Replaced my A/C this weekend, results of downsizing by 1/3.
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  1. #1
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,132

    Replaced my A/C this weekend, results of downsizing by 1/3.

    Recently my 13 yr old Carrier A/C devolved a leak and I chose to replace the system rather than track down the leak and repair the coil. I had been reading about doing manual J calculations and how contractors are scared to death to undersize. I had been talking to co-workers about this and it's an uphill battle getting them to think different than 1 ton per 500sq ft. So I decided to take confidence in correct sizing and "put my money where my mouth is" and install a correctly sized A/C.

    House: 1587sq ft located in Oklahoma City, built 1999.
    OLD A/C: Carrier 3 ton 12SEER system original to the house.
    New Condenser: Thermalzone (Rebadged Rheem/Ruud 2 ton 14.5 SEER) 2 ton 14.5 SEER.
    New coil: 2 ton RFCL 13-16SEER with TXV http://www.rheem.com/documents/rcfl-...ication-sheets

    Results:
    Temperature the old unit cycled until it waswas above 105, clearly oversized for our area. New 2 ton unit can keep 76F inside with a 100 degree outdoor temperature, but runs continuously to do so.
    Humidity is HIGHER in the day with the new system even though it runs more. 45% vs. 40% for the old system. I've already reduced the 3 ton blower to it's lowest speed.
    As a test I've captured the condensate in a bucket to measure humidity removal. The 3 ton system with piston metering pulled more humidity by day, less by night. The new TXV system pulls more humidity at night than day. Both units keep humidity about 45% at night.
    Power Old unit consumed 3,000 to 3,500 watts depending on outside temperature. New unit consumes 1,800 to 2,500watts depending on outdoor temperature. Power use is on my in home wattmeter and includes blower consumption.
    Delta T Old unit got about 18 degrees most of the time, day or night. New unit gets 14 degrees in day, 18 degrees at night. Keep in mind the blower speed was lowered when the new A/C was installed.
    Cycle time Cycles were running about 6 minutes with the old unit, new unit runs about 8-10 minutes per cycle.
    Noise New unit is WAY quieter, both units have 8 pole motors. Scroll compressor can barely be heard compared to the recip compressor of the old unit. With the new unit you can barely hear the fan, very impressed. Neighbor still has his 3 ton builder unit with the 1125 RPM fan and it's twice as loud as my old unit.

    The coworker that helped me install it was impressed with how quiet the unit is and how much cooling it put out considering that it's only a 2 ton. He was a bit skeptical that the 2 ton could pull it off, but it's doing so with flying colors IMHO. So, believe the load calculations and don't fudge the input number to get the result you want. Yes they may seem low, but they DO work !!!

  2. #2
    Join Date
    Aug 2012
    Location
    Palmyra, Missouri, United States
    Posts
    224
    new unit 14 degree split. sounds terrible.

  3. #3
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,132
    Quote Originally Posted by drife678 View Post
    new unit 14 degree split. sounds terrible.
    Split is measured return grille to supply, not across the coil itself. Don't know how much of a difference it makes.

  4. #4
    Join Date
    Nov 2008
    Location
    DFW,TX
    Posts
    2,169
    Quote Originally Posted by 54regcab View Post
    Split is measured return grille to supply, not across the coil itself. Don't know how much of a difference it makes.
    it typically makes a difference as some r6 or less insulated ducts running through and attic pick up 2-4 degrees of heat..I always try and take my reading with astick thermometer right near the entering return plenum and the supply plenum..

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,832
    Sounds like a lot of duct gain chewing up a good chunck of your capacity. I'd look into stopping that.

  6. #6
    Join Date
    Aug 2012
    Location
    Palmyra, Missouri, United States
    Posts
    224
    Id like to see a minimum of 18 degree split

  7. #7
    Join Date
    Nov 2008
    Location
    DFW,TX
    Posts
    2,169
    Quote Originally Posted by drife678 View Post
    Id like to see a minimum of 18 degree split

    where at the return/supply plenum or at the supply duct and near return plenum opening in home? Ive always wondered how much delta T reallty mattered but do use it for at least a start on whether i should look for issues or in combo with sc/sh and pressure readings..

  8. #8
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    5,974
    Possibly why the humidity rises with the increased run times. Infiltration?

  9. #9
    Join Date
    Nov 2007
    Location
    Mississippi
    Posts
    1,239
    14 degrees delta t could be on target. All acording on indoor dry bulb and wet bulb. There are charts for target delta t.

  10. #10
    Join Date
    Nov 2008
    Location
    DFW,TX
    Posts
    2,169
    Quote Originally Posted by socotech View Post
    14 degrees delta t could be on target. All acording on indoor dry bulb and wet bulb. There are charts for target delta t.
    where are these charts?

  11. #11
    Join Date
    Dec 2003
    Posts
    2,348
    Maybe you are moving a little to much air? You said you lowered the fan speed, but it sounds like you have more air flow than ideal for 2 tons of cooling. Your rh is dependent on how cold you get the air. Maybe put in a more restrictive filter and see if the rh comes down.

  12. #12
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,132
    To be fair it was over 100 degrees when I took the daytime delta T reading, it's 5 degrees above design conditions for our area. I would think if I had duct leakage issues it would have affected the 3 ton units day/night offset also. I did notice the suction line felt warm when I took the daytime test on the new unit. I need to do a test with real thermometers across the coil, I question the accuracy of the infrared temperature gun. I'm thinking airflow is probably a little high even on the lowest speed, the 45% humidity is acceptable but I'm curious to see what happens when it's cool/rainy.

  13. #13
    Yeah, I wouldn't trust an infrared to do a Delta T. They only measure the surface temp of what you point it at. You need the actual air temp.

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