Servicing a Thermostatic Expansion Valve
Thermostatic expansion valves generally offer worry-free operation and are highly reliable. If they’re malfunctioning it’s usually not because of something wrong with the valve itself, but rather a problem somewhere else is expressing itself symptomatically in the TEV. If you’re going to have to be servicing a TEV it means you’re either fixing an issue somewhere else, or you’re going to be replacing the TEV due to an improper installation. If you’re lucky however, the issue could be fixed with a superheat adjustment.
Causality in Servicing TEVs
More often than not when you’re on a service call and dealing with a malfunctioning TEV you’re not going to be doing much, if any, fiddling with the TEV itself. Flooding, starving and hunting can be caused by a direct malfunction of the TEV by debris in the lines getting caught inside the inner workings, from an improperly installed valve, or improperly set superheat. Installing a filter-drier ahead of the valve can remedy any issues with debris in the valve body.
Servicing Thermostatic Expansion Valves
For this you’re going to need your gauges and your thermocoupler. In order to diagnose an issue you will need to attach your gauges and you’ll also want to measure amp draw. A restricted TEV will cause amp draw to be low since the compressor doesn’t have to work as hard and a flooding TEV will cause amp draw to be high. Remember to let the system run for at least five minutes before attaching your gauges.
With gauges attached and system running, locate the bulb and remove it from the line.
You’re going to need to replace it in exactly the same position to mark, photograph, or remember how and where it was attached. Hold the bulb in your hand and watch the gauges. The high side should drop substantially as the TEV opens to supply more refrigerant. If the pressure does not change substantially then the TEV is likely stuck. Next, immerse the bulb into a cup of cold water.
The high side pressure should increase as the TEV closes to restrict the flow of refrigerant. If it doesn’t, again it is likely stuck.
If the TEV is hunting excessively, this is caused by either an oversized TEV or an improperly installed one. Replacement will be necessary if that is determined to be the case.
Remember to reattach the bulb to the line set before packing up. Remember, if the line is 7/8 or smaller, install the bulb at 12 o’clock. If it is larger than 7/8 then install the bulb at 4 or 8 o’clock.
For more information about servicing TEVs or HVAC systems, follow the All Climate Heating & Air Conditioning blog to stay on top of the technology.