Nebraska WaterFurnace 7 series Negative Subcooling

Discussion in 'Maintenance and Troubleshooting' started by Krishna, Sep 1, 2016.

  1. Krishna

    Krishna New Member

    Is it possible to have a negative subcooling?

    I noticed in my unit that I saw -3 for subcooling, does it sound right?

    I always thought that subcooling should be positive. Could someone explain me this?

    I have desuperheater if that makes any difference.
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Verify readings first. Then we can go down this path.
  3. parrisjr

    parrisjr Member

    Here's one!

  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Was more thinking - verify readings with field instruments. Negative subcooling is one issue.

    Incorrect algorithms or field monitoring setup is another.
    Deuce likes this.
  5. geoxne

    geoxne Active Member Forum Leader

    We have no idea what the programmer had in his head when he coded an algorithm. Perhaps he has never held a set of gauges and TP chart. Or was too lazy to add an absolute value routine to a calculation. Or perhaps the negative number helps in his coding to control of the system. Or the "minus" is just an indicator of "sub" cooling.

    Probably the same guy who listed E15 as an error code.
  6. parrisjr

    parrisjr Member

    Got plenty of those also, limits that is!

    Last edited: Sep 3, 2016
  7. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I think it is a coding issue - inappropriate inclusion of a "-" (negative sign). Here is why - At a discharge pressure of 324 psig R410a has a saturation temperature (the only temperature corresponding with that pressure at which liquid and gas may co-exist) of just under 102*F. WF has programmed that into the displayed values - 101.7*F

    Since the measured liquid temperature is 98.2*F the refrigerant is in fact slightly subcooled; about 3.5 degrees.

    Subcool and superheat values are always positive. Liquid refrigerant can't exist in a negatively subcooled state (it would all be vapor, and superheated by an amount equal to the "negative subcooling"), nor can gaseous refrigerant exist in a negatively superheated state (it would all be liquid, and subcooled by an amount equal to the "negative superheat")

    I'm leaving out considerations of glide, fractionation, bubble and dew points; those do apply since R410a is a mixture, but the effect is quite small and can safely be ignored.

    In an effort to explain / support the impossibility of negative subcool / superheat, consider the example of ice and liquid water at sea level atmospheric pressure: Ice won't exist at any temperature above 32*F. Likewise, liquid water won't exist at a temperature above 212*F.

    In gaining an understanding of what's happening inside an opaque tube containing refrigerant, the temperature / pressure chart is your best friend. Accurately measuring and understanding the meaning of just two pressures and just two temperatures gives great insight into what is going on within the refrigerant circulation system.

    I don't understand the reasoning behind displaying superheat in both degrees and percent. The figures above have low side pressure at 131 psig. That corresponds to a saturation temperature of about 45.5*F
    HVAC Technician likes this.
  8. geoxne

    geoxne Active Member Forum Leader

    I believe the percent value is the controlled % opening of the Electronic Expansion Valve (EEV) to achieve a calculated target superheat.

    Your explanation is much appreciated in a process that is often misunderstood. Whenever I am asked to explain the heat pump process, in most cases eyes glaze over within 30 seconds. In these cases the inquirer would have been perfectly satisfied with an explanation of magic.
    HVAC Technician likes this.
  9. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Glad to have been useful.

    I'm perpetually amazed by how few "techs" with years of experience in HVAC are able to reliably define and describe the roles of superheat and subcooling.

    Your hypothesis as to the % opening of the EEV seems plausible.

    "Any sufficiently advanced technology is indistinguishable from magic" - Clarke's (Arthur C.) third law.
  10. mrrxtech

    mrrxtech Member

    Subcooling exists if you have a positive number, but if it is a negative number you would be that many degrees above being subcooled. That works in other areas of science but I don't know what the intent is of the geothermal monitor you have is.

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