Sensing Bulb Location — What is the correct position for the TXV sensing bulb?
The placement of the remote sensing bulb of the TXV onto the suction line is critical to proper TXV performance. A lot of “hunting”, “flooding”, “starving” and other problems can be corrected by making sure the TXV is properly located on the suction line of the evaporator coil. Too often, the TXV is mis-diagnosed as being bad simply because it was not responding correctly. I have had calls where I get asked, ” Are you experiencing a lot of defective TXV’s? I’ve changed 2 on this coil and it still is not working correctly. Why don’t you use adjustable valves?” only to find out the bulb is not making good contact, it was not properly insulated, and believe it or not, the bulb isn’t even attached. Contractors figure that because the coil comes with a TXV factory installed that they have nothing to do. In most cases, manufacturers will install the TXV on the coil but leave the sensing bulb installation to the installer based on how they run their line set, etc. The important points to follow in regards to the TXV bulb are:
Clean the suction line near the outlet of the evaporator. Even on new installs, this is very important.
The entire length of the sensing bulb must be in contact with the cleaned portion of the suction line. It must be clamped to the suction line to maintain good contact.
The sensing bulb should be placed several inches upstream of the external equalizer connection.
- The sensing bulb should be attached at 12 o’clock on any suction line of 7/8-inch diameter or smaller. On lines larger than 7/8-inch diameter, the bulb should be placed at either 4 or 8 o’clock. The bulb should never be placed at 6 o’clock.
- Always insulate the entire sensing bulb after installation.
A sensing bulb can be installed on a vertical suction line if necessary, but never place the bulb downstream of a trap. Bulb placement before a trap (upstream) is recommended. Remember, with R-410A, traps are no longer necessary.
- When putting the bulb on a vertical line — always — TAIL END UP! And — when clamped to a horizontal suction line — always TAIL END DOWN. The reason for keeping the tail end up on vertical lines and down on horizontal lines is to assure that the refrigerant charge in the bulb stays in the bulb and allows it to react better and quicker to changes in the suction line temperature. Even when clamping the bulb at the 4 or 8 o’clock positions on larger horizontal lines, keep the tail rotated so it is always down to keep the charge in the bulb.
A lot of TXV’s are misdiagnosed and replaced because of refrigerant flow control issues and all that really is wrong with the TXV is the “installation”. Keep these little steps in mind when diagnosing TXV’s problems.