Zones and sizing to replace two units with one

Discussion in 'HVAC Heating & Cooling' started by Hillel, Aug 12, 2009.

  1. Hillel

    Hillel Member

    Messages:
    33
    Location:
    Cleveland, OH
    Hello Everyone,

    When I bought my house, it had two relatively old HVAC units each effectively running one floor of a two floor 2600 sq ft house. There is a 3 ton A/C unit that originally ran the whole house, and now runs just the second floor bedrooms. The first floor kitchen, dining room, and family room are run off a separate 2 ton unit installed when the family room was added as an addition.

    We liked the way the two separate units performed, and we had the thermostats programmed so that it was extremely rare for both units to kick on A/C at once. Our climate is such that it was quite frequent in the winter for both furnaces to kick on at once.

    Both A/C units are now dead. The furnace part of the larger unit is also dying. Time to replace the whole thing. I might also put in a whole house A/C at the moment and replace the furnace later in the winter, but may as well come up with the whole plan now.

    Budget requirements make it impossible to replace both with separate systems, and I would not mind getting back the chunk of my basement utility room taken up by having a second furnace. The problem we come to is A/C sizing.

    A friend of mine who is an HVAC pro calculated the whole house at a 4 ton (13 SEER) unit. He installed the same unit at the neighbor's house which is almost identical, and it is working perfectly there. The problem is that he has never encountered a customer with as disciplined a programming schedule as we are used to. Unlike just about all the neighbors, we do not feel a need to keep the entire house cold all the time during the summer. We are used to only cooling half of the house at a time, and not very extreme cooling at that. It is possible that both old A/C units would go on at the same time maybe 5% or less of the total cooling time in a year. Now, the solution to this is to use zones, and he has installed those before. It is easy when the duct work is already set up for two zones with the old trunks to each separate unit. The real challenge is what does a programming schedule that uses zones to only cool half of the house at a time do to the A/C sizing requirements?

    I am concerned that a 4 ton unit is too much for just half of the house at a time, and would end up short cycling just cooling one zone at a time. We could try to go with a smaller A/C unit, but the rare occasion to push both zones at once could kill a smaller unit. My friend is willing to go down to a 3.5 ton, but he is afraid to go any smaller, but even a 3.5 ton would be oversized if it only had to cool half of the house. A smaller unit could also hurt the resale value of our house as there is no guarantee a buyer would want to maintain such a disciplined schedule.

    For this reason I am considering going up to a 2 stage 4 ton unit. The energy tax credit for a 16 SEER A/C unit would likely cover the price difference, assuming I wait for the beginning of 2010 to replace the furnace part to get the credit separately for that. Then it could run in 2 ton 50% mode unless the draw was really huge from both zones at once, and it could cover the house in any programming schedule.

    Does anyone know if the 2-stage A/C units are really worth it? Has anyone experienced more breakdowns and maintenance hassles due to the increased technology requirements? Would a 2-stage 4 ton unit give better performance with less risk of over/under working than a 3.5 ton with the zoning discipline of only using one zone at least 95% of the time?

    Thanks.
    ---Hillel
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,015
    Location:
    New England
    Depending on how many rooms you have, you might consider a split system. It doesn't use the ductwork, but uses multiple compressors in one housing that run to individual evaporator, fan units.
  3. PEW

    PEW DIY Senior Member

    Messages:
    487
    You will possibly find you have to replace the ac and heater at the same time to get the tax credit. I just tried, but a 4 ton ac unit alone would not qualify for the refund.
  4. Hillel

    Hillel Member

    Messages:
    33
    Location:
    Cleveland, OH
    Tax credits

    The AC should qualify on its own if it is 15 SEER or better. The 4 ton 2-stage unit I mentioned above is 16 SEER and would qualify on its own. The single stage units considered are 13 SEER, which would not qualify. In fact, the resulting credit of the 2-stage AC just about pays for the difference in price.
  5. PEW

    PEW DIY Senior Member

    Messages:
    487
    One would think so, but it is not true. Most of the 16+ units have to be coupled with an appropriate heater to take advantage of the higher seer.

    You will find cases where a 2 ton unit may qualify but, a 4 ton unit of the same seer ratings will not.

    Our government at work!!



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    -- Mark Twain
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,641
    Location:
    Cave Creek, Arizona
    units

    Two story structures without a means of sealing off the stairway between them do not typically lend themselves to a single unit with one thermostat. If it is downstairs it will respond to the cool air flowing down from the upper level, and if it is upstairs the rising heat will activate it. This usually means that the lower level is cooler, or the upper level is hotter, than the thermostat setting.
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