In our industry, we have had the pleasure of working with a few different metering devices when it comes to air conditioning. Capillary tubes, Pistons and TXVs (thermal expansion valves), and, becoming more used by manufactures as technology evolves, EEVs or Electronic Expansion Valves. Cap tubes and pistons were generally most common for residential AC and, did a pretty decent job when it came to metering the flow of refrigerant. So long as we did the job right, and adjusted the charge correctly. The TVX has been around since the 1920's and has been increasing in popularity by manufactures and contractors over the last couple of decades or so. This is mainly due to the chase for efficiency and reliability, as using the TXV can easily provide both, but again, so long as we are doing the job correctly. 

There are plenty of contractors who will do there best to avoid using TXVs at all costs, complaints of breaking, and not working right are usually the excuses. It is true in that a TXV can "break", after all, there are moving mechanical parts inside, and, the TXV can all of a sudden stop working right, 100% true. But there is usually a reason why this happens. I reached out to Parker/Sporlan, and was able to get some inside information regarding their TXVs that were return for warranty due to failure. What we learned was pretty shocking actually, I was told that around 80% of the TXVs they evaluate for warranty defects, are not true defects, but failures related to the install and service practices of technicians. Things like sludge (system contamination) and debris get lodged in the TXV causing it to break or not meter properly. Brazing with out nitrogen, adding leak stop, and not pulling a proper vacuum will most certainly cause a TXV to "break". Now, on to the good stuff.

So why should we be using TXV's? Efficiency for starters. With systems becoming more efficient, you will be hard pressed to find anything tested/rated at 15 seer or above with out using a TXV. Another, and probably best reason for using a TXV is for compressor protection. After all, its YOUR equipment and reputation on the line, so why not protect it? The TXV is designed to monitor and maintain superheat, keeping it a constant. When you think of superheat from here on out, focus more on it being about compressor protection. Too much SH, too little SH, and we have compressor failure, plain and simple. As long as the Subcooling is set correctly (making sure there is liquid refrigerant in the liquid line), the TXV will "dial in" the superheat. Resulting in a system that is operating at peak performance.

The other benefit to using a TXV is how much easier it makes life when it comes to charging the system. When using pistons or cap tubes, we charge the system by required superheat, the amount of required super heat can change daily (if set once correctly, it should follow daily demand). Determining this can take a little bit of time. But when using a TXV, we charge the system by Subcooling, a number that is provided to us by the manufacture (usually on the rating plate, no guessing). The manufactures also predetermines the Superheat value on the TXV to be between 8-14*F (guess work also eliminated). So as long as we set the SC, the SH should fall right into place.  Again, making life easy. 

Many manufactures are ahead of the curve and building evaporators with TXVs already installed which also makes life easy for us. Because of this, and the chase for efficiency,  I recommend getting used to the TXV, understand it better, and use it to your advantage. Keep in mind that regardless of preferred metering device, NONE of them will work right, if the system itself is not clean, dry and tight, or if the airflow is not set properly. 

Carlos S.