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New Home HVAC Configuration

I’m building a new two-story 2900 sq ft home in Massachusetts. The original HVAC design appears to have been sized correctly based on a Manual J showing a total cooling capacity of approximately 3.5 tons. The original design included two separate systems, first floor at 2 tons and second floor at 1.5 tons. I recently learned that the HVAC contractor modified the design and has now installed a single 3.5 ton unit with 3 zones; one zone for the Master Suite (~450 sq ft), one zone for the balance of the first floor (~1550 sq ft) and the third zone dedicated to the second floor (~900 sq ft).

I have the following questions/concerns and would like the community’s input:

1. The modified design was installed without my knowledge. Will the modified single system design with 3 zones work, or should I push to retrofit with the original design with 2 units?
2. The modified HVAC system contains a single stage non variable speed fan. Is not having a variable speed fan an issue in a multi-zone configuration?
3. I am concerned that the HVAC contractor didn’t size the ducts correctly for the different zones:
a. It seems that the Master Suite will cause the system to short cycle, thoughts?
b. If all three zones are running, will the fan be strong enough to supply adequate cooling to the second floor given the pressure drop for the longer upstairs supply line?

Zone Pictures:
Master Suite

First Floor

Second Floor


    HVACNUT Member Posts: 5,478
    Was there design/location issues for the second floor system?
    What was his reason for opting just one system?
    A variable speed ECM motor should be used. With a new build I'm surprised it wasn't spec'd. 
    Without an ECM motor, a barometric bypass is a must. 
    Staging is preferable.
    Personally, I'm am not a fan of residential zoned air systems. However, I see and service them all day, every day on the south fork of Long Island. Heaven forbid my 30 room house doesn't have 30 thermostats. 
    Even the best zoned setup isn't as efficient as a single zone system. 
    I would toss them just for using 2 different brand zone dampers. 
    Find out why they did what they did. If the attic was not a viable option, then the zoned system should be upgraded. With matching parts.
  • pecmsg
    pecmsg Member Posts: 4,389
    2 stories 2 systems!

    Its hard enough to get single duct systems correct 3 zones triple the issues.

    Basements need heat and dehumidification while the 1st & 2nd floors require A/C how are they addressing that?
  • SuperTech
    SuperTech Member Posts: 2,024
    I agree that two systems would have been better.  That setup requires everything that @HVACNUT outlined in his post. Another thing I have noticed about zoned systems is that they tend to run a lot of short cycles, adding to wear on the system components.  Plus you have much more to possibly go wrong. 
    The two different dampers is definitely a bad look.
  • unclejohn
    unclejohn Member Posts: 1,833
    Two system's for sure. Looks like some dampers were on sale as they say.
  • BostonGTR
    BostonGTR Member Posts: 18
    Thanks for all of the valuable feedback.

    The second floor does have space for an air handler unit. The builder apparently decided to save themselves some money by going with a single unit vs the original design which had specified two units.

    Given the above, what would you recommend:
    1. Install a second floor system and remove the interfloor ducting between the basement and attic (very difficult, will probably have to engage a lawyer)?
    2. Have HVAC contractor install appropriate bypass or install a variable speed ECM fan or both?
    3. Any other suggestions/modifications without involving a lawyer?
  • unclejohn
    unclejohn Member Posts: 1,833
    Has he been paid? Start with the general contractor and find out why plans were changed and you were not notified. Get the architect involved, they hate it when their plans aren't followed.
    HVACNUT Member Posts: 5,478
    Someone needs to be held accountable. And the G.C. should be aware of what's happening on his or her site. It's not like the white on the base molding doesn't match the white on the crown molding. There were Sawzall's involved here. Demand it be installed the way it was intended. Maybe change them to variable speed. You won't be sorry. 
    At least the installers look capable from seeing the ductwork. 
  • motoguy128
    motoguy128 Member Posts: 394
    Demand a 4 ton modulating system with modulating zone control (Rheem, carrier ) or demand 2 systems. A single stage system with 3 zones will never work well. Will need to dump to non calling zones even with a bypass. Which defeats the point on low load days.
  • TAG
    TAG Member Posts: 751
    Trying to do three mismatched sized zones with a static single speed system is problematic ...my first house was a typical large 4 square with a large rear addition. Single speed system -- worked very well because each zone was almost exactly the same. 2nd floor -- first floor -- addition. It was installed by an old school oil dealer that knew his stuff. Before I sold it back in the mid 00's the same oil dealer (from the 50's) did a 2 stage w/ VS blower in it.

    The new modulating stuff is fantastic -- I have done a few Carrier systems. My last was a 5 speed with three zones (up/down and office) The key is to size the ductwork correctly -- smallest zone must be 25% over the min CFM of the system --- don't oversize the overall system. The new systems learn the ductwork.

    The key to any system is proper duct