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  1. #1
    Join Date
    Apr 2013
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    Confused Sizing concerns - 2.5ton, go to 3? Design issues.

    Well, looks like its about time to replace my old furnace and finally install A/C into his old home as well. Considering a couple of units and solutions currently. Did a load calc on the house using our standard sheets from work and came up with 30610BTU heat gain, and then went ahead and double checked using just out of curiousity on how close it would be. Few extra options there with shading and vinyl windows, came out to 30475 cooling load, 29675 sensible and 800btu latent load. Pretty close. Design temps used were 97* outside, 75* inside. Looking at installing a Lennox SL280 and likely go with the XC17 outside unit, I'm coming up with AHRI ratings of 28600btu capacity using a 2.5ton unit. I also have a few large fish tanks in the home, so I'm not sure how that is going to affect the cooling and humidity removal capacity of the machine on those hot days.

    Is the 2.5ton going to cut it? Or should I step up to a 3ton? Maybe take a look at the XC16 and go with two stage cooling?

    The house design itself leaves not a lot of room to work with ductwork as its inaccessible without tearing the hall ceiling out - a modified Eichler-esque home. No crawl space and no attic. Also curious on where might be the best place to mount the condenser, I could place it on the side of the home in the shade and run a lineset over the roof or should I consider placing it on the foam roof?

  2. #2
    Join Date
    Apr 2013
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    Thread Starter
    Not so much as an opinion on this? Maybe I'll just go with the 3ton, humidity here is fairly low in the summer.

  3. #3
    Join Date
    Jan 2004
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    Never going to get 29,675 sensible BTUs from a 2.5 ton.
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  4. #4
    Join Date
    Jun 2005
    SW Wisconsin
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    In that dry climate, I assume Phoenix AZ, you should be very comfortable at even 80-F & 40% Relative Humidity indoors.
    Personally, I would not design for 75-F indoors. However, that is your personal decision.

    I would also see what I could do to reduce your home's heat-gain to a point where the 2.5-Ton would handle the heat-load. Again, that is your prerogative. Otherwise, the 3-Ton may be the better option; depending on the sizing of the duct system & return filter area. Our target is .5" ESP.
    Those factors are very important due to the more fins per inch & higher pressure drops of the higher efficiency evaporator coils.
    I am looking at a 3-Ton coil for 3-Tons, @1200-CFM, a dry coil has a .29" pd; wet coil .36 pd.

    A Merv 14, 4" deep has a .29 pd @300-fpm vel.; @500-fpm vel., the pd is .40" of an inch

    At .40 clean filter &.36 wet coil, that is .76" of an inch; without registers, grilles, volume dampers, etc.
    A MANUAL D ought to be properly conducted on all systems...

    This is why I say, use (2) two Return Air filter areas to reduce the velocity in half & reduce that pressure drop, so the system has a chance to function properly. No matter what the filter sizing a mfg'er lists; this needs to be our absolute NEW M.O.
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