioners and heat pumps is that many manufacturers have gotten away from cheaper orifice and capillary tube metering devices and gone back to the old industry standard of Thermostatic EXpansion Valves (TXVs or TEVs).
If you don’t understand refrigerant metering, liquid refrigerant coming from the outside (condensing) unit is metered into the indoor lower-pressure (evaporator) coil, which results in the cooling. The more efficiently and regularly the refrigerant is metered into the coil, the more efficient the system. Yet in past years, achieving the lowest-cost product in “builder model” air conditioners and heat pumps became the goal for almost all (if not all) HVAC manufacturers. The first thing to go was the high-priced TXV metering device (which ensures optimum filling of the evaporator), to be replaced by simple restriction devices which aren’t as efficient, but are much cheaper to manufacture.
I checked with representatives of two of the major residential equipment manufacturers (Trane and Carrier) and was told that they’re now using TXVs across their lines - GREAT! However, this changes the way that technicians charge the systems.
With orifice or capillary metering, the amount of refrigerant entering the evaporator coil was determined by the pressure difference between the high-pressure liquid entering the metering device and the lower-pressure liquid and evaporated gas leaving it. So, the temperature of the air flowing through the outdoor coil (the higher the temperature, the higher the pressure) and the total temperature of the air flowing across the indoor coil (the lower the temperature, the lower the pressure) had a great effect on how much refrigerant flowed through the equipment. Thus, charging charts had to be created to show the different pressures at different indoor and outdoor air temperatures, which most technicians never used, so most residential air conditioners are poorly charged and few actually meet their rated efficiencies.
However, with TXV metering, the charging methods are much simpler and the results are far more reliable. TXVs have a sensing bulb that feels the temperature of the refrigerant leaving the evaporator, and it adjusts its flow to maintain a constant rise of around 12°F and 14°F (depending on the system efficiency) between the lines entering and leaving the evaporator.
Also, the charging process is far less critical and much easier to perform, since the complicated charging charts are no longer necessary. About all that is really required (although I’m sure some manufacturers will argue with this) is to slowly charge the system until the temperature of the liquid line (as read with a thermometer) is somewhere around 12°-14°F cooler than the condenser coil internal temperature (as read on a gage) - it’s the old “subcooling” method. Because liquid-line subcooling can’t be determined through a sight glass, it is ineffective for use when charging such systems.
R410A is a blend of three refrigerants, so any leakage will cause a change in the blend, resulting in reduced system efficiency. All refrigerant must be removed and replaced with new refrigerant.
The manufacturer-provided charge (inside the condensing unit) is the method most commonly employed in the field on new installations. But remember that longer line lengths, oversized liquid lines and the addition of filter-driers require additional charge to be added to obtain full system capacity and efficiency.