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  1. #1
    Join Date
    Jul 2013
    Posts
    18

    REPLACE OVERSIZED COMPRESSOR OR AIR HANDLER?

    Greetings:
    I recently had a new heat pump system installed to replace an old 2 ton system which on the hottest days of the year could hold indoor 77 deg F with low humidity.
    By accident the contractor installed a 2.5 ton Compressor and AHU - which I am concerned will cycle more often than a 2 ton system.
    In fact the only time it runs for more than 20 minutes is when it's over 90 degrees outside by keeping indoor at 75F.
    The contractor has being very nice about it and has offered to redo the entire system if needed but would like to try first replacing one component to a 2 ton size.
    So, my question is which makes more sense as a first try - outside 2 ton, inside 2.5 ton or outside 2.5 ton, inside 2.0 ton?
    Lennox specs say either arrangement is a proper match. Will a 2.5 ton AHU matched to a 2 ton compressor operate at a higher indoor coil temp and not dehumidify as well?
    Lennox XP17 single stage scroll compressor. 2.5 ton AHU is one speed set to lowest speed and is probably putting out around 950CFM, higher than old system.
    Thoughts anyone?
    Thanks!

  2. #2
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    Post model numbers for both outside condenser and inside air handler.

    What thermostat?

    Did dealer set up "dehumidify on demand" feature?

    Post back.

    IMO

  3. #3
    Join Date
    Jul 2013
    Posts
    18
    Outside XP17-030
    Inside CBX27UH-030
    Thermostat ComfortSense 7000
    I do not think dehumidify on demand was set up.
    Thanks!!

  4. #4
    Join Date
    Jul 2013
    Posts
    18
    Location: NYC suburb

  5. #5
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    You lose a bit of efficiency but really not a lot.


    3839713 Active Systems XP17 SERIES LENNOX INDUSTRIES, INC. XP17-024-230* CBX27UH-030-230*+TDR 800 24800 14.70 17.70 22000 9.50 14200 1 HRCU-A-CB 170 408 Yes

    3839841 Active Systems XP17 SERIES LENNOX INDUSTRIES, INC. XP17-030-230* CBX27UH-030-230*+TDR 1000 29200 13.70 16.20 26800 9.00 17300 1 HRCU-A-CB 218 538 Yes

    Both qualify for Fed Tax Credit.

    I really think you should run the system for a few weeks. If you are concerned about humidity, then you should take some readings. And especially with the heat wave that is occurring this week for your location.

    Not familiar with Lennox thermostats. Perhaps others can comment. Since this is high eff air handler but not var speed, I don't believe DOD can be set up.

    The larger condenser will help you on heating costs. I would want to verify that your ductwork system can handle the extra CFMs both supply and return. Any noise issues? This system should be very quiet.

    What size heat strip was installed?

    IMO

  6. #6
    Join Date
    Jul 2013
    Posts
    18
    Humidity is not currently a problem - although on cloudy days with high outdoor RH, indoor is higher than old system by about 4-6% I am basically concerned that by cycling more often I will pay more in electricity and the useful life of equipment will be shortened and might require more repairs. There is/was a noise issue with the higher CFM that was fixed by removing one diffuser on a ceiling register - if we stay with 2.5 AHU I will replace diffuser with a simple grate - no big problem. Heat strips are 10KW.
    Thanks so much!!!

  7. #7
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    I really think you are fine and if I thought otherwise I would tell you.

    What size living area?

    Are you comfortable?

    What kind of runtimes are you getting? You might want to document over the next several days. Note time, outside temp, inside thermostat setting, and runtime. I suggest morning, afternoon, evening. Just an idea. As long as you are getting decent runtimes, I would not be worried about system cycling.

    IMO

  8. #8
    Join Date
    Jul 2013
    Posts
    18
    2 Story townhouse - about 900 sq ft per floor with about 300 sq ft of that being cathedral ceiling.
    Runtimes on the 87F Sunny days is roughly 10-15 minutes on and 5-8 minutes off.
    Cooler days or early AM or late evening 5-9 minutes - overnight, less than 2 minutes.
    Yesterday, a very hot day second floor was freezing at 75 Tstat (nonstop compressor) and at 77T stat it would cycle 15- 20 minutes and indoor RH was bout 55%.
    We also have a heat pump for first floor - it is an old 2 ton that will eventually be replaced.
    I just keep reading all over the place that with heat pumps, it is better to be slightly undersized than to be oversized so that during the high/low seasons they run for long periods. My old pump would run 8+ hours non-stop on a lot of days. I realize that the new system might be right on the edge.
    Thank you for taking the time to respond!!!

  9. #9
    Join Date
    Jul 2013
    Posts
    18
    p.s. I would still love to know what you think about which component to switch first if we are going to try. Thanks!!

  10. #10
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    You want RH to be no higher than 50. 55 not acceptable. You might ask dealer what speed he set the air handler blower. No higher than 400 CFMs/ton and possibly lower. If dealer can not get RH below 50, I think I would consider asking him to change out to the 2 ton. Runtimes though seem OK.

    IMO

  11. #11
    Join Date
    Jul 2013
    Posts
    18
    p.p.s. - turns out that cant match 2.5 ton XP17 compressor to single speed 2 ton AHU so only single component swap option is to go down to 2 ton compressor. So I guess my comcern is will indoor coil stay cool enough to dehumidify with high CFM - about 200 + more than needed. I guess if it doesnt work, we will just have to replace AHU as well. Thanks for your time - very helpful!!!!

  12. #12
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    Something not correct about your last reply. You have a single speed air handler. It is a high efficient unit but not var speed. And yes, the CFMs/ton can be adjusted. I would want to know what your air handler speed is set at.

    Is air handler blower fan set to auto or fan on?

    IMO

  13. #13
    Join Date
    Jul 2013
    Posts
    18
    The AHU is set to the lowest electric tap - Tap 1 which gives CFM between 475 to 1145 depending on static - which has not been measured. See attached. The more I think about it, the less Im happy with the amount of air being forced thru system - even with register removed system is not very quiet.Name:  AHU Taps.jpg
Views: 139
Size:  84.3 KB

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