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  1. #1
    Join Date
    Jun 2008
    Posts
    6

    One system or two?

    Hi. In the market for a new system or two and hoping that you can help settle a discussion between my wife and I.

    Our current system consists of two 10 SEER gibson units about 10 years old. Downstairs unit is 2 tons and upstairs is 2.5 tons. The downstairs unit has worked fine for the year that we have lived in the house. The upstairs unit recently developed it's second siginificant freon leak in the condensing unit. We are going to replace with a new R-410a unit rated at SEER 15 or better. Additionally, we have had two load calcs done with the same conclusion - our current units are not even close to the correct size. The load calcs suggest a 3.5 ton unit downstairs and only a 1.5 ton unit upstairs
    Would you recommend replacing both units at the same time? What are the key factors we should consider in making the decision to replace the downstairs unit?

    Thanks,

    Matt

  2. #2
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,570
    Quote Originally Posted by MRT View Post
    Hi. In the market for a new system or two and hoping that you can help settle a discussion between my wife and I.

    Our current system consists of two 10 SEER gibson units about 10 years old. Downstairs unit is 2 tons and upstairs is 2.5 tons. The downstairs unit has worked fine for the year that we have lived in the house. The upstairs unit recently developed it's second siginificant freon leak in the condensing unit. We are going to replace with a new R-410a unit rated at SEER 15 or better. Additionally, we have had two load calcs done with the same conclusion - our current units are not even close to the correct size. The load calcs suggest a 3.5 ton unit downstairs and only a 1.5 ton unit upstairs
    Would you recommend replacing both units at the same time? What are the key factors we should consider in making the decision to replace the downstairs unit?

    Thanks,

    Matt
    If you have an open floor plan do not believe the second floor calc. Do you reach setpoint easily on the second floor when it is really hot?
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  3. #3
    Join Date
    Dec 2006
    Location
    Dallas
    Posts
    189
    Go with a single 15-17 SEER 2-speed unit with zoning. This will hit the best ROI sweet spot. You'll have only 1 unit to purchase, install and service. Be sure your lineset is replaced if there's any shadow of a doubt for needing to. Plan to splurge on a new Zero leakage R8 duct system with zoning.

  4. #4
    Join Date
    Jun 2008
    Posts
    6
    Yes, it is an open floor plan, the upstairs is actually a loft open to the downstairs. The current system seems maintain the set temp. I think our current system was a DYI special, so it's more about thinking our current system is not designed properly. I think the calcs are probably correct.

    Does the single unit with zoning esentially replicate having two units?


    Thanks for the feedback.

    Matt

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,727
    Something seems wrong with the load calcs.
    You have a 2 ton for the first floor that cools it fine, but the calc says its 1.5 tons too small.
    Seems the current 2 ton is proving that calc incorrect.
    So I would question the calc for the second floor also.

    How long did they say it will take to redo your first floor duct work to handle the extra 600CFM of air that the 3.5 ton will need.

    Open floor plans tend to let the warmer air of the first floor rise to the second floor, and the cooler air of the second floor drop to the first. I don't thing your calcs have taken any of this into account.

    Since your ductwork is already set up for 2 systems, its not cost effective to have all the extra ductwork done to zone it.
    Once the dutwork cost, and zoning controls and dampers are added in, may cost more then 2 systems.

    Plus, if you only have one system, if it breaks down, your whole house is effected until they come out to fix it.
    With 2 systems, one part of the house is still conditioned if one unit breaks down.
    Plus, smaller units tend to be closer to their rated efficiency then larger units.
    Some 4 and 5 ton up to 19 SEER units are closer to 15 then to 19.
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  6. #6
    Join Date
    Nov 2004
    Location
    North Carolina
    Posts
    443
    If you go with these load calculation's, your wife is going to whup you. If it's maintaining now it doesn't need to be bigger.

    Stick with two systems. Beenthere explained the reasons well.

  7. #7
    Join Date
    Jun 2008
    Posts
    6
    Interesting point on the duct work. I will have to follow up.

    I actually just received the formal quote from the fist contractor. He is now recomending a 3 ton unit downstairs and a 2 ton unit up (he told me up front the day he measured that he was just guessing on size, and would follow up once he ran the load calc). He also said the down stairs calc came out to 2.85 tons and the up stairs was 1.7 tons. The actual calc equates to about the out put of out current system. Our upstairs unit always seemed to run more, I guess this was due to the upstairs unit compensating for the downstairs unit?

    Would it be a correct assumption that if we replace just the broken upstairs unit with the proper size (thus become undersized in total) the system may have trouble cooling?

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,727
    With an open floor plan. The upstairs unt is going to run often, and for long period of times.
    If your first floor unit is cooling that floor fine. Then the second floor unit isn't compensating for it.
    I don't want to say stray from the load calc.
    But, before you downsize that second floor unit. How has it maintained the humidity in your house. Has it been keeping the second floor humidity less then 55%.
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  9. #9
    Join Date
    Jun 2008
    Posts
    6
    I live in South Jersey & it can get quite humid here in the summer. I don't specifically track it, but it is ususally pretty comfortable inside, especially when compared to out side. Not sure this will help much though.

    So, you think we should stick with the 2.5 ton unit upstairs?

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,727
    Was your upstairs unit working during the heat wave we had a couple weeks ago.
    If so, did it cycle on and off when it was 98 outside, or did it pretty much run all day.

    If it was cycling on and off once or twice an hour during the day at 98, you can most likely go down a half ton. If it pretty much ran constantly for hours during the day, I would stay with the same size.
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  11. #11
    Join Date
    Jun 2008
    Posts
    6
    Unfortunalty it was broken during the heat spell

  12. #12
    Join Date
    Dec 2006
    Location
    Dallas
    Posts
    189
    Some 4 and 5 ton up to 19 SEER units are closer to 15 then to 19.
    Beenthere - Do you mean per average install that's not properly installed?

    Isn't it safe to say the manufacturer's are regulated by some governing entity not to inflate the advertised SEER of a the equipment beyond their true performance if installed per specifications?

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,727
    No.
    When you see an ad saying up to 21 SEER. The 5 ton won't be close to it. Maybe 16 SEER.

    The key phrase itYp To, they don't say all sizes are 21 SEER.

    How did it do other years for cooling the second floor.
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