How to Charge An Air-Source Heat Pump In Winter

By Phil Rains
HVACRedu.net
About Phil!

Much interest has been shown in the correct requirements and/or procedures necessary for winter heat pump charging. The correct method(s) necessary for accomplishing winter charging are often included in an overall charging description, typically devoted to summer charging. In this article, we discuss only winter charging criteria.

Always remember that the ASHP (air-source heat pump) must contain the correct refrigerant charge to be able to transfer heat appropriately and meet the structure needs.



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.

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). Weighing in charge is recommended, but, “topping off” is allow in most cases. If less line is used you should recover the excess refrigerant.

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.

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 Temperature Rise between supply and return)]. In other cases, measurement of external static pressure is helpful.

When you must charge an ASHP during the heating mode, you must always use the “sub cooling” method of charging.

Only liquids and solids can be sub cooled. Sub cooling is any temperature of a liquid or solid below its saturation temperature. As an example, consider water. Liquid water at sea level and atmospheric pressure (14.7 PSI) has a saturation (boiling) temperature of 212ºF. At this boiling temperature (and pressure) and above, the water will be in a vapor state (or “superheated”). At any temperature below 212ºF (and atmospheric pressure in this case), the water would be in a liquid state. The resulting temperature of the water would be below the boiling temperature for water of 212ºF. If the water was at 200ºF, we would say that it is actually sub cooled 12 degrees. Liquid refrigerant can be under the same situation within an operating heat pump. In the condenser (the indoor coil during winter), after the vapor refrigerant condenses to a liquid, it will continue to decrease in temperature as more heat is rejected, or be sub cooled.

When referring to sub cooling, we are always concerned with the amount of liquid refrigerant that is in the “condenser”. During winter, the “condenser” is the indoor coil, not the outdoor coil as in summer (for either heat pumps or air conditioners). Inside a heat pump system’s “condenser” (indoor coil) during winter, conversion of vapor to liquid involves removing heat from the refrigerant at its saturation condensing temperature. Any additional temperature decrease is called sub cooling. Finding liquid line sub cooling requires determining the condensing pressure and two temperatures - the condensing temperature at the measured condensing pressure, and the temperature of the refrigerant at the outlet of the “condenser” on the liquid line. The liquid line temperature involves measuring the surface temperature of the refrigerant line at the outlet of the “condenser”.

Utilize the following steps to determine the refrigerant sub cooling value (remembering that the condenser is the indoor coil during winter). You don’t have to find entering wet bulb during winter as when finding superheat during summer, but, you do have to determine entering dry bulb at the return. You must take pressure readings at the service valves at the outdoor coil (as no service valves are typically located at the indoor 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:



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 in the heating mode to allow pressures to stabilize. Install a reliable temperature analyzer (thermometer) on the liquid line leaving the indoor coil with adequate contact and insulate for the best possible reading.

· Measure and record the outdoor ambient temperature with a reliable temperature analyzer.

· Measure the entering dry bulb temperature at the indoor coil (at the return grille). Use manufacturers’ extended performance data to determine the pressures expected at the inspection conditions (heating mode). You should be within ± 5 PSIG if the system is correctly charged. An example of heating data is shown below:

· Find the liquid line temperature (leaving the indoor coil) 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 within the condenser (indoor coil).

Sub Cooling = Discharge Saturation ºF - Liquid Line ºF

· Refer to the manufacturer’s data sheets for required sub cooling target operating values. Sub cooling during winter should be typically 8 to 15 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.

· 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. Never vent it into the atmosphere!

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), and charge into the low side of the system. Throttle the refrigerant to a vapor either by “hand-throttling” using the hand valve and the compound gauge, or use a commercially available throttling device in the low side line. This method assures the zeotropic blends in the R-410A refrigerant will not fractionate, and is required by the EPA and UL.

Copyright © Phil Rains

About the Author: Phil Rains is Master Trainer/Lead Technical Writer for HVACRedu.net. He has over 38 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in all 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. For information concerning online HVACR courses, please our website at HVACRedu.net.