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  1. #1

    Different SEER rating between Upstairs and Downstairs, Matter?

    Potentially looking at replaceing my upstairs unit.
    Upstairs = 3.5 ton 10 SEER aire-flo ~1100sqft
    Downstrs= 3.5 ton 10 SEER aire-flo ~1600sqft
    2700sqft, Phoenix, AZ.

    If I end up replacing the upstairs unit it would be with a 13 or 14 SEER 3.5 ton unit/air handler, the whole thing. Single stage likely same as before.

    The way I understand it, it should not negatively affect the other unit. They both blow the same 3.5 tons (granted the older unit may blow a little less), just the new unit would do it with less energy.
    Would this be a correct assumption?

  2. #2
    Join Date
    Apr 2012
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    winnipeg
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    indeed
    it was working.... played with it.... now its broke.... whats the going hourly rate for HVAC repair

  3. #3
    Join Date
    Nov 2006
    Location
    Athens, Ohio
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    1,694
    The tons refer to Btu's of cooling, that's the size of the unit. SEER is for the efficiency of the unit. Higher SEER equipment will use less electricity. There will be no interaction between the systems unless you have an open balcony or large stairway which would mix the air from the two systems. Even then there should be no negative effect.

    However, I question whether you need 7 tons of cooling for a house that size. Have any contractors calculated the heat load for the house? Many people think bigger is better but it is not. The PROPER size is better.

  4. #4
    Join Date
    May 2012
    Location
    San Diego, CA
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    352
    That is a correct assumption. Technically the units don't care what the other is doing... although if one is on and the other is off.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  5. #5
    I am the 2nd owner, the house was built in late 2005 or early 2006 and the units are what were installed at time of construction. No heat load calculation that I am aware of. I agree on the sizing that proper is better, no arguement.

    I will ask about a heat load test when speaking with my next calls on units.

    Off to google more about heat load tests and what is involved.

  6. #6
    Join Date
    May 2012
    Location
    San Diego, CA
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    352
    Quote Originally Posted by mrdeath2000 View Post
    I am the 2nd owner, the house was built in late 2005 or early 2006 and the units are what were installed at time of construction. No heat load calculation that I am aware of. I agree on the sizing that proper is better, no arguement.

    I will ask about a heat load test when speaking with my next calls on units.

    Off to google more about heat load tests and what is involved.
    Look at ACCA manual J. This is usually used to find residential heat loads.

    https://www.acca.org/Files/?id=68
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  7. #7
    I was just reading their sample form, holy geez. Some more searching it's starting to make more sense.

    Being in AZ and the East/west facing I would expect higher numbers. No low-e windows, just plain dual panes. But my estimate there sounds a lot like what I seem to find complaints about; Estimates with no paper/measurements behind them.

  8. #8
    Join Date
    Jan 2008
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    1,968
    Being in AZ - I would go with properly sized, high seer(18+), 2 stage equipment.
    We have no idea what size is needed, but your comfort level will be quite nice if it's sized right.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  9. #9
    Don't get me wrong, we've been comfortable with out 10 SEER 3.5 ton units for the (almost) 4 years we've owned the home.

    But we're not opposed to doing some wallet comfort math too and going with a higher SEER unit. Not sure if a 2 stage system would bring up the original question if I only replaced one unit.

  10. #10
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    no problems... both units are completely independent from each other if both are on separate thermostats or controls. One can be super efficient 21 SEER with 2 stage ac and furnace and it wouldn't hurt the other unit.

    Arizona does get pretty hot... you guys are in the 100s right?
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  11. #11
    It was 102 at 7pm today... it will be 108 tomorrow.

  12. #12
    Join Date
    Aug 2010
    Location
    West Monroe, LA
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    1,529
    No is does not matter that the systems are different seer. That just means one is more energy eff. Then the other. I would get a proper load done to confirm the sizing is correct. While I agree two stage equipment in your climate would be great might want to consider mid range 15-16 seer for both upstairs and downstairs. I would suggest variable speed for both systems to help with Humdity control.

    The reason I say might want to consider 15-16 seer with a variable speed is to help your wallet out and give you options to replace both upstairs and downstairs systems at the same time. While (2) stage is great most of the time the payback is not there but the comfort is.

  13. #13
    Join Date
    Jan 2009
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    Keokuk, IA
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    Quote Originally Posted by 2old2rock View Post
    Being in AZ - I would go with properly sized, high seer(18+), 2 stage equipment.
    We have no idea what size is needed, but your comfort level will be quite nice if it's sized right.

    I would think in AZ you want high EER as the priority. Effciency in lower tempratures is less critical. However, being an upstairs unit, it will be used more in the evening and nighttime, so you'll get long run times which will even out temperatures upstairs at night.

    In dry climates you want to run the system at a higher CFM level. The ductwork should be evaluated. You might find for example that you can get equal performance running a 3 ton at 450CFM/ton than a 3.5 Ton at 400CFM/ton and it will use less energy. Nearly the same airflow.

    The EER (efficiency at 95F outdoor conditions (hot)) is usually the same for a 15 or 16 SEER single stage as a 2 stage 16-18 SEER unit.

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