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  1. #14
    Join Date
    May 2004
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    Sounds like you need to get some accurate load calcs.
    What's the budget look like and all that stuff.
    I would get dedicated systems for each floor too but that's just a personal selection for me.

  2. #15
    Join Date
    Sep 2004
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    2 systems is the only way to go. You'll be glad you spent the extra money. I see so many houses around here that have 1 system for 2 floors. Its always a lot warmer upstairs than downstairs. 4 tons for the whole house seems very small.
    Life is like a jar of jalapenos. What you do today might burn your ass tomorrow.

  3. #16
    Join Date
    Nov 2004
    SW FL
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    Talking Value is in the mind of the H.O.

    Originally posted by bat1029
    I have a brand new home being built (3600 Sq.FT., 1800 1st fl. and 1800 2nd fl.) and got estimates from two respected HVAC firms in my area (northern NJ)
    One suggested a 4ton unit for second floor and a 3ton unit for first floor using trane units
    The other suggested using a Carrier Two Stage 4 ton unit for the whole house Any input would be appreciated
    4-tons is too little for 3,600 S.F. for many days in N.J.
    However, if you don't mind 82'F inside temp for many afternoons, go for it. Just make sure you have enough ceiling fans.

    How much is your comfort worth?

    TWO 3-ton systems should not be that much more $
    than ONE zoned 5-ton system.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  4. #17
    Join Date
    Mar 2004
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    A single unit can do two stories just fine, but you need a good zone control system, good duct design, and preferably two-stage equipment to do so.

    If the load calc indicates that a single 4 or 5 ton unit can do the job, I wouldn't hesitate to use a Carrier Infinity zoning system. It will usually cost less up front than two systems, take less energy to run, less cost to service (one system instead of two), and with the capacity modulation and variable air volume capabilities, should be most comfortable too. Plus you can control and/or program all the zones from a single point (or multiple points, if you set it up that way). You can put in a single high-end zoned system for the same money (or less) than two mid-range systems.

    A single zoned system can often be of slightly less capacity than you would use in twin systems, because the load peaks in different zones often occur at different times. You may not care that the bedrooms are a little warm at 6 pm, for example, because you never go to bed before 10pm. By having one small system work hard, you can get much closer to its theoretical maximum efficiency than by having two systems that are forced to cycle on and off more of the time.

    1. Yes, you lose the redundancy of being able to retreat to the other floor should one system fail.
    2. Zoning works very well when done properly (like many things, it's much easier to do it wrong than to do it right!) and used with reasonable expectations. It will do fine at maintaining an even temperature between upstairs and downstairs. It can also maintain somewhat different conditions between the two (5-8 degrees). If you're looking at a situation where you may not want to heat or cool one floor for long periods (the classic empty nester with master-on-main-floor setup), though, zoning will not work as well, and then you should absolutely have two separate systems for the two floors.
    3. The floorplan will have to allow some space for ductwork to go from one floor to the other; this may not be necessary with two systems.

    As for sizing the equipment, don't forget that cooling equipment is rated based on the assumption of an 80 degree indoor temperature. So even if your system designer used the temperature you requested- say 75- for the system to maintain during cooling season, that only can account for how much cooling the structure needs to hold 75. The AC equipment has less capacity when the indoor temp is 75 than it does at 80. So if you want an indoor temperature of less than 80 during cooling season, you need to de-rate the capacity of the cooling equipment by 835 btu, per degree below 80, per 1000 cfm. So for a "five ton" system, which today is typically more like 56,000 BTU at 2000 cfm, if you want 75 degrees, you have to derate the equipment capacity by 835 btu * 5 degrees * 2, or 8350 BTU- more than half a ton. That leaves you with 47,650 in this sample case. So a Manual J can easily say you need four tons, but if you want much less than 80 degrees indoors during cooling season, you will often need 5 tons.

    I think this is the primary reason that people mistrust Manual J; they think it will tell them what size equipment to use, don't correct for equipment capacity ratings, and then can't figure out why the system can't hold the desired temperature in the summer. It's not Manual J's fault, they're just not interpreting the results correctly. They may well have done the measurements and calculations perfectly, and still had poor results because of this issue. Manual J tells you what the structure needs; this correction tells you what the equipment can actually do under the intended conditions.

    All that said, if after Manual J and correcting for equipment capacity you discover that 5 tons won't be quite enough, then you will need two systems regardless

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