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  1. #1
    Join Date
    Feb 2012
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    33

    Lennox XC-17 AC / Too Much Temperature Drop Across Coil?

    Hi everyone,

    I just had a 2.5 ton Lennox XC-17 installed with the matching cased-coil. My system has a discharge air temperature sensor and I notice that the discharge air temp is anywhere from around 40 to 50f. Based on the manual I should be shooting for a temperature drop around 15 to 20f by doing both wet and dry bulb tests.

    My temperature drop is anywhere from 20 to 30f (sometimes a bit more). This is much higher than the "15f" temp. drop I am supposed to have. I called out the HVAC guys who installed the unit and they weren't much help. They did no temperature readings and only increased the blower speed to 1400CFM. It brought the temperature drop down a bit, but not much. They said that it is working fine.

    I don't trust these guys as they didn't even know how to properly setup the iComfort thermostat. I had to read up and do it myself.

    My questions are... is 1400CFM too much air to be pushing through a 2.5ton coil? Does anyone have any idea what could be causing such a high temperature drop? Also, as long as the coil isn't freezing, is having such a high temperature drop ok? Will I lose efficiency?

    Any help is appreciated. Thank-you!

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,590
    A 2.5 ton system should have around 1000 CFM blowing through it. More in a dry climate, a bit less in a humid climate. 1400 CFM is out of line. It would cause a low TD and higher humidity. High TD is low airflow. I wonder if they wired it wrong and you are getting 1st stage of airflow and not full blast for a 2.5 ton. One of my colleagues found this Tuesday. Actually I figured it out and told him what to look for

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    Quote Originally Posted by nkforum View Post
    Hi everyone,

    I just had a 2.5 ton Lennox XC-17 installed with the matching cased-coil. My system has a discharge air temperature sensor and I notice that the discharge air temp is anywhere from around 40 to 50f. Based on the manual I should be shooting for a temperature drop around 15 to 20f by doing both wet and dry bulb tests. (What were the readings?)

    My temperature drop is anywhere from 20 to 30f (sometimes a bit more). This is much higher than the "15f" temp. drop I am supposed to have. I called out the HVAC guys who installed the unit and they weren't much help. They did no temperature readings and only increased the blower speed to 1400CFM. It brought the temperature drop down a bit, but not much. They said that it is working fine.

    My questions are... is 1400CFM too much air to be pushing through a 2.5ton coil? Does anyone have any idea what could be causing such a high temperature drop? Also, as long as the coil isn't freezing, is having such a high temperature drop ok? Will I lose efficiency?
    Any help is appreciated. Thank-you!
    The temp-split is directly related to the percent of relative humidity in your home.

    The lower the RH the higher the temp-split; the higher the RH the lower the temp-split.

    Do you have that much variation in your indoor RH?

    Air flow CFM through the coil has less of an effect on temp-split & Btuh absorbed than humidity levels.

    Use a throw away filter with a low pressure drop.

    For example a 16-SEER 3-Ton unit at 50% indoor RH & 400-CFM of airflow per to Ton of cooling (1200-CFM) unit will have around a 22-F indoor-temp-split.

    Check the condensers outdoor temp-rise-split compared to outdoor temp.
    Need indoor percent humidity or, both accurate (difficult to get) wet bulb & dry bulb temps at both supply & return air; then if I had its CFM I could ballpark Btuh of unit. That is different from checking the Actual Delivered Btuh to & from all the rooms.

    Just because the settings say it is supposed to be getting that 1400-cfm does not mean it is, unless, you have the actual airflow readings through the coil.
    Last edited by udarrell; 06-03-2012 at 04:48 PM. Reason: Some words left out, etc.

  4. #4
    Join Date
    Feb 2012
    Posts
    33
    Quote Originally Posted by udarrell View Post
    The temp-split is directly related to the percent of relative humidity in your home.

    The lower the RH the higher the temp-split; the higher the RH the lower the temp-split.

    Do you have that much variation in your indoor RH?

    Air flow CFM through the coil has less of an effect on temp-split & Btuh absorbed than humidity levels.

    Use a throw away filter with a low pressure drop.

    For example a 16-SEER 3-Ton unit at 50% indoor RH & 400-CFM of airflow per to Ton of cooling (1200-CFM) unit will have around a 22-F indoor-temp-split.

    Check the condensers outdoor temp-rise-split compared to outdoor temp.
    Need indoor percent humidity or, both accurate (difficult to get) wet bulb & dry bulb temps at both supply & return air; then if I had its CFM I could ballpark Btuh of unit. That is different from checking the Actual Delivered Btuh to & from all the rooms.

    Just because the settings say it is supposed to be getting that 1400-cfm does not mean it is, unless, you have the actual airflow readings through the coil.
    The manual for my unit is here:

    http://www.completeheating.ca/manuals/XC17.pdf

    On page 30 there is a chart. Here are my readings using 1000CFM

    Indoor humidity - ~45%
    Discharge Air Temperature - 48f
    Dry bulb (return air) - 70f
    Wet bulb (return air) - 66.3f
    Temperature drop - 22f
    (my return air is 70f but the thermostat shows 74f - I think this could be because of the large amount of return air coming in from the basement cold air-returns and the fact that the thermostat is on the main floor).

    Based on these numbers, the temperature drop should be 14f as per the lennox chart on page 30. Even if they move the blower speed to 1200cfm or 1400cfm, I can't get to the recommended temperature drop.

    We do have some humidity variation. Either very sticky and humid outside (say 70% humidity) or it hovers around 45%. I'm a bit worried as when the iComfort senses humidity around 50% it will slow the blower speed to 70% of the high speed value (so it my case if cfm is set at 1000, it slows the blower to 700cfm). That results in the temperature drop becoming even larger. I've seen discharge air temp as low as 41f.

    ... or maybe I am just being paranoid and things are working fine even though they are out of spec as per lennox.

    Based on these numbers should I keep the cfm set to 1400?

  5. #5
    Join Date
    Jan 2004
    Location
    PA
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    68,759
    How are you getting your WB reading for the return.

    70F DB and 66F WB, is a RH of 81%, not 45%. 57F WB would be 45%RH. And by the chart, a 19 temp drop at 400 CFM would be ok.
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  6. #6
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    Quote Originally Posted by beenthere View Post
    How are you getting your WB reading for the return.

    70F DB and 66F WB, is a RH of 81%, not 45%. 57F WB would be 45%RH. And by the chart, a 19 temp drop at 400 CFM would be ok.
    Hmmmm... Well, I used a thermometer and wrapped the bulb with a wet piece of cloth and left it in the return vent for about 10 minutes to get the reading. How do you calculate the wet bulb temperature with the temperature and humidity? I have a digital thermometer I can place in the return vent that will show me both - and then I can recalculate.

  7. #7
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    I use a digital sling to take WB readings.

    To find the WB from the RH use posted, I used software.

    Good chance your readings are not correct. 70DB at 66WB, would make it feel like it 80 degrees in your house. Does it feel that hot in your house when your getting these readings.
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  8. #8
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    Feb 2012
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    Quote Originally Posted by beenthere View Post
    I use a digital sling to take WB readings.

    To find the WB from the RH use posted, I used software.

    Good chance your readings are not correct. 70DB at 66WB, would make it feel like it 80 degrees in your house. Does it feel that hot in your house when your getting these readings.
    House does not feel that hot... you're right, I probably messed up the WB readings.

    Ok - I just placed my digital thermometer in the return air supply right before the filter. This is the reading I got:

    Temp - 69.1f
    Humidity - 48%

    Do you know what the WB temperature would be with those readings? I can then figure out if things are actually ok over here.

  9. #9
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    57.2F WB.
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  10. #10
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    Quote Originally Posted by beenthere View Post
    57.2F WB.
    Much appreciated...

    I guess by the chart I am within spec. Now my question is, if it is 75f on the main floor, why is my return air temp 69f? Is this normal or do I have something else going on here?

  11. #11
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    Could be pulling in some supply air, or duct leakage pulling in basement air, or just cooling off in the duct work..
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  12. #12
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    Quote Originally Posted by beenthere View Post
    Could be pulling in some supply air, or duct leakage pulling in basement air, or just cooling off in the duct work..
    Ok great. I just added a 6" flex duct return in the basement in an attempt to get the temperatures evened out (basement is probably 6 degrees cooler than the top level). That's probably why return air is so cool.

    Thanks for all your help. I think things are within spec.

  13. #13
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    Jun 2005
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    Quote Originally Posted by nkforum View Post
    The manual for my unit is here:

    http://www.completeheating.ca/manuals/XC17.pdf

    On page 30 there is a chart. Here are my readings using 1000CFM

    Indoor humidity - ~45%
    Discharge Air Temperature - 48f
    Dry bulb (return air) - 70f
    Wet bulb (return air) - 66.3f (Wrong!)
    Temperature drop - 22f
    (my return air is 70f but the thermostat shows 74f - I think this could be because of the large amount of return air coming in from the basement cold air-returns and the fact that the thermostat is on the main floor).

    Based on these numbers, the temperature drop should be 14f as per the lennox chart on page 30. Even if they move the blower speed to 1200cfm or 1400cfm, I can't get to the recommended temperature drop.

    We do have some humidity variation. Either very sticky and humid outside (say 70% humidity) or it hovers around 45%. I'm a bit worried as when the iComfort senses humidity around 50% it will slow the blower speed to 70% of the high speed value (so it my case if cfm is set at 1000, it slows the blower to 700cfm). That results in the temperature drop becoming even larger. I've seen discharge air temp as low as 41f.

    ... or maybe I am just being paranoid and things are working fine even though they are out of spec as per lennox.

    Based on these numbers should I keep the cfm set to 1400?
    Since beenthere got you on the right track getting more accurate readings, that 22-F temp-split is within a normal range with that humidity percentage level.

    Doubt it is running at 1400-CFM... think it running closer to 400-cfm per ton of cooling.

    I am back on dial-up (hopefully temporary) & it takes forever to get a pdf to load. Will look at the pdf later...
    Last edited by udarrell; 06-03-2012 at 08:27 PM. Reason: Typo...

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