Charging Residential a/c


From Brian Vick (via e-mail)

I would like to know the proper way to charge a home central a/c unit. Rule of thumb was always to take the ambient at the condensing unit and add 30°F, then convert it to psig. Is this still the norm? I did this on a Janitrol 3.5-ton and a 3.5-ton Sears unit, and they both are running well.

Also, what is the best way to charge car a/c properly by high pressure? Any rule of thumb for the temperature difference on the temperature across the coil, and how much more to add before converting to psig?

I would also like to know how to join my local RSES chapter.


From Daniel Kramer Specialist Grade RSES Member and professional engineer

To answer your first question, you have stated that your rule of thumb for charging a home central a/c unit is to measure the ambient temperature, add 30, find the equivalent saturated pressure for that temperature from your P-T chart, and simply charge until that pressure was reached.

I personally think that your system is too simple. For instance, would you observe liquid floodback to the compressor or a badly starved evaporator by just looking at the head pressure? Also, some of the more expensive and more efficient units might have design condensing temperatures of 20° over ambient instead of 30°. Your charging system would defeat the extra condenser by flooding it until your 30° temperature differential (TD) was reached. Yes, you will fix some systems, but there could be shortened life.

I would install both high- and low-side gauges and slide some of the insulation back so I could get my hand or a thermometer (or its digital equivalent) on the suction line. I would also want to be able to get my hand on the liquid line. I would note the indoor air temperature and the outside ambient.

Then I would ask, why does the system need gas? Is it empty? Then there’s a big leak. Better find and fix it. Is it operating, but the head and suction pressures are low? Then there is only a small leak. Find out from the owner when it was last serviced and whether gas was added then. Maybe the leak is too small to find. Check your rules as to whether you can just charge it without finding and fixing the leak.

If this were a new system, I would be sure that it had been evacuated to 1,000 microns or less. I would weigh my refrigerant cylinder and write down the weight. Then, I would break the vacuum and get the pressure up to about 10 psi before I started the compressor.

Then, new or old, I would charge until I had a suction superheat of about 20° or until the suction line got good and cold, but not flooding back. With experience you can tell when there is liquid in the suction line because it will chill your hand. If you are not sure, observe the suction line temperature.

While you are charging and watching the low-side pressure and temperature, you must also be watching the high side or liquid line pressure and feeling the liquid line temperature. In warm weather, the liquid line should get quite hot — 10°, 15°, or even more over the outdoor ambient temp.

If the high-side pressure begins to rise and the liquid line begins to cool at the same time, you are overcharging. Stop charging, even if the suction pressure is not high enough or even if the suction line is still warm. Open the expansion valve to flood the evaporator, cool the suction line, and raise the suction pressure (but not flood back). If opening the expansion valve doesn’t help, replace the drier.

If the evaporator floods out while the suction pressure is still low, check to be sure the evaporator fan is running at high speed. Maybe the air filter is plugged. If there is no air filter (because the homeowner found it clogged and took it out, meaning to get a new one but forgetting), you can bet the coil face is covered with lint or dirt. Clean it. That’s always a tough job. Charge plenty.

When finished, I would weigh the refrigerant cylinder again and write that weight down. Subtract the final from the initial weight. That’s how much I’d put in. Put that weight on your bill.

As for how to charge an auto a/c, there are no rules of thumb. Install the high- and low-side gauges. Put some gas in. Where’s the leak?

Open the car doors so it stays hot inside. I find the best way is to get a thermometer on the condenser outlet line. While watching the pressures, charge until the liquid-line temperature reaches a peak and starts to drop. That means you are beginning to flood the condenser. Pull out a little gas. Stop.

To join RSES, call the main office at 847-297-6464, or check the Web site at

EPA Requirements


From Herb Bloomer Carrollton, TX

I have some recovery and EPA questions. First, you must fill out a form and send it to the EPA indicating the make, model, and serial number of your recovery equipment; do you do this every time you buy a new recovery machine or only for the first one? I am on the third generation now.

Also, regarding hoses for your manifold: Do they need a valve of some kind near the end? Does everyone have to have a recovery machine on their truck at all times, or can two or three men share one?


By Julius Banks Stratospheric Protection Div., EPA Washington, DC

The questions that you’ve asked deal with regulations covered under Code of Federal Regulations (CFR) 40 Part 82 — Protection of Stratospheric Ozone, Subpart F — Recycling and Emissions Reduction.

To answer your first question, the EPA requires that persons servicing or disposing of air conditioning and refrigeration equipment certify to the appropriate EPA regional office that they have acquired (built, bought, or leased) recovery or recycling equipment. This certification must be signed by the owner of the equipment or another responsible officer and sent to the appropriate EPA regional office. Owners are not required to submit a new form each time they add recycling or recovery equipment to their inventory.

The following items need to be submitted to the EPA recycling program manager in order to be listed by the EPA as a certified refrigerant reclaimer:

1. Complete name and address of the reclaimer;

2. List of the equipment used to reprocess the refrigerant; if the reprocessing equipment was purchased off-the-shelf, please provide the make, model, and serial number. If the equipment is generic or was manufactured by the reclaimer, please provide a description and photographs of the equipment and processes.

Note that equipment used to recover refrigerant directly from air conditioning and refrigeration equipment must be certified by the Air-Conditioning & Refrigeration Institute (ARI) or Underwriters Laboratories (UL) under the ARI 740-1993 Standard.

3. A list of the equipment used to analyze the refrigerant; if an independent lab is used to analyze the refrigerant, submit the name, address, and phone number of the laboratory, and the list of equipment used by the lab to analyze the refrigerant.

4. Certification that the refrigerant will be returned to at least the standard of purity set forth in ARI 700-1993, Spec-ifications for Fluorocarbon Refrigerants;

5. Certification that the purity of the refrigerant will be verified using the methods set forth in ARI Standard 700-1993;

6. Certification that no more than 1.5% of the refrigerant will be released during the reclamation process;

7. Certification that wastes from the reclamation process will be disposed of in accordance with all applicable laws and regulations (40 CFR Part 82);

8. Certification that the information given in the certification is true and correct.

Regarding your second question, the EPA requires that recovery or recycling machines manufactured after Nov. 15, 1993, possess low-loss fittings. A low-loss fitting is defined as any device that is intended to establish a connection between hoses, air conditioning and refrigeration equipment, or recovery or recycling machines. The low-loss fittings are designed to close automatically or manually when disconnected, thus minimizing the release of refrigerant from hoses, air conditioning or refrigeration equipment, and recovery or recycling machines.

In regards to your third question, although owners of recycling and recovery equipment are required to list the number of trucks based at their shops, they do not need to have a piece of recycling or recovery equipment for every truck.

For further information concerning stratospheric protection, copies of regulations guidance documents, or fact sheets, please call the Stratospheric Protection Hotline at 800-296-1996. The EPA’s Hotline operates between 10 a.m. and 4 p.m. EST, Monday through Friday, except federal holidays. This information is also available on the Stratospheric Protection web page at

R-409A as R-12 Replacement?


From Jim Wise Texarkana, AR

Can R-409A be used in automotive systems to replace R-12? If so, how? What is required to be done to the system — oil change, pressure readings, head pressure readings?


By Dan Tranchina F.T. Industries Franklinville, NJ

The answer is no, R-409A cannot be used in an automobile for the purpose of cooling in the passenger compartments. The reason for this is that R-409A is currently not an approved alternative refrigerant in the SNAP program (Significant New Alternatives Policy) established by the EPA. The EPA established SNAP in 1994.

The EPA examines new substitutes for ozone-depleting substances like CFC-12, looking at global warming, flammability, and toxicity characteristics. The EPA has done an excellent job putting together a very informative website on this subject at title6/snap/macssubs.html.

It is important to understand alternative blends and the considerations before using these blends. One thing to keep in mind is that a motor vehicle air conditioning (mvac) system is very different from a stationary (hvac) system. Some of the differences are the hoses vs. copper tubing or other types of piping used in a stationary system. Many of the SNAP-approved alternatives have R-22 in the blend, which will permeate through non-barrier hoses at a much faster rate than the other refrigerants in the same blend.

The problem with R-22 escaping by itself is that the characteristics of the blended refrigerant can be dramatically altered, even if the percentage of R-22 in the blend is low, which in return will most likely cause the efficiency of the refrigerant to drop greatly.

For every alternative refrigerant being sold for automobiles, there must also be a unique set of permanent fittings and a label attached to the automobile. This is done to help eliminate the cross-contamination of other refrigerants that may be in the autos you may service.

Keep in mind that in the automotive industry, a separate recovery or recycling machine must be used on each unique refrigerant, no matter how close they are in their blends.

R-409A may work well in stationary equipment, but it is important to understand that it is illegal to use R-409A, along with many other refrigerants, in this type of automotive application.

Refrigerant Glide


From Ralph White Danville, CA

My question for the Hotline pertains to the new ternary blends of near-azeotropic refrigerants. I am told that these have a characteristic called “glide,” which is described as evaporation and condensation of their three different components at different temperatures, although operating at a common pressure. It is further stated that, for best results, the glide should not exceed 10°F.

My first question has to do with superheat control. Since it is unlikely that all three refrigerants in the mixture would pass through the final liquid-to-vapor phase at the same point, how is superheat control achieved?

My second question has to do with the strong possibility that a refrigerant leak in any part of the vapor phase of the cycle would upset the critical quantity balance of the charge and lead to the necessity of removing the entire refrigerant charge and replacing it with a new charge with a proper percentage balance. It seems as if, with a fairly large system, this could result in abnormal service costs, couldn’t it?

My third question is whether or not the operation of this kind of charge could be successfully shown on a pressure-enthalpy chart. At this time I have not seen any such illustration.


By Paul Reed DuPont Wilmington, DE

You are essentially correct regarding your statements on glide. The term “ternary” refers to the fact that there are three components in the blend. There are also “binary” blends containing two components, “quaternary” blends containing four, and so on.

At a constant pressure, the mixture of liquid and vapor compositions changes slightly from the start to the end of boiling. The term “glide” is used to note this effect. This effect would only be noticeable if you were to sample different points in the evaporator and check the composition, for example. In reality, the system operation will appear normal, although inlet and outlet temperatures may be slightly different than with a single-component refrigerant.

There is no danger to having glide greater than 10°; some refrigerants sold today have glides slightly greater than that, such as R-409A or -407C. We successfully charge and service these refrigerants on a regular basis.

As for your first question, as long as all of the liquid is boiled off prior to leaving the evaporator, superheat control is achieved in the same way for ternary blends as for any refrigerant. All manufacturers have pressure-temperature charts that show the saturated liquid and vapor pressures for different refrigerant blends. The amount of superheat is calculated from the saturated vapor temperature on the chart. (Customers who desire a more detailed explanation can refer to the Suva Refrigerants Technical Reference Manual, available from DuPont authorized distributors or directly from DuPont at 800-235-7882 or

Regarding your second question, a vapor leak from the system will result in no change in composition of any part of the charge. In fact, in operating systems, leaks that occur from either vapor or liquid lines will cause no change in composition, and the refrigerant can be recharged or the system topped off the same way you have handled refrigerants in the past.

We do recommend, as do all manufacturers, that blends be removed in the liquid phase from the charging cylinder. This will prevent any slight change in composition from occurring in the cylinder. Once removed, the refrigerant can be charged as a liquid to the system, or flashed through the manifold gauges if needed to prevent liquid damage to the compressor.

To answer your final question, pressure-enthalpy charts are available for nearly all of the Suva refrigerants available today. In addition, commercial software is available that can plot these refrigerant mixtures. The only difference seen is that the constant temperature line, or isotherm, is not horizontal across the vapor dome, which takes into account the glide.