As I've talked about before, a TXV is designed to maintain a specified and constant superheat at the outlet of the evaporator coil. It does this through a balance of forces between the bulb pressure (opening force), equalizer pressure (closing force), and spring (closing force). It is the spring pressure that can be adjusted on some valves, but why and when would this be done?
For the quick, cut to the chase version, turning the adjustment on the bottom of an adjustable valve clockwise equals higher superheat while turning it counterclockwise equals lower superheat. However, before you start messing with the adjustment, I suggest you read on.
First, the valve must be an adjustable type. Many valves on small equipment are not adjustable and have no hex cap at the base.
Here are some other items you need to consider first.
Before an expansion valve can function properly and do its job, it must have a full line of properly subcooled liquid refrigerant all the way to the inlet. On a split system, checking the subcooling at the condensing unit is a good start but you also need to make sure there isn’t a significant temperature drop all the way up the expansion valve inlet. Keep in mind that some valves have a screen right at the valve inlet, so a restriction even at that point will cause operational issues.
REQUIRED PRESSURE DROP
For an expansion valve to function there needs to be a significant pressure differential between the evaporator design pressure and the liquid pressure entering the expansion valve (In many cases 100 PSIG +). During cooler times of the year, the outdoor condensing pressure/temperature may drop to the point that the required difference in pressure may not exist, and in these cases, the valve may no longer be able to maintain the target superheat. While low ambient controls may be employed to rectify the issue in some cases, in many cases, you must simply be aware that the valve will not function as expected.
IMPROPER BULB PLACEMENT
Ensure that the bulb is mounted on the suction line flat and tight with a proper strap. It is never a bad idea to insulate the bulb. Anytime it is exposed to ambient air, it is a necessity.
WHEN TO CONSIDER ADJUSTMENT
Now you are at the point where you can consider whether the valve could use some adjustment. First, read your superheat right at the evaporator outlet in the same general location as the TXV bulb and equalizer. In most cases ,the superheat at that point should be 5-8 degrees, but refer to manufacturers specs when in doubt. In some cases, you will not have a pressure port at the evaporator, so you must rely on a pressure reading outside. Use common sense when assessing the situation and realize that there may be some pressure drop on a 100 foot line set and there should be very little in a 10 foot line set. Make some allowance according to the situation.
If the system is running very low or very high suction pressure and /or superheat readings are way out of range, it is very unlikely that adjusting the valve will remedy it. Usually, valve adjustments are only for small superheat changes up or down.
THE FORCES AT PLAY
The bulb pressure is the opening force of the valve, so when the bulb is warmer, it exerts more opening force resulting in a more “open” orifice. And, when it’s cooler, it exerts less opening force resulting in a more “closed” orifice.
The equalizer is a closing force, so the higher the suction line pressure, the more the valve is forced closed while the lower the suction line pressure the more the valve is forced open.
The spring is also a closing force and on an adjustable valve increasing the spring tension/force results in lower flow and higher superheat, decreasing the spring tension/force results in more flow and lower superheat. In short, counterclockwise equals lower superheat, clockwise equals higher superheat.
MAKING AN ADJUSTMENT
Before you adjust anything, the system must have been running for a good long while, and you have observed that the superheat has stabilized. You must then check the entire system and surmise that everything else is functional — that the valve is being provided with a fully liquid, properly subcooled, high enough pressure feed of refrigerant. If at that point you find it is out of range, then you can make adjustments by following these steps:
- Carefully remove the hex cap from the base of the valve with a properly sized wrench and a backing wrench exposing the adjustment screw;
- Turn 1/2 turn at a time clockwise to increase superheat or counter-clockwise to decrease superheat;
- After a 1/2 turn adjustment, replace the panels and allow the system to run and stabilize;
- Recheck the superheat and not the change; and
- Repeat as needed until the maximum setting is reached. Never force the adjustment screw too far, it should require minimal force to turn other than possibly initially to “unstick” the screw.
Adjusting a TXV / TEV is an advanced skill for a technician who has a good grasp on their readings and the forces at play. Tread carefully.
Publication date: 7/23/2018