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Thread: On average, when will a coil freeze?

  1. #1
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    On average, when will a coil freeze?

    Not trying to ask a dumb question, but when will a coil freeze over?

    Given the filter is clean and airflow is properly set, coil is clean, return air is 72-75 degrees... Will an evaporator freeze up if it runs long enough with a 31.9 degree saturation?

    Would it have to get down into the 28 to freeze? 26?

    I understand every situation would be different to some degree.

    The question spawns from some servicing some older equipment, 10 SEER and older, that is still functional, but the saturation is 33-34 degrees... They don't complain about any freeze ups and it is just a clean and check... I'm sure the coil is probably dirty, but if it isn't broken, I can't justify the expense of pulling and cleaning a coil on something that old.

  2. #2
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    If properly set up NEVER!

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  4. #3
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    Willing to bet airflow isn’t correct.

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  6. #4
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    my opinion is whenever the slt is below 32* or the suction pressure is below 57# it has a very good possibility to freeze up. the colder it is outside and the colder the return air is in the house the better the chances. Also the lower the air flow is for what ever reason the better the chances are.

  7. #5
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    Walkin coolers have 20 deg coils ....

    Some small reach-in coolers for like coke and pepsi have 10 deg coils

    Pretty crazy eh

  8. #6
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    I agree on A/C .Coil should never freeze up.On refrigeration.Its different.You need a Timeclock and defrost.

  9. #7
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    Thread Starter
    Quote Originally Posted by Gleng View Post
    I agree on A/C .Coil should never freeze up.On refrigeration.Its different.You need a Timeclock and defrost.
    I agree as well, that a freezing coil temp should not happen on A/C. I just have seen several situations where the suction saturation temp isn't but a degree or two above freezing and the blower is on high and filter is clean. Most likely it is low airflow and is due to a 20 or 30 year old coil that is dirty. If it hasn't caused any problems yet and I'm just there to clean it and check it out and it hasn't had an issue... I typically advise them of the situation and also tell them it isn't worth the expense of pulling the coil to clean it on a 20 - 30 year old system. I was just curious as to how low the suction saturation can get before a problem is noticed.

  10. #8
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    Evaporator approach
    Air flow
    Wet bulb/ dew point
    Saturation temp

    R-400 series use mean temp for approach so they could frost sooner

  11. #9
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    Quote Originally Posted by mjohnson2981 View Post
    Not trying to ask a dumb question, but when will a coil freeze over?

    Given the filter is clean and airflow is properly set, coil is clean, return air is 72-75 degrees... Will an evaporator freeze up if it runs long enough with a 31.9 degree saturation?

    Would it have to get down into the 28 to freeze? 26?

    I understand every situation would be different to some degree.

    The question spawns from some servicing some older equipment, 10 SEER and older, that is still functional, but the saturation is 33-34 degrees... They don't complain about any freeze ups and it is just a clean and check... I'm sure the coil is probably dirty, but if it isn't broken, I can't justify the expense of pulling and cleaning a coil on something that old.
    When the temp gets to 32 degrees F -100 calories.

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  13. #10
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    I don’t remember the whole discussion in class, but in a 2nd or 3rd year apprentice class the instructor (worked on large commercial built up systems) claimed that you can run a coil down to 22 saturated. Supposedly he said that coils are engineered to a 10 difference between the saturated temp to the end of the fins where the condensate is. I am just really glad I have somewhere to dump that discussion, it’s been bugging me for 4 years now.

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  15. #11
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    The metering device will make some difference. I don't believe a TXV will be a apt to freeze as conditions change as a fixed will. You did specify a 72-74 degree return,however I have seen a 72 degree set point with a 65 degree return at the unit. If the temp drops enough at night they can sometimes run long enough to freeze. I have always assumed that a coil would start to freeze at the tube turn where there is less airflow on a correctly operating system that froze when it hadn't happened before and you can find no reason.

  16. #12
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    Quote Originally Posted by oldguy1949 View Post
    When the temp gets to 32 degrees F -100 calories.
    Yep. Figure out what is causing the coil to run at <32*. Could be airflow, could be charge, could be load, could be restriction, could be thermostat set too low, could be welded contactor...and so on.
    Officially, Down for the count

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  17. #13
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    Quote Originally Posted by mjohnson2981 View Post
    Not trying to ask a dumb question, but when will a coil freeze over?

    Given the filter is clean and airflow is properly set, coil is clean, return air is 72-75 degrees... Will an evaporator freeze up if it runs long enough with a 31.9 degree saturation?

    Would it have to get down into the 28 to freeze? 26?

    I understand every situation would be different to some degree.

    The question spawns from some servicing some older equipment, 10 SEER and older, that is still functional, but the saturation is 33-34 degrees... They don't complain about any freeze ups and it is just a clean and check... I'm sure the coil is probably dirty, but if it isn't broken, I can't justify the expense of pulling and cleaning a coil on something that old.
    I see this was now a couple days ago. You were there to do a PM and the owners probably was not expecting to pay anything extra, since they did not tell you they have been having any issues. I'd explain to them similar to what you did here. Explain the unit is now 20+ years old, and most parts are at the end or beyond the normal service life. The evap coil probably needs cleaning, which is not included in their service and not inexpensive. While it's working OK today you can't tell them for how much longer it will be OK. They should consider doing additional maintenance or be looking to replace in the near future. Then I'd say, if you decide to do additional maintenance or replace please give you a call.

  18. #14
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    Quote Originally Posted by Pnasty View Post
    instructor claimed that you can run a coil down to 22 saturated. Supposedly he said that coils are engineered to a 10 difference between the saturated temp to the end of the fins where the condensate is.
    Thats quite technical to be honest. And there has to be some sort of difference between saturated temp and the end of the fins.....

    That said I rarely look at saturated temps. I spend much more time watching SLT. I dont need to hook up gauges, if SLT is below 40* you got a potential freeze up and horrible airflow. Lets go spend some time figuring that out instead of wasting time with the gauges....

  19. #15
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    Quote Originally Posted by mjohnson2981 View Post
    Not trying to ask a dumb question, but when will a coil freeze over?

    Given the filter is clean and airflow is properly set, coil is clean, return air is 72-75 degrees... Will an evaporator freeze up if it runs long enough with a 31.9 degree saturation?

    Would it have to get down into the 28 to freeze? 26?

    I understand every situation would be different to some degree.

    The question spawns from some servicing some older equipment, 10 SEER and older, that is still functional, but the saturation is 33-34 degrees... They don't complain about any freeze ups and it is just a clean and check... I'm sure the coil is probably dirty, but if it isn't broken, I can't justify the expense of pulling and cleaning a coil on something that old.
    Running as you describe, 31.9^F Sat. is 44^F lower than a 75^F return air temperature. You are the edge of freeze-up. This hard on the compressor when it freezes up. Increase the air flow or lighten up on the air filter!!
    Regards Teddy Bear
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    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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