I’d like to talk about a common strategy that many service techs use to charge or check the charge of air conditioners. Some of these common methods include: Ambient plus 30, 20 degree split, 40 degree evaporator, 75/250, sight glass, cool back or sweat back, system or compressor amps, and finally pressure charts. None of these charging techniques work. So if you use any of these methods and get your feelings hurt easily this is not the article for you. For the rest of you, can we talk?
Let’s take a look at the first charging method in the list, ambient +30. For those of you that don’t know this method let me explain with some amount of trepidation out of fear of spreading the ambient +30 rule like a virus. The rumor goes that if you measure the temperature of the air entering the condenser coil while operating the system in cooling and then add 30 degrees to that temperature the resulting sum will give you the discharge saturation (condensing) temperature at which the system should be operating. Example: 100 degrees entering air temp plus 30 gives you 130 degrees discharge (head) saturation temperature. Take a look at the picture of the high side gauge on a manifold set (Fig 1). Notice the saturation temperatures on the inner three rings. The inner ring represents the saturation (condensing) temperature for R-22. If you line up the needle with 130 degrees you will see that the needle will point to about 300 pounds. According to the ambient +30 rumor the machine should have a 300-pound discharge pressure.
Now let’s take a look at a pressure chart from a system to see if this rule of thumb really works. Fig 2 is a pressure chart that lends itself to this kind of inquiry because it has more information on it than most others. You’ll notice that the first column represents the condenser entering air temperature. The second column is the indoor wet bulb temperature which is an expression of the total amount of heat in the evaporator air (latent and sensible). The lower the wet bulb temperature the lower the amount of heat in the air and so the lower the system pressures will be. The next column shows the system’s temperature split at the selected ambient and indoor wet bulb temperature. The next three columns show the suction pressure, head pressure and system amps respectively. If you were charging this system at a 100 ambient the rule would dictate a head pressure of about 300 pounds. Looking at the chart to see what the manufacturer says the head pressure should at 100 degrees you can see that at no time does this system operate at 300 pounds head pressure.If you were to charge this machine until the head pressure was 300 pounds you can see that the system would be undercharged regardless of the amount of heat in the indoor air. There are a couple of things you can deduce from this little exercise. As an example, notice that at 100 degrees ambient the head pressure changes depending on what the indoor wet bulb is. If, when servicing this system it was 100 degree ambient and 59 degree indoor wet bulb (low indoor load) the head pressure should be 321 pounds. On the other hand if the indoor wet bulb was 71 degrees (high indoor load) the head pressure should be 337. Air conditioning systems do not follow simple rules of thumb. Refrigerant systems are very dynamic, responding to many variables such as airflow, voltage, indoor airborne moisture and indoor and outdoor temperature. If during operation any of these variables changes the system pressures will change so it doesn’t make sense that you can predict the proper operating pressures based upon only one of these variables such as ambient temperature. The second thing you can glean from this example is that different systems operate at different pressures from one another at any given load condition. If you repeat this exercise with different pressure charts you will find that no systems will follow this simplistic charging shortcut. There are some variations of this shortcut that you may have heard such as ambient +25 for ten seer systems and ambient +20 for very high seer systems. Of course these rules of thumb don’t work either for the same reasons.
Let’s take a look at another commonly used charging shortcut, 20 degree split. This charging method supposedly says that if a machine is charged properly the difference between the return air temperature and the supply air temperature should be 20 degrees. Looking at the same charging chart you can see that the temperature split at any given ambient temperature changes depending on the amount of moisture in the indoor air as represented by the indoor wet bulb column. You can see that as the indoor wet bulb temperature goes up (more moisture in the indoor air) the temperature split goes down. Look at Fig 3. This is a temp split chart for a 3-ton package machine. If we select 95 degrees outdoor ambient and 63 degrees indoor wet bulb we can see that at 75 degrees indoor temperature the split is 18 degrees. If we keep the indoor wet bulb and the ambient the same but look at 80 degree indoor temp you can see that the temperature split is about 19% greater (22 degrees). What this means to us in the field is that the temperature split changes depending on all three loads on a machine. If the ambient, wet bulb or the indoor dry bulb changes the split will change. In addition these split charts are based on the assumption that the system is moving 400 cfm per ton of cooling over the evaporator coil. So if the airflow is off the split will be off. The conclusion here is that the 20 degree split rule just doesn’t work to determine if a system is charged properly. If you would like to know more about charging properly consider attending my upcoming charging class.www.totaltechtraining.com/pages/5/index.htm. Have fun out there.
