For many years residential air conditioners used pistons as metering devices, and you will still commonly encounter these systems. The piston looks like a small brass or bronze “bullet,” with a calibrated hole drilled through the center. The size of the hole, along with the pressure drop across the hole, determines how much refrigerant flows through the system. Most of the systems with piston metering devices are charged by the suction superheat method. Manufacturers of these systems typically provide a charging chart or slide rule to properly charge their systems. Sometimes these charts are available from your wholesaler. If you’re fortunate, they may be included inside the service panel of the unit that you’re servicing. Normally the charts will require an indoor wet bulb temperature reading as well as an outdoor dry bulb temperature reading. 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.

The outdoor dry bulb temperature will affect how well the system can reject heat to the outdoor air. In the case of the outdoor air there’s no latent heat involved. Once you determine the indoor wet bulb and outdoor dry bulb temperatures, check the manufacturer’s charging chart to determine the proper suction superheat. To measure suction superheat, attach your gauge manifold to the suction service port on the outdoor unit. 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. 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. Note that most gauges have the commonly used refrigerants shown on the gauges. This is especially true of the newer digital gauges. Next, attach an accurate digital thermometer to the suction line near the suction gauge port. It’s usually best to insulate the thermometer probe so that it’s not affected by the ambient air.

The temperature that you measure should be warmer than the saturated refrigerant temperature. The difference between the saturated suction temperature and the measured suction line temperature is the suction superheat. Add refrigerant to lower the suction superheat. Recover refrigerant to increase the suction superheat. Note that you should never add refrigerant if the superheat is already 5F or less, even if the charging chart shows 0F. You don’t want to overcharge the system if your thermometer or gages are not perfectly accurate. Overcharging can damage the compressor and will also reduce both the capacity and efficiency of the system. Note that if the suction superheat is correct and the suction pressure is low, the system probably has low airflow. Correct the airflow problem and check the charge again. On many newer systems, especially high efficiency systems, thermostatic expansion valves (TXVs) are used. Expansion valve systems are normally charged by using the subcooling method.

Liquid sub-cooling is required so that you have only liquid entering the expansion valve with no bubbles present. Vapor bubbles present in the refrigerant will cause low refrigerant flow. Low refrigerant flow will cause a loss of capacity and efficiency in a cooling system. Liquid sub-cooling is normally measured at the liquid line service valve. jual ac portable malangThe manufacturer will usually specify the required sub-cooling on the outdoor unit service panel. 10 ton rooftop ac unitIt’s usually between 5F and 15F, but always go by the manufacturer’s requirements. window unit air conditioners at lowesTo measure liquid subcooling, attach your gauge manifold to the liquid line service port. If you use a quick-connect fitting on the end of your hose, make sure it’s a low-loss fitting.

I’ve seen some quick-connect fittings that allow pressure loss, which can cause inaccuracies in the charging procedure. Take a pressure reading. Use a temperature-pressure chart to convert the pressure to the saturated condensing temperature of the refrigerant. Next, attach an accurate digital thermometer to the liquid line. The temperature that you read with the thermometer should be lower than the saturated condensing temperature. The difference between the measured liquid line temperature and the saturated condensing temperature is the liquid subcooling. Add refrigerant to increase subcooling. Recover refrigerant to reduce subcooling. Note that if the subcooling and superheat are correct, and the suction pressure is low, the system probably has low air flow. When charging by the subcooling method, you should be sure to check the suction superheat as well. If the expansion valve goes bad, you can have a very low suction superheat when you have the proper subcooling. In some cases it’s not possible to achieve the required subcooling without having a superheat of zero degrees.

If you get zero degrees superheat with a TXV, then the TXV is defective and will need to be replaced. One other method of charging is the weigh-in method. The weigh-in method can be very accurate if you know the exact length of the refrigerant lines. The outdoor unit usually comes charged with enough refrigerant for the outdoor unit, a standard indoor unit, and 15 or 25 ft. of line set. You must add refrigerant for any line length over what is specified by the manufacturer. You may also have to add or remove refrigerant depending on what indoor unit or indoor coil is used. Even if you charge by weigh-in it’s still a good practice to check the charge using the subcooling or superheat methods, to ensure that everything is operating properly. Kevin ONeill, CM, is the co-owner of ONeill-Bagwell Cooling & Heating, Myrtle Beach, SC. He has 35 years of experience in the HVAC service business and is a 28-year member of Refrigeration Service Engineers Society (RSES). Air Conditioning Refrigerant and Refrigerant Leaks

The cooling medium of a air conditioning or refrigeration system is called its refrigerant. In most residential air conditioners and many refrigeration systems it is R-22 or Refrigerant which is a chlorofluorocarbon or CFC. Refrigerant is an Environmental Protection Agency controlled substance which is deemed hazardous if released into the environment and will damage the Earths ozone layer if not properly contained. Air conditioners do not consume Refrigerant and you should never need to add or change the Refrigerant in your air conditioning system unless there is a leak. According to environmental laws any refrigerant that is removed must be extracted and and filtered by a refrigerant recovery or recycling machine. According to environmental laws any refrigerant that is purchased, installed, or removed must be extracted and and filtered by a certified technician using a certified refrigerant recovery or recycling machine. To become certified technicians must successfully pass a written examination for the type of equipment on which they wish to work.

There are three different types of certification, Type I for small appliances, Type II for high pressure refrigerants, and Type III for low pressure refrigerants. Exams are given by organizations which have been approved by the Environmental Protection Agency. It is extremely important that the coolant or Refrigerant level of an air conditioning system be checked often. A 10% loss of Refrigerant or coolant will cost the home or business owner 20 percent more in electrical costs and can cause undue wear and tear on your unit. The Air Conditioning Contractors of America or ACCA recommend checking once a year to maintain the proper Refrigerant levels. If your Refrigerant levels are low either the proper charge was never added to your system or there is a leak which should be repaired. Low levels of Refrigerant can cause compressor failure, reduce the efficiency of your air conditioner, can freeze the evaporator coil, and many other problems. Refrigerant leaks will not get smaller and will usually increase in severity over time so a repair can save money on service as well.

The laws on CFC's do not allow an air conditioning contractor to add Refrigerant to a leaky commercial system if the leak is within 30% of the units capacity. They are required to fix the leak in the system. Violation of this law may cause contractors to loose their license. Leaks can be caused by many problems including unit vibration, physical damage (from lawn movers, pets, etc.), stress, worn service valves, etc. Methods for locating leaks include visual inspection, high viscosity liquid leak detector, electronic detection equipment, ultraviolet leak detection and a nitrogen isolation test. Of all of the methods the nitrogen isolation test is one of the most effective methods for locating all the leaks in the a system. In a nitrogen isolation test the system is isolated into four parts. The condensing unit, suction line, liquid line, and evaporator coil. Access fittings are installed and sealed shut to allow nitrogen pressure of several hundred pounds PSI (per square inch) is added to each component.

Pressure readings are taken and recorded and if there is a pressure drop it indicates the presence of a leak. For very small leaks it may be necessary to leave the unit pressurized for up to 24 hours. After the leak is repaired the system is reconnected, vacuumed, and charged with refrigerant Click here to Order Parts from Ra-Jac Services Online Air-Conditioning and Heating Store Take the guesswork out of shopping for high quality air conditioning and heating parts for your home or office. We offer only top quality parts that the professionals are using. Let our 40+ years in the air conditioning, heating, ventilation and refrigeration industry work for you. Click here for 10 Reasons to choose Ra Jac as your Air Conditioning and Heating Company for Repair and Installation Service. Ra-Jac Air Conditioning & Heating has proudly served Galveston County TX since 1966. We pride ourselves on providing top notch service on residential and commercial air conditioning, heating, and refrigeration in the Southern Texas area.