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Thread: One story home zoned A/C

  1. #1
    DIY Junior Member Talisker's Avatar
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    Default One story home zoned A/C

    In a one story home can i do a zoned A/C? I will have Trane 4 or 5 ton unit. the house is 3000 sq. ft. Thx.

  2. #2
    Plumber jimbo's Avatar
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    A single forced air unit cannot really be zoned...you just have to balance the loads. Did someone tell you that you need 5 tons for 3000 square feet?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    From a comfort standpoint, an oversized a/c unit is both uncomfortable and inefficient. Now, whether that's oversized is the big question. Before buying, have someone do a real energy assessment - not something that just says, 3000 sq ft at 600 sq/ft per ton! Depending on where in TX, humidity may be the biggest issue...an oversized a/c can get it cold, but will NOT purge the humidity required to make things comfortable...to do that, you need the a/c to run long enough on a regular basis to extract lots of moisture. When it's too big, it doesn't run long enough and does not dry the air. It's better to be slightly undersized as you'll be more comfortable than too big and be cold and damp!
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member Talisker's Avatar
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    I am in Richmond, Tx. The inspector said most likely we should be getting 5 ton unit.

  5. #5
    In the trades Dana's Avatar
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    That's probably an insane level of oversizing, unless you have 500-600 square feet of "sunset view" west facing clear-glass single pane windows. Old-school rule of thumb was a ton per 500' of conditioned space (which would put you at 6 tons), but even that was overkill unless you had the leakiest uninsulated ducts running in the attic above the (crummy R19 batts) insulation layer, with no wall insulation and single pane windows throughout.

    In code-min new housing a ton per 1000' usually works, (which would be 3 tons), with reasonably sealed and insulated ducts. If the ducts are inside of conditioned space rather than in a 140F attic, it can easily be as low as a ton per every 1500', which would put you at 2 tons.

    If this is a new house you have the option of pulling the ducts inside, or even going ductless, if the floor plan is reasonably open. A 1-1.5 ton mini-split for the main area, and maybe a 3/4 ton for the master bedroom suite would fill bill for MANY new houses, and fix the zoning issue. And being fully modulating systems with a large turn down ratio the idle along at low-speed most of the time, which does wonders for taming those horrific muggy gulf coast latent-loads in (You could heat the place with them too, if you bought the heat pump versions, which aren't that much more expensive than cooling only.)

    The only way to get a handle on the sizing and zoning is a room by room heat gain/loss calculation. The "ton per xxx" rules of thumb really don't cut it, especially if you're looking to maximize comfort. With AC, maximizing comfort will often maximize efficiency. The only reason any body would oversize by that much is if they were they type who turn off the AC in the AM, then come home at 4PM to a 95F house and want to cool it off fast. With a right-sized modulating ductless it works best and most-efficiently with a "set and forget" approach to the temperature, the house never gets torrid or sticky, and you achieve an SEER >>20 (if you buy a better-class ductless.) Most houses could do pretty well with a couple of ductless heads, but if need be there are 2-2.5 ton 3 & 4 head multi-splits out there.

    Modulating ductless systems set to a single temp are nicer to the power company too, since instead of having a 4-5PM load that's 5x higher than it needed to be for the instantaneous late afternoon cooling load, the peak loads will be much lower, and occur 3 hours earlier. The part-load efficiency of these things is just insane- almost too good to believe until you've reviewed third party lab test data.

  6. #6
    In the Trades Tom Sawyer's Avatar
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    You certainly can zone our home.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  7. #7
    DIY Junior Member Failure2Comply's Avatar
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    You can zone a single unit system, but dealing with the amount of cold bypassed air can be an issue. It is very easy to grow mold in a zoned system because of the bypass. When you bypass high volumes of cold air (most zones closed and only one or two calling for cooling) the lower temps cause the casing to finally get cold enough to start condensing moisture on the outside cabinet and it creates a breeding ground for mold inside and outside of the cabinet. Sizing of the equipment is critical and less (tonnage) is more when it comes to dehumidification (Latent Heat removal) which is the primary function of an air conditioning system. Cooling is a byproduct of the process (Sensible Heat removal). Dry air enables your body's cooling system to evaporate more moisture from your skin which is the key to true air conditioning.

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