When should a TXV be used and how do I select it? TXVs for air conditioning is pretty much the same as TXVs for refrigeration. They work off of bulb pressure that opens the valve and increases refrigerant flow to the coil. A combination of adjustable spring pressure and suction pressure work together to overcome bulb pressure to drive the valve closed to reduce refrigerant flow. The suction pressure is measured by the valve at the coil inlet for small coils (internally equalized) or at the coil outlet for larger coils (externally equalized). The internally equalized valve will have only two connections while the externally equalized valve will have three. Most A/C coils will utilize an externally equalized valve due to the large pressure drop across the coil. An internally equalized valve should never be substituted for an externally equalized valve.
The TXV determines which refrigerant can be used with a coil. Most new coils can be used with either R22 or R410A providing that the correct TXV is used. A TXV can also be added to a piston type coil to improve efficiency. This is a good idea when installing a new "dry R22 unit" on an existing system. You may not achieve 13 SEER with the old coil but you will improve efficiency. When adding a TXV to a piston type coil, an equalizer line must be added. The equalizer line should be connected to the coil outlet near the sensing bulb location.
Most OEM TXVs have special connections, Aeroquip or Chatleff, so a standard off the shelf sweat TXV may not work as a replacement. Many A/C coils can be used for heat pump as well as A/C so the OEM TXVs will have a bypass check valve to allow for the reverse flow of refrigerant during the heating mode. Again the standard off the shelf valve will have no internal check valve so it won't work for a heat pump application.
You can use the OEM valve for the evaporator coil if it is available or you can use a Parker Universal A/C or heat pump TXV. Select the valve you need:
1½ - 3 ton R22 040727-01
3 - 5 ton R22 040728-01
1½ - 3 ton R410A 040771-01
3 - 5 ton R410A 040772-01
Select the correct adaptor to fit the existing coils connection, Aeroquip, Chatleff or mail flare. Wrap the valve with a wet rag and braze the adaptor to the TXV you selected. You now have an OEM style valve that will fit most any coil for A/C or heat pump applications. The Parker Universal A/C TXVs have internal check valves for use with heat pumps. They also have an equalizer line with ¼ in. flare just like the OEM TXV. Always attach the sensing bulb securely to the suction line on the coil outlet and wrap with insulation. These valves are adjustable but adjustment should rarely be needed.