Suction pressure low, unit freezing
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  1. #1

    Suction pressure low, unit freezing

    hello, thankful this site is around as it has helped me several times in the past although i have just become a member. Question is: i have a split coil carrier unit with approx. 100 ft run, installer chose to use 7/8 instead od 1 1/8 suction line. Unit is 410a and running around 100 psi suction, below freezing point hence the coil freezing. txv is controlling superheat. Wondering if it is possible to install a parallel suction line (3/8") to achieve the same pipe I.D. as the 1 1/8" would have been, and save myself from running all new pipe. i would duplicate the existing trap as it will be needed for oil return. Has anyone done this before or have any suggestions. Txv is working correctly, airflow is not an issue. Suction pressure is low and fvreezing coil, unit is not low on charge as subcooling is 10 f. thanks in advance.

  2. #2
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    An undersized suction line would tend to cause the opposite problem to what you are describing.

    "fine" isn't an airflow value.

    Who measured airflow, how was it measured and what, exactly was it when measured?

  3. #3
    Quote Originally Posted by jpsmith1cm View Post
    An undersized suction line would tend to cause the opposite problem to what you are describing.

    "fine" isn't an airflow value.

    Who measured airflow, how was it measured and what, exactly was it when measured?
    First off, i never said airflow was "fine". Niether did i say the suction line was "undersized", although it is close to the distance where a 1 1/8" would be the only choice. And yes a smaller suction line on a long run will cause lower suction pressures. Please take the time to read, and learn basic principles before you comment.

  4. #4
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    Quote Originally Posted by ol' blue View Post
    First off, i never said airflow was "fine". Niether did i say the suction line was "undersized", although it is close to the distance where a 1 1/8" would be the only choice. And yes a smaller suction line on a long run will cause lower suction pressures. Please take the time to read, and learn basic principles before you comment.
    No, a smaller suction line will not cause lower pressures at the evaporator.


    I am telling you that your suction line isn't the problem.

  5. #5
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    Quote Originally Posted by jpsmith1cm View Post

    Who measured airflow, how was it measured and what, exactly was it when measured?
    I apologize for misquoting you, but you are still ignoring my basic question.

  6. #6
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    Super heat is good with TXV,, sub cooling is according to condenser data plate?.

    Check airflow.

    Check the airflow.

    Check airflow, Blocked airflow: Dirty filter, dirty evap coil due to use to cool during construction, factory filter left in blower compartment while using a filter at the return grill. Closed registers (5 closed out of 7 did it for one customer), insulation fell down inside of evap coil (this happened twice on carrier) or insulation in blower compartment blocking airflow (countless times), collapsed ducts, crushed ducts. No dampers opening on a zoned system. G terminal not energised when Y is energised (some designs do this to slow blower to half speed to remove additional humidity) . 2 stage split system ac not going to higher blower speed upon call for y2.
    Furnace is misconfigured - 5 ton AC set for 1200 cfm of airflow.
    Undersized furnace on a 2 system home (furnaces installed to wrong AC).
    Wrong blower pulley installed at factory, wrong sheave or misadjusted sheave on blower motor. I haven't seen many 410 split carrier systems with belted blowers. Could just be my market.
    Fire damper closed in commercial building.

    Lots of fun
    I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.
    ― Benjamin Franklin

  7. #7
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    If your line set run is 100 foot long. Then from your coil to the condenser, the 7/8" vapor line is only losing 4 PSIG, if its a 5 ton unit. Which is only a capacity loss of 1758 BTUs an hour, and a velocity of 1744FPM, which is fine for R410A. So that is not causing your freeze up problem.

    If your liquid line is 3/8", then it has a pressure loss of 17PSIG on a 100 foot run.

    According to what little info you posted. Your system is not charged correctly. And yes, I read where you said tht its charged correctly, and has 10 of SC.

    Is the evap above or below the condenser?


    PS: The pressure losses I posted are for straight runs of pipe, no fitting or vertical rise pressure drops are included.
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  8. #8
    The airflow was measured via a anemometer, well over 3400 cfm. the condensor is above the evap, if i remember correctly the would be 14 or so elbows (90 degrees) there abouts on the liquid line aswell. Subcooling is 10 as i said before, and the sight glass is clear. This is not my question though, i was asking if anyone had ever ran a parallel suction line? thanks again

  9. #9
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    Quote Originally Posted by ol' blue View Post
    The airflow was measured via a anemometer, well over 3400 cfm. the condensor is above the evap, if i remember correctly the would be 14 or so elbows (90 degrees) there abouts on the liquid line aswell. Subcooling is 10 as i said before, and the sight glass is clear. This is not my question though, i was asking if anyone had ever ran a parallel suction line? thanks again
    In certain applications, short runs of parallel suction lines are used.

    I've never heard of it being used for the entire length of a lineset, though.


    That said, given the symptoms described so far, installing the additional suction line will be a waste of time. It will not solve your icing problem.


    Are you a technician or the equipment owner/operator?

  10. #10
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    The vapor line is the right size for oil return. Double risers are used sometimes, but not for the full length of the line set. And not to get the vapor pressure up.

    What size unit is this, and what brand. 3400 CFM is enough 9 tons.
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  11. #11
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    Also. the 14 ells on the vapor line would equal at least another 20PSIG of pressure drop.
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  12. #12
    Sorry, busy weekend, on call and doing landscaping. lots of fun. So what your saying Beenthere is approx pressure drop of 37 psi? what would you suggest? The pressure drop is what i think my issue is, hence why i was asking my original question. Any thought on where to go?

  13. #13
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    Quote Originally Posted by ol' blue
    Sorry, busy weekend, on call and doing landscaping. lots of fun. So what your saying Beenthere is approx pressure drop of 37 psi? what would you suggest? The pressure drop is what i think my issue is, hence why i was asking my original question. Any thought on where to go?
    A 37 pound pressure drop would put your evaporator safely out of freezing temps.


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