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“Is it true that charging an R-410A system has to be done differently than R-22 systems because R-410A is a blend?”
I think the best place to begin answering this question is to address the issue of R-410A and what we know about blended refrigerants in general. Many refrigerant blends are actually made up of three different refrigerants that are of three different pressure levels, and they are referred to as ternery blends. These three-part blends are subject to two things that we don’t find in single-compound refrigerants….Temperature Glide and Fractionation. Fractionation simply means that if a three-part blend leaks out of a refrigeration system, the refrigerant segment that is of the highest pressure will leak out (we call this leaking in uneven amounts) and the integrity of the blend will no longer exist. Temperature Glide refers to the concept that matching up information on a temperature/pressure chart isn’t the same as it is with single compound refrigerants. On an R-22 chart, for example, you can match up a 40-degree temperature listing with a pressure of 68.5 PSIG, and that’s all there is to it. In the case of a three-part blend, the chart isn’t read so directly, meaning that there will be a range of temperatures listed for a given evaporating or condensing pressure, and we call this Temperature Glide.
In the case of R-410A, it is also properly referred to as a blend, but as a binary rather than ternery blend, consisting of HFC-32 and HFC-125, which means that when it comes to temperature glide and fractionation potential, the potential for this is so small (less than 0.3 degrees F temperature glide) that it’s a non-issue.
However, the fact that R-410A is so close to being what we refer to as an Azeotropic Blend, and behaves similarly to a single compound refrigerant doesn’t mean that we can “top off” an R-410A system in the same manner in which we would an R-22 system, so the short answer to your question is “yes, it has to be done differently”.
R-410A refrigerant must be introduced into the system as a liquid from the charging cylinder or tank by either allowing short bursts of liquid into the low side of the refrigeration system through your gauge set by manually controlling the flow with a hand valve, or employing a throtting device as is shown in Figure One below.
Following proper charging procedures will allow the liquid to flash off (vaporize) as it enters the suction side of the compressor. If the refrigerant tank is equipped with a dip tube, then it can be in an upright position as we’re showing here. If it isn’t equipped with a dip tube, then the tank has to be inverted in order to get the liquid out. Also, when charging an R-410A system, keep in mind that the standard procedures of superheat and subcooling apply in order to achieve the proper charge, while attempts to clear the bubbles in a sight glass could result in an overcharge since any blended refrigerant…even R-410A… can still show some bubbles even though the charge is correct.
“How do you check a reversing valve to find out if it’s leaking?”
The process for checking a reversing valve is a simple temperature test with an accurate digital device. Before getting into the specifics of how that’s accomplished, however, I want to say that it’s relatively rare to encounter a ‘leaking’ reversing valve, which technicians often consider as the source of the problem in the event that a customer is complaining that their heat pump is not cooling or not heating enough. More often that not, the reason behind the low capacity complaint is an air flow problem, and, in some cases, a refrigeration system problem. That said, the illustration in Figure One shows the temperature test points employed when checking a reversing valve in the cooling mode.
Note that the test points are on what is referred to as the permanent or ‘true’ suction line, which is the tubing connection in the center of the valve and on the connection on the valve that allows refrigerant flow from the indoor coil. The test points are about 5 inches away from the body of the valve. If you’re using a test device that employs a Type K thermocouple sensor, then you should find some way to insulate the probes so they can provide an accurate reading. If you’re using a device that employs a clamp-type probe, covering and insulating won’t be necessary.
From a practical standpoint, when a test like this is being accomplished accurately, the temperature difference between the inlet tube of the valve and the outlet tube that routes refrigerant back to the compressor should be no more than 2 degrees. In the event that your differential readng is higher than that, it indicates that the valve is, in fact, leaking, such as the example shown below in Figure Two.
In this situation, a compromised seal is allowing hot gas to leak into the suction line assembly inside the valve, and the temperature differential is 10 degrees, which is far beyond what you would normally measure at these test points. With this type of leakage, the performance of the heat pump will be diminished to the point where the customer will be complaining about exessive run time and poor performance.
In addition to leakage that causes performance issues in the cooling mode, a valve that is leaking can also affect the ability of the heat pump to perform in the heating mode. Figure Three below shows you the test points for a heating mode test, and what the numbers would look like if the valve was leaking.