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Steven Simmons

Technical Services Manager Southwest District

Indoor TXV Failure in Heat Mode

This article is in reference to Residential Heat Pumps.

                Everyone in the HVAC industry knows that the indoor TXV does not do anything in the heat mode, right? -or does it do something? Let us discuss it!

                In the Heat Mode, the reversing valve reverses the refrigerant flow through the system. Now, the indoor coil is the condenser, and the outdoor coil is the Evaporator. This means the outdoor coil is COLD due to a metering device like a piston or TXV, and the indoor coil is going to be HOT. Since the refrigerant is flowing backwards it comes right out the discharge of the compressor and goes to the indoor coil with HOT gas. This gas passes through the indoor coil nonstop without using a piston nor TXV and gives up heat to the air in the home; when heat is removed from this gas, it turns into a warm liquid.

                In the description laid out in the paragraph above, you see that the indoor TXV has no job in the Heat Mode, so you think. It does have a job though; its job is to “bypass.” There is a bypassing check valve built into Heat Pump TXV’s. If this bypassing check valve fails, then the TXV cannot do its “bypassing” job.

What would this failure look like, or how would this look like on a system?

                If this bypassing check valve fails, then the refrigerant will slow down/ or stop in the indoor coil. Diagnosing this uses the liquid line pressure.

Typically, there are two ways the bypassing check valve fails in Heat Mode:

  • Completely shut.
    1. This causes the head pressure (large vapor pipe) on the unit to rise and trip the high pressure switch open and/or the low-pressure switch will open because the unit will be trying to pump down.
    2. True suction may be pulling down like a pump down.
    3. If you put a gauge on the liquid line (small pipe), then you would see the pressure is extremely low and mimicking the suction pressure.
    4. Sometimes there is a temperature drop across the indoor TXV.
    5. Liquid line at the outdoor unit will be colder than the indoor temperature.
  • Partial shut. This is a little harder to notice and diagnose.
  1. This will cause the outdoor metering device (piston or TXV) to freeze up and typically misdiagnosed: either as restricted or the unit being low on charge.
  2. The suction pressure will be extremely low for the conditions outside.
  3. The discharge will be higher than normal for the indoor conditions.
  4. There will be a temperature difference across the indoor TXV.
  5. Liquid line at the outdoor unit will be colder than the indoor temperature.
  6. If you put a gauge on the liquid line, you would see the pressures favoring or being close to the suction pressures need to be.

So, what SHOULD your liquid line pressures be in heat mode?

                The liquid line pressures in heat mode should favor or be near (ish) the head pressure. As you know, the head pressure is variable depending on indoor and outdoor temperatures. Unfortunately, there is no statistical data provided by any manufacturer for what the liquid line pressures should be in heat mode. However, we do know what they should not be. They should not be pulling down and looking like suction pressures nor anywhere close to those pressures. The liquid line TEMPURATURE should not be colder than the indoor dry bulb temperature of the home.

Remember, if the bypassing check valve in the TXV is doing its job, it should freely flow refrigerant right through the valve with no temperature drop across it.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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