I’ve seen some quick-connect fittings that allow pressure loss, which can cause inaccuracies in the charging procedure.Ģ. If you use a quick-connect fitting on the end of your hose, make sure it’s a low-loss fitting. Attach your gauge manifold to the liquid line service port. WATCH THIS VIDEO FROM YELLOW JACKET FOR EXTRA TIPSġ. It’s usually between 5☏ and 15☏ but always go by the manufacturer’s requirements. The manufacturer will usually specify the required sub-cooling on the outdoor unit service panel.
Liquid sub-cooling is normally measured at the liquid line service valve. Low refrigerant flow will cause a loss of capacity and efficiency in a cooling system. Vapor bubbles present in the refrigerant will cause low refrigerant flow. Liquid sub-cooling is required so that you have only liquid entering the expansion valve with no bubbles present. Expansion valve systems are normally charged by using the subcooling method. On many newer systems, especially high-efficiency systems, thermostatic expansion valves (TXVs) are used. Correct the airflow problem and check the charge again. Note that if the suction superheat is correct and the suction pressure is low, the system probably has low airflow. Overcharging can damage the compressor and will also reduce both the capacity and efficiency of the system. You don’t want to overcharge the system if your thermometer or gages are not perfectly accurate. Note that you should never add refrigerant if the superheat is already 5F or less, even if the charging chart shows 0☏. Recover refrigerant to increase the suction superheat. Add refrigerant to lower the suction superheat. The difference between the saturated suction temperature and the measured suction line temperature is the suction superheat.Ĥ. The temperature that you measure should be warmer than the saturated refrigerant temperature. It’s usually best to insulate the thermometer probe so that it’s not affected by the ambient air. Next, attach an accurate digital thermometer to the suction line near the suction gauge port. This is especially true of the newer digital gauges.ģ. Note that most gauges have the commonly used refrigerants shown on the gauges. Take the pressure reading and use a temperature-pressure chart to convert that reading to the saturated temperature of the refrigerant that the system uses. Do not use the common suction pressure port on a heat pump because the pressure drop through the reversing valve will affect the pressure reading.Ģ. A ttach your gauge manifold to the suction service port on the outdoor unit. Once you determine the indoor wet bulb and outdoor dry bulb temperatures, check the manufacturer’s charging chart to determine the proper suction superheat.ġ. In the case of the outdoor air, there’s no latent heat involved. The outdoor dry bulb temperature will affect how well the system can reject heat to the outdoor air. The indoor wet bulb reading reflects the total heat of the air and, therefore, the total loading on the indoor coil, both sensible and latent. Normally the charts will require an indoor wet bulb temperature reading as well as an outdoor dry bulb temperature reading. If you’re fortunate, they may be included inside the service panel of the unit that you’re servicing. Sometimes these charts are available from your wholesaler. Manufacturers of these systems typically provide a charging chart or slide rule to properly charge their systems. Most of the systems with piston metering devices are charged by the suction superheat method. The size of the hole, along with the pressure drop across the hole, determines how much refrigerant flows through the system. The piston looks like a small brass or bronze “bullet,” with a calibrated hole drilled through the center. EDITOR'S NOTE: This article was updated to include photos and a tutorial video.įor many years residential air conditioners used pistons as metering devices, and you will still commonly encounter these systems.
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