HVAC Training - Underfeeding & Overfeeding TXV

A TXV or thermal expansion valve is becoming the go to metering device for most modern air conditioning systems. TXVs help the HVAC manufacturers meet the new government mandated efficiency standards. So as the years pass, it is conceivable that all systems will have TXVs.

When an air conditioner or heat pump has a TXV that works, it can work wonders. Great cooling over a wide range of operating conditions. But, when they are not operating they can be a headache to diagnose. Many times what looks like a bad TXV is not, so diagnose carefully. One of the problems a TXV has is underfeeding in the cooling mode. Here's a short lesson on underfeeding TXVs, symptoms and possible causes:

Symptoms

• Frost on the valve or indoor coil
• Low pressure reading at the suction service valve
• High superheat
• High liquid pressure at the the liquid line valve

Causes 

• Clogged TXV port or screen
• Restriction in the liquid line
• Clogged filter drier
• Bad power head

Another possible cause in and undercharged system. All of the symptoms listed above are the same except there is LOW liquid pressure at the liquid line service valve.

Another TXV problem in the cooling mode is overfeeding.

Symptoms

• High suction pressure
• Low superheat
• Low to normal liquid pressure

Causes

• Overcharged system
• Loose TXV sensing bulbe
• Plugged external equalizer
• TXV stuck open

A TXV can overfeed from restricted air flow, return air blocked, dirty evaporator coil, dirty air filter, or a bad indoor fan.

Symptoms

• Low suction pressure
• Low superheat
• High to normal liquid pressure

As you can see, there are many outside factors that can cause a TXV to have exhibit problems other than the TXV itself. While it can be done, it is difficult to check a TXV. When troubleshooting TXV systems ; measure, observe, and record system temperatures and pressures. Compare them with the known operating values of the system you are working on.

Remember - TXV equipped systems must be charged using the subcooling method.

by Ron Walker