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Thermal Bulb in the airstream

Hello Professor,

In another thread you mention that the TXV bulb should be insulated even if it is located in the air stream. I have purchased equitment that did not have an insulated bulb, and when I called the MFGs tech support was told not to wrap it. When the system was started superheat and sub cooling were exactly what they were supposed to be.

To wrap or not to wrap - that is the question.

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  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    Exellent Point

    The "one line" job description of the TXV is to maintain constant evaporator superheat. If the thermal bulb is located in the air stream and not insulated, the bulb will be exposed to a temperature that is somewhat higher than the suction line temperature. How much higher depends on the location of the bulb. If the bulb is located in the return air stream the temperature of the bulb will be much higher than the suction line temperature, as apposed to the bulb being exposed to the supply air. In this case, the bulb temperature will be a little higher (maybe 6 degrees) than the suction line.

    At any rate, this increased temperature will cause the valve to open more than if the bulb was installed on the suction line and insulated. The opening of the valve causes more refrigerant to enter the coil, while still maintaining the desired amount of superheat. This enables the evaporator to operate with more liquid in it, making the coil, and therefore, the entire system, more efficient.

    So, to make a long story a little longer, if the manufacturer intended for, and designed the system to operate with, an uninsulated bulb, then that is what you should do.
  • RadPro
    RadPro Member Posts: 90


    My problem with this situation is what do you do? Rule of thumb only gets you so far! I don't mind calling the OEM, but when they can't answer your question 'cause it's too old or maybe they did not attend that session, it does get kinda of frustrating!!

    Maybe they should all go to exv and then you don't have to worry about "bulb" placement. (only senser placement).

    Don in SEIA
  • will smith_4
    will smith_4 Member Posts: 259
    TXV bulbs

    You gotta let us know what kind of equipment this was on-I'm curious, anyway! I've never seen any literature from the manufacturures I deal with that call for the bulb to be free hanging/uninsulated, but if it's out there, share the wealth brother!
  • Empire_2
    Empire_2 Member Posts: 2,343
    The way I look at it...

    The bulb is to sense line temp only!!! I do not want ANY interference from air stream or anything else for that matter.

    Mike T.

    Hey Will where Ya been? Haven't seen you on the wall lately.
This discussion has been closed.