Some useful tips on refrigerant charging


Here are some useful and helpful tips on refrigerant charging pertaining to your aircon.

  1. Refrigerant charging takes time, do not get in hurry or you will increase the chances of making a mistake.
  2. Do not put liquid refrigerant into the system, compressor damage may occur.

With your gauges hooked up and a third hose hooked to the refrigerant tank you open the tank slightly with the valves closed on your gauges and unscrew the tank hose close to the gauge manifold to release the air in the hose then tighten the hose securely. Do the same for the two hoses hooked to the system, open the valve on the tank and open the low side valve on your gauges allowing the vapor in the tank which is at a higher pressure than the low side of your system to flow into the refrigerant circuit. You need to be monitoring the temperature readings on your subcooling or superheat (preferably both) this will tell you the effect of the refrigerant that you are adding. If you are charging a TXV you should be looking for a high pressure reading corresponding to a temperature 20-30 degrees above your outdoor temperature (ex. 275psi r-22 =125 degrees this would be a good reading on a 95 degree day). If you are charging to superheat use the same method of adding refrigerant and as your suction or low pressure rises the difference in the refrigerant temperature and the actual temperature will decrease what you have to watch out for is that as you add refrigerant to the system the system performance improves and your indoor temperature drops changing your conditions and therefore changing the temperatures you are looking for if you don’t stop every few minutes and take new temperature readings so you know what superheat to look for you could be in danger of making a mistake, take your time, do it right and re-evaluate often.

The reason this such a critical function is that most of the mistakes made by contractors involve this process and if it is not done right best case your unit is not operating efficiently and worst case you could overcharge the system flooding liquid refrigerant out of the evaporator into the compressor and causing thousands of dollars in compressor damage.

Here are some other helpful guidelines. The goal for outside is to get a full liquid line while keeping the head pressure as low as possible. On a newer unit 190-225 PSI of head is normal, on some older units (25+ years) 300 PSI of head on a 90 degree day is not out of the question and may be normal. Anything higher wash the coil (you’re right the coil should have been washed before you started, just checking to see if you were paying attention) and consider the unit may be overcharged.


* Warning: There are situations where due to the conditions of your system you may have lower than normal suction pressure and/or higher than normal head pressure. Usually lower than normal suction pressure due to low airflow (either intentional or due to restrictions) and any attempts to raise the suction pressure by overcharging will cause damage by flooding the compressor. The trick is to determine if this pressure is normal for this system. For example some systems 50-55 PSI (30-35 degree evaporating temperature) is normal. The secret to being a good aircon servicing technician is to determine if this is a normal pressure and leave it alone.

Information courtesy from Mike Newberry from the Aircon Repair

For more useful tips, please refer here – Aircon Guide


Recharging the Aircon System


This is the continuation from the refrigeration topic on previous article where we have touch a bit on recharging the system. Note: Information here may be a little technical but if you are a trained aircon servicing technician, then it should be no problem for you. So let’s get started.


You need to understand what metering device your system uses, there are several different types of devices used to control the flow of refrigerant but for our purposes we need only be concerned with two, 1. Capillary tube or fixed orifice, 2. TXV or thermal expansion valve. The CT or fixed orifice device is basically a small hole that does not change and is not adjustable and the pressure on the low side is controlled by the size of the hole and the pressure pushing on the high pressure side. The TXV is a self adjusting device that controls the flow of refrigerant by taking a temperature reading on the suction line exiting the evaporator and adjusting refrigerant flow to keep the proper level of liquid refrigerant is the evaporator. Before we move on, you might also want to check out our article on refrigerant charging.


After you determine what type of metering device you have then you need to take a pressure reading to start the process of evaluating the present condition of your system, as a general rule if your metering device is fixed you will be using super heat to charge the system and if you have a TXV you will be using sub cooling. Your manufacturer will have a data sheet to tell you the recommended charging procedure for your piece of equipment, this data should be in the installation instructions or service literature that came with your unit if you do not have this data you will have to call the manufacturer they all have 1-800 numbers and they will be glad to send you anything you need and with faxes and email you should be able to get it very quickly.


With an outdoor temperature of 95 degrees, an indoor temp of 82 you put your gauges on the system (low side gauge on the left hose, should be connected to either the port marked low or a tap on the larger cooler line) you get a low side reading of 48psi (the outermost ring of numbers on the gauge) the temperature of the refrigerant at that pressure is on the gauges also r-22 is noted by the green numbers. Your high pressure reading 190psi you have already determined that you have a fixed orifice so you take a reading of the suction line temperature as close to the evaporator exit as possible your suction line temperature is 85 degrees your gauges tell you that at 48psi the temperature of your refrigerant vapor is 24 degrees this would mean that you would have 61 degrees super heat 85-24=61. Your manufacturer info will tell you under these temperature conditions you will need somewhere between 15 and 25 degrees super heat this means you are undercharged.


Same outdoor conditions with a TXV your liquid line temperature 150 degrees your high pressure gauge is reading 155psi and your gauges tell you that at 155psi r-22 temperature is 82 degrees, your manufacturers info tells you that at that outdoor temperature (95 degrees) your high pressure should be about 275psi and you should have at least 12-14 degrees sub-cooling which means your liquid line temperature should be about 111 degrees because the temperature of r-22 at 275psi is 125 degrees so 125-111=14 degrees sub-cooling, this system is undercharged.

Information courtesy from Mike Newberry from the Aircon Repair


What causes aircon refrigeration problems

refrigeration problems

This article on aircon refrigeration is written mainly for aircon servicing technicians or someone who has interest in the technicality of air conditioning. Many refrigeration problems can be due to lack of airflow, either from dirty filters or dirty coils, as noted above. If the evaporator coil is freezing up you would check the filter first, indoor coil second then the fan, these are easy preliminary checks to be made. Another cause of low airflow can be a belt /pulley problem, if your equipment has this sort of motor / fan drive, as the belt may be slipping and reducing the fan air output. This is easily fixed by tightening the belt, but you should check your manual, as over tightening may reduce the life of the bearings.


Low refrigerant charge is a common cause of coil freezing as the pressure of the refrigerant in the system determines the temperature, and a lower pressure corresponds to a lower temperature. Over the years you may expect that some refrigerant is lost through joints and fittings, particularly with an older system. Also, most equipment is designed to be used in hot weather. If the outside temperature is not high, say below 65ºF, but your system still needs cooling because of high internal heat loads, you may find that you get freezing, and this is a result of the equipment manufacturer’s decisions for design of the unit. This condition can be overcome with various fan or refrigerant controls.


Recharging Your System

Recharging the refrigeration circuit on your system is one of the trickiest mechanical operations to perform because there are so many variables to be taken into account. I don’t tell you that to scare you I just want you to keep in mind that you need to be looking at more than one or two things when you perform this task. You must know these things before you begin!!! The indoor and outdoor coils Must Be Clean, (air is your heat transfer media so limited airflow means limited transfer).


The outdoor temperature where your outdoor unit is takes the reading where the air enters your outdoor coil. The indoor temperature of the air as it enters your indoor coil (if attic or crawl space or outside air is infiltrating your ductwork a reading at the filter will not be accurate), you really need to know the dry bulb and wet bulb temperature but many of you will not have a pshycrometer to determine wet bulb. The suction or low side pressure of the system, the liquid or high side pressure of the system, the suction temperature and the liquid temperature, these readings can be gathered through the use of a set of hvac-r gauges and an inexpensive surface probe thermometer.


To properly charge the refrigeration circuit you will have to become familiar with a few terms, two of these are super heat and sub cooling. Super heat is heat added to a vapor after it has boiled, if the steam coming off your boiling pot is 220 degrees and the boiling point of the water is 212 degrees then that steam has 8 degrees of super heat or additional heat added to the vapor. Sub cooling is heat taken away from a liquid after it has condensed, if that steam at 220 degrees was allowed to cool and condense back into water and the water was further cooled to 198 degrees that liquid would have 14 degrees of sub-cooling. These two terms are used as guide posts for charging your ac system the reason for their importance is that you cannot see what is happening inside the refrigeration circuit and the ratios of liquid and vapor are critical to the long term health and efficient operation of your equipment.

Information courtesy from Mike Newberry from the Aircon Repair