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The TXV Powerhead and What it Does
We’ll never run out of content to discuss as long as thermostatic expansion valves exist. Deal with it.
Today, we’re going to focus on the powerheads of TXVs. This article will focus on what the powerheads do, how they work, and some basic troubleshooting procedures involving TXV powerheads.
What is a powerhead, and where is it?
The powerhead (sometimes called the element) sits on top of the TXV and attaches to the sensing bulb with a capillary tube. Some powerheads are replaceable, so they just thread on top of the valve to keep them secure enough for operation yet easy to replace.
In short, the powerhead causes the valve to open by relaying the force from the sensing bulb to the diaphragm. Both the bulb and the powerhead have some refrigerant charge inside them, so the charge’s temperature and pressure dictate the TXV function according to the PT relationship of a mixed vapor/liquid.
When the temperature and pressure of the bulb’s charge rises, the powerhead provides the force to open the TXV. When the temperature and pressure of the bulb charge decreases, the TXV closes. As you can see, the sensing bulb determines if a TXV opens or closes. The powerhead is simply there to relay the pressure from the bulb and provide the proper opening force against the spring and equalizer forces.
How can I identify and troubleshoot a failed powerhead?
The good thing about powerheads is that it’s usually pretty easy to tell how they failed. Like the sensing bulb, the powerhead contains a refrigerant charge. It should not be too difficult to notice a leak, which will be one of the main symptoms of powerhead failure.
For example, a common problem with powerheads is a cracked capillary tube between the powerhead and the sensing bulb. On top of charge leakage, the cracks themselves are often quite apparent. Steel powerheads are also prone to corrosion, which makes them especially susceptible to failure.
In those cases, there’s usually little you can do except change the powerhead. As we said earlier, some powerheads are easily replaceable because they just thread right into the top of the valve. Some techs may replace the entire TXV because of a faulty powerhead, which is more time-consuming and labor-intensive than necessary. They aren't ALL replaceable though… especially in residential.
More troubleshooting procedures and tests
Suppose that the sensing bulb feels light, but you can’t detect any leaks. You suspect that something’s wrong with the powerhead, though. Maybe the evaporator superheat also seems higher than normal, which also indicates a problem with the TXV. What can you do?
Here is a short troubleshooting procedure that you can perform to help you diagnose your TXV powerhead issue. This process came from Emerson’s contractor service tips, which you can subscribe to on their website if you’re interested.
1. Before doing anything to the TXV or its components, check your subcooling in the condensing line. It should be normal up to the valve’s opening. Otherwise, you may not have a powerhead issue at all.
2. On the valve with an external superheat adjustment, turn the adjustment nut entirely to the counterclockwise position (fully open). Verify that the evaporator superheat is still too high.
3. Remove the bulb from the suction line. Hold it in your hand for several minutes to warm them up. Observe the suction pressure and note if there is an increase. An increase indicates that the powerhead still contains charge. If there’s no increase, then it’s very likely that the powerhead has failed.
Another test you can try is to press the diaphragm with your fingers. The diaphragm should not have any give. If the diaphragm depresses under your touch, then it’s reasonable to believe that your powerhead has run out of charge.
Again, while troubleshooting is important, you normally don’t have to waste lots of time looking for a single appropriate fix for each possible thing that could go wrong. Replacing the powerhead is usually quite easy and inexpensive, but there are still a few good testing procedures that will help you identify the powerhead problems’ root causes.