Before I start on this one At HVAC School we focus on a wide range of topics, many of them are very basic. My experience as a trainer for over 16

Transcription

1 Refrigerant Charging Basics For Air Conditioning & Heat Pump Systems Before I start on this one At HVAC School we focus on a wide range of topics, many of them are very basic. My experience as a trainer for over 16 years has taught me that no matter what I assume others SHOULD know, it doesn t change that fact that

2 they often do not. This write up is very basic but you may

3 above 32 degrees (freezing), don t leave a system with a below 32 saturation suction without doing more diagnosis even if the superheat/subcool looks correct. If you see a liquid line pressure that is more than 30 degrees saturation above outdoor temperature (like a 440 psi liquid pressure on an R410a system on a 90 degree day), do not proceed until you have further addressed the possible causes of high head, regardless of what the superheat or subcool might be reading. Always purge your hoses to prevent introducing air into the system and never mix gauges when using low loss fittings and different types of refrigerants. Charge in the liquid phase (tank upside down) and add the refrigerant slowly and carefully to ensure you do not flood/slug the compressor mo 欀 n6 㘠呣 吊䉔 m 夲ㄴ 漀漀搀 猀氀甀爀敯 od/sre add

4 Most if not all manufacturers have a charging chart available with their respective units. With the information you have gathered on indoor and outdoor temperatures, you can calculate the recommended superheat or in a pinch, you can use a superheat calculator such as the Trane superheat calculator or a free app like our superheat calculator or even better the MeasureQuick app. A good calculator will require that you determine the wet bulb temperature in the return air stream. Without a sling or digital psychrometer or hygrometer, you will not be able to determine wet bulb temperature. Once you know the target superheat you can adjust the system charge to hit it. Let s say, the recommended superheat was 18 degrees, you would add/remove refrigerant to the system until the actual temperature of the suction line was 18 degrees above the indicated saturation temperature from your low-pressure gauge. Adding charge will decrease the superheat and recovering refrigerant will increase the superheat. Subcool Charging To charge a system using subcool, you will need to monitor the actual temperature of the liquid line and indicated saturation temperature on the high-pressure gauge. Information on the entering temperatures is not necessary to charge the unit by the subcooling method. Most manufacturers have recommended subcool charging information with the units if it is designed for a TXV (TEV). If for some reason, there is no information with the unit, or if it has worn off, you can set a typical residential air conditioner charge to 10 to 12 degrees of subcooling which is a relatively safe range to use. Let s say for example the manufacturers recommended subcool is 14, you would add enough refrigerant to the system so the

5 actual temperature of the liquid line was 14 degrees less than the saturation temperature, as indicated on the high-pressure gauge for that particular refrigerant. Adding more refrigerant will increase the subcool reading and recovering refrigerant will decrease the subcool reading. Approach Method Lennox factory information asks that we charge by the approach method on TXV systems. I suggest charging to at least a 6 subcool before even attempting to calculate the approach method. The approach method is a calculation based on the relationship of liquid line temperature to outdoor temperature. To calculate approach, subtract outdoor ambient from actual liquid line temperature. The outdoor temperature used to calculate approach should always be taken in the shade and away from the hot condenser discharge air. To increase the approach differential you would remove refrigerant to decrease it you would add refrigerant. Some Lennox heat pump systems come with a subcool chart next to the approach chart. This subcool chart is for < 65. This means the subcool chart is only valid when the outdoor temperature is below 65. Follow the instructions on the unit carefully when charging in subcooling in <65 temperatures. The method requires that you block sections of the coil to achieve higher head pressures before setting by subcooling. Heat Mode Charging for Heat Pumps In most, if not all, cases you will charge a unit in heat mode

6 according to the manufacturer s recommendations. In those cases where no information is available, there are other indicators that you may use to set a proper charge in heat mode. First, make sure you switch your hoses so the suction gauge is reading off of the common suction port that taps in between the compressor and reversing valve. You may put your high side gauge on either the discharge or liquid (on most systems) depending on what you are checking. Before doing any heat mode charging use common sense, if installing a new system the best bet is to calculate line distance and weigh in any additional charge before moving on to the detailed testing phase. The first one is the 100 over ambient discharge temperature rule. The general rule to this is that a properly charged unit will have a discharge line temperature of 100 above the outdoor ambient temperature. If the discharge line is too hot. you would add refrigerant which would lower the discharge temperature. Alternately, if the discharge line were too cool, you would remove refrigerant to raise the discharge

7 below outdoor ambient temperature. This means if it is 50 outside the suction saturation temperature would generally be between 25 and 30 on a functioning system. Remember that in heat mode the colder it gets outside, the lower the suction pressure and the hotter it gets inside, the higher the head pressure. Since the roles of the coil are reversed in heat mode, if you notice an abnormally high head pressure it may be due to a dirty air filter or evaporator coil. A dirty condenser coil would cause the suction pressure to drop below normal and also cause superheat problems. Once heat mode a charge is set, whether by manufacturer specification or an alternative method, you can still verify the subcool and superheat on the unit in some cases. Do not confuse the superheat or subcool methods recommended by the manufacturer though when running in heat mode. These are only used for setting the charge in cooling mode and not in heat. Look for heat mode specific or low ambient guidelines. Finally and most importantly is ALWAYS TEST EVERYTHING. Airflow, Delta T, Superheat, Subcool, Suction Pressure, Head pressure, Amps, Incoming voltage, Filter etc Read manufacturers specs, understand the units the units you are working on, only then will guidelines and rules of thumb help instead of hinder you. Bryan