Saturday, June 20, 2009

How to Charge an Air-Source Heat Pump (ASHP)

As an HVAC/R technician, you will have to charge air-source heat pumps (ASHPs) during your HVAC/R career. Often, there are those who never fully learn the correct requirements and/or procedures necessary for meeting this need. Many learn “rules of thumb” to charge a system and hope it is correct. Always remember that these methods, even though sometimes useful, are never precise. The ASHP must contain the correct refrigerant charge to be able to transfer heat appropriately and meet the structure needs. We will briefly address these requirements in this article to help HVAC/R technicians perform these procedures correctly in the field.

An ASHP will achieve its rated energy efficiency only when it contains within a few ounces of the proper refrigerant charge per original equipment manufacturers’ (OEM) criteria. An ASHP that is either undercharged or overcharged cannot achieve its rated capacity. And, an improper refrigerant charge places an ASHP under additional stress and may shorten its service life. When the charge is correct, specific refrigerant temperatures and pressures listed by the manufacturer will match temperatures and pressures measured in the field.

You will have to charge an ASHP under various conditions that can include: new installations, pre-season start ups, and replacement of a component in an installed ASHP refrigerant circuit. Depending on the type of metering devices that are installed in the system the superheat or sub cooling charging method can be used for field charging or checking the existing refrigerant charge in a system. You should charge by weight as well using a scale for accuracy.

Today’s ASHPs typically will include two metering devices. These may be orifices/pistons or thermostatic expansion valves (TXVs) in most applications. There even may be two orifices/pistons, two TXVs, or one of each in some ASHPs. Most modern ASHPs will have at least one TXV, and most likely prior to the indoor coil. In fact, most new R - 410A systems will specify a TXV prior to the indoor coil. These systems will either have an orifice/piston at the outdoor coil, or another TXV.

Adequate refrigerant charge for matching coils and 15 feet of line set is typically supplied with most split-system ASHPs. However, because each installation is different in terms of indoor air flow, refrigerant line length, and duct variations, etc., the manufacturer’s charge may not be correct for every application. To assure the best performance from the ASHP, the refrigerant charge should be checked and adjusted when needed on each installation. NOTE: Some manufacturers provide different line set lengths so always check with the supplier and the installation and operation manuals. In most cases where the line set exceeds 15 feet in length, refrigerant should be added at.3 to .6 ounces per foot of liquid line (again, check the installation and operation manuals). Again, weighing in charge is recommended, but, “topping off” is allowed in most cases. If less line is used you should recover the excess refrigerant using accepted practices.

Always be aware that all refrigerant in an operating ASHP is under pressure. Plus, some ASHPs will use different refrigerants. Many ASHPs have R-22, but newer systems will use R - 410A. You must guard against any refrigerant spraying into your face or on your skin. Always wear protective equipment, i.e. safety glasses or goggles and gloves, when working with any refrigerant.

For years knowledgeable technicians have charged R-22 ASHPs using the superheat method during the cooling mode as most systems had two orifices/pistons, one prior to each coil. This method is applied only in the cooling mode for systems with non-TXVs on the indoor unit coil. It involves charging refrigerant into the suction line as a vapor with the compressor running. This is done by keeping the refrigerant cylinder right side up. If the cylinder is on its side or upside down, you will be charging liquid refrigerant and it could damage the compressor. Of course, manufacturers typically charge empty systems liquid to the high-side on the assembly line as it is faster. But, in the field this could result in problems, unless you first recover any refrigerant and have an empty system. Evacuating the system before charging is recommended.

During summer, with a TXV prior to the indoor coil, you can still check the superheat to determine if the TXV is operating properly, but you can’t field charge or check the existing refrigerant charge in a system only using the superheat method. The TXV controls refrigerant flow by maintaining a constant superheat (for instance, 8ºF to 12ºF). You must also check the condenser side for the operating sub cooling value. This will help determine the level of refrigerant in the outdoor coil (condenser) during the cooling mode. The sub cooling method is typically applied in the cooling mode for systems with a TXV prior to the indoor coil (evaporator), or in the heating mode for systems with a TXV prior to the outdoor coil (evaporator). Remember, when checking in the heating mode during winter, the condenser is the indoor coil, so you must determine the operating sub cooling value at the indoor coil.

When charging or checking charge, always check for clean coils, clean filter(s), and proper air flow. Indoor air flow should be 350 to 450 CFM per ton of cooling, based on the size of the outdoor unit. This approximate CFM amount should also be moving during the heating mode as well. The most common way of establishing indoor air flow of an ASHP is the emergency heat temperature rise method. Indoor air flow will then be: (heating output of electric heater in Btus) / (1.08 x temp. rise between supply and return). In other cases, measurement of external static pressure is helpful.

The basic requirements for checking charge and/or proper charging using the superheat method (assuming orifice/piston prior to indoor coil (evaporator) during the cooling mode) are:

· First, purge your manifold gauge lines. Then, connect the gauge manifold to the base-valve service ports. Run the ASHP at least 10 minutes to allow pressures to stabilize. Install a reliable temperature analyzer (thermometer) on the suction line near the compressor. Typically, the thermometer should be located within 4” to 6” of the compressor. Ensure that the thermometer makes adequate contact and is insulated.

· Always measure and record the outdoor ambient temperature with a reliable temperature analyzer when charging or checking charge.

· Measure the entering wet bulb (with a “psychrometer”) and entering dry bulb temperatures at the indoor coil (at the return grille). This step is very important because it helps you determine the evaporator load, and therefore will have a significant effect on system pressures. Use manufacturers’ extended performance data to determine the pressures expected at the inspection conditions. An example of this data is shown below. You should be within ± 5 PSIG if the system is correctly charged.


· Obtain a pressure-reading of the suction vapor leaving the indoor coil (evaporator) to get the refrigerant saturation pressure-temperature. Refrigerant saturation temperature is the pressure-temperature when the refrigerant is turning from a low-pressure liquid to a low-pressure vapor (absorbing heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature.

· Convert the suction pressure to temperature with a P/T chart or the saturation scale on the compound gauge.

· Take a temperature reading at the leaving suction line of the indoor coil (evaporator). Clamp-on thermometers are recommended for accuracy. Compare both the suction saturated temperature and the leaving suction line temperature. Subtracting one from the other, the difference is the amount the refrigerant gas has heated past its saturated temperature, or superheat.

Superheat = Suction Line ºF - Suction Saturation ºF

· Most manufacturers provide a system superheat table for checking charge for an ASHP using an orifice/piston at the indoor coil during the cooling mode. A typical system superheat table is shown here. Refer to the table for proper system superheat, or a specific superheat target temperature specified by the manufacturer.


· Add charge to lower superheat or recover charge to raise superheat.

· If you need to remove R-22 or R - 410A from a system, you must recover the refrigerant based on EPA criteria. You can’t just vent it into the atmosphere. It must be recovered.

The basic requirements for checking charge and/or proper charging using the sub cooling method (assuming a TXV prior to the indoor coil (operating as the evaporator during the cooling mode) are:

· First, purge your manifold gauge lines. Then, connect the gauge manifold to the base-valve service ports. Run the ASHP at least 10 minutes to allow pressures to stabilize. Install a reliable temperature analyzer (thermometer) on the liquid line near the service valve with adequate contact and insulate for the best possible reading.

· Always measure and record the outdoor ambient temperature with a reliable temperature analyzer when charging or checking charge.

· Measure the entering dry bulb temperature at the indoor coil (at the return grille). If you are checking in the cooling mode, you must also measure the entering wet bulb temperature at the indoor coil as well. Use manufacturers’ extended performance data to determine the pressures expected at the inspection conditions (cooling or heating mode). You should be within ± 5 PSIG if the system is correctly charged.

· Find the liquid line temperature and subtract it from the discharge saturation temperature from the saturation scale on the discharge gauge, or a P/T chart for the refrigerant being used. Subtracting one from the other, the difference is the amount the refrigerant gas has condensed and cooled past its saturated temperature, or sub cooling.

Sub Cooling = Discharge Saturation ºF - Liquid Line ºF

· Refer to the manufacturer’s data sheets for required sub cooling target operating values. Sub cooling should be typically 7 to 14 degrees ± 3 ºF (manufacturers will typically determined a target sub cooling operating value for various combinations of equipment and publish these values in the installation and operation manuals, or provide sub cooling charts). Contact your supplier or manufacturer if no target values are provided. An example of a sub cooling chart is shown below.

· If sub cooling is low (and superheat is normal due to the TXV working correctly), add refrigerant while checking sub cooling until normal levels are reached.

· If sub cooling is high (and superheat is normal due to the TXV working correctly), recover refrigerant while checking sub cooling until normal levels are reached

· If you need to remove R-22 or R - 410A from a system, you must recover the refrigerant based on EPA criteria. You can’t just vent it into the atmosphere. It must be recovered.

If you have to field charge or check the existing refrigerant charge in an ASHP during the heating mode, you must also use sub cooling. You simply follow the previous steps except the condenser is now the indoor coil. You don’t have to find entering wet bulb during winter, but, you do have to determine entering dry bulb at the return. You take pressure readings at the service valves at the outdoor coil, but the liquid line temperature is taken at the liquid line leaving the indoor coil. If the manufacturer provides you with winter sub cooling targets use them. If not, typical sub cooling targets can be utilized, if you understand they are not precise. The following values have worked in most cases:


When charging an R - 410A system, you must charge from the refrigerant cylinder in the liquid form (pull the liquid from the canister in the upside-down position). Then throttle the refrigerant to a vapor either by hand using the hand valve and the compound gauge, or use a commercially available throttling device in the low side line. The charge is then loaded into the low side (suction) of the operating system. This method assures the zeotropic blends in the R - 410A refrigerant will not fractionate, and is required by the EPA and UL.

Phillip A. Rains
Copyright © Phil Rains



About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.

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