Bryan explains why and how to adjust a TXV and what to consider before doing it.
A TXV maintains a constant superheat value at the evaporator outlet, and it needs proper refrigerant flow to do its job right. Some of them have screens in the inlet, and those screens may provide a restriction that prevents a TXV from doing its job.
TXVs also require sufficient liquid pressure to meter the refrigerant into the evaporator; the TXV drops the refrigerant pressure before it enters the evaporator. We have a target range of evaporator temperatures and pressures because we don't want the evaporator temperature to be too low in A/C and medium-temp refrigeration applications.
Before adjusting a TXV, consider checking for restrictions that resulted from contamination. Check to make sure the sensing bulb has been properly strapped to the suction line. The bulb should be fastened snugly to the line; if it's too loose, then it can't do its job properly. If the line is insulated, then the bulb may be insulated WITH the suction line.
Adjusting the TXV won't directly change the pressure; it will affect the superheat. So, adjusting the TXV is NOT the way to deal with low suction pressure. You only adjust a TXV to make it reach the target superheat. Also, see if you're measuring superheat inside or outside; the outside superheat may give you an inaccurate reading due to the likelihood of higher ambient temperatures.
Not all expansion valves are adjustable. You can tell if a TXV is adjustable by looking to see if they have a nut at the bottom. That nut covers an adjustable stem, which affects the position of the spring on the inside. (The spring pressure is a closing force that opposes the bulb's pressure, an opening force.) A non-adjustable TXV would have a flat bottom instead.
If you want to decrease the superheat and feed more refrigerant into the evaporator coil, you would turn the stem counterclockwise to loosen it. If you want to feed less refrigerant into the coil and increase the superheat, you'll tighten the stem and turn it clockwise. It's a good practice to adjust a valve in half-turn increments so that you don't overcorrect the problem. Once you make an adjustment, let the system run for a little while to make sure you're meeting your superheat target.
Allow the system to run for at least 15 minutes before adjusting the valve; there needs to be sufficient head pressure for you to get a good idea of the TXV's status. You'll also want to measure the superheat before you think about adjusting the TXV.
Check out an updated version of the video: • How to Adjust a TXV, T...
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