Replacing a Piston with a TXV Using the Danfoss TR6 Kit
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The system in the video contains R-410A refrigerant; Danfoss also has TXV kits for systems that use R-22 refrigerant. There are also three different kits for different tonnages; we use the small one for 1.5-3-ton systems.
The Carrier system’s data plate indicates that a TXV should have been installed, but it has a piston instead. So, we start off the metering device replacement by getting a TXV for 3-ton systems. However, we have to make an external equalizer port and mount the bulb during the job, and the system did not start off with the means to start using a TXV.
When we take measurements on the unit, we get ~1,050 CFM, meaning that the system is delivering 350 CFM per ton and putting out just over 31,000 BTUs. We also determine that the evaporator temperature is 41 degrees (Fahrenheit) and that the condensing temperature is about 20 degrees over ambient, which is appropriate. The superheat is around 8 degrees and the subcooling is around 3 degrees.
Before we change anything out, we pump down the system to avoid contaminating it. The system has a scroll compressor, so we don’t pump it down completely. We then recover the remaining refrigerant from the system.
We then remove the piston, which has a Teflon seal. In a heat pump, the piston can seat or unseat itself depending on the operating mode; these can become restricted or clogged, but they can’t “fail.” By comparison, TXVs can fail because they calculate superheat using refrigerant pressure in the bulb vs. the suction line. However, TXVs allow you to set the subcooling and control the capacity to a much greater extent than a piston, meaning that you can get more efficiency out of your system.
We decide to sweat in new copper and cut out the line-drier it is sometimes not a great idea to unsweat a line-drier because you can release all of the contaminants inside of it. Bryan and Jesse also create a groove for the external equalizer port (using the edge of a file and a scratch-all, not a drill).
The TR6 kit comes with the TXV, Aeroquip fittings, a flare assembly, and a chatleff fitting. We use the chatleff fitting. We sweat in the inlet and connect in the chatleff (making sure to protect the o-ring; that is what does the sealing). While brazing anything in, make sure the valve stays cool; it’s good to use a wet towel or heat-blocking putty. We strap the sensing bulb to the side (though on top is generally acceptable as well) and insulate it.
Since we’re working with a straight-cool system, we only need to sweat in a one-way filter-drier. Before we walk away from the job, we do a standing pressure test with nitrogen and bubble-test potential leak points. When we pull the vacuum, we remove the cores and pull the vacuum through the hoses directly, not the gauges. Our decay test shows an acceptable leak rate.
We use the data plate to set the charge; we try to aim for 12 degrees of subcooling. We use the recovered refrigerant to charge the system (bleeding our hoses first).
After taking our final measurements, we determine that everything looks good and that the capacity is better with the TXV instead of the piston (33,000 vs 31,000 BTUs). Since everything that we’re looking for is in range, we don’t need to adjust the TXV immediately after installation.