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Community and Q&A

HVAC supply registers in a mostly heat dedicated system.

Eric Schroeder | Posted in Energy Efficiency and Durability on

I have one other question to pose in regard to the HVAC system which is now being installed in the house which is under renovation right now. The house is a side split with each level five to six steps up from the next making it difficult to run ducts. The system proposed is a hydro forced air system with two air handlers split into two zones each. One unit would be in the attic which will be part of the conditioned space and one in the basement. the unit in the basement was supposed to cover the finished basement and the first floor above. The attic unit was to cover bedrooms (half a level above the first floor and attic a half floor above bedrooms. Unfortunately, the ducting for unit in the basement will take up quite a bit of wall and ceiling space. To avoid that the hvac contractor suggested putting a larger unit in the attic and running most of the supply registers into the ceilings of the rooms from the attic with the returns low to the floors. I was always under the impression that in colder climates not only were registers close to the floor and interior walls, but the supplies were also in the floors near windows and exterior wall directed upward sort of washing the windows or areas with the most heat loss potential and the down draft of air from these areas. So finally to my question, if the suplies are located in the ceilins and firing down will that potentially make heating the house more difficult and inefficient with improper mixing? I forgot to mention that the ducts both share a/c and heating.

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  1. GBA Editor
    Martin Holladay | | #1

    There is no simple answer to your question. Basically, it's a question about occupant comfort, since the proposed HVAC system should be able to satisfy the thermostats without any problem.

    The answer depends on the home's air leakage rate (leaky homes will be more uncomfortable with this set up than tight homes) and on the type of registers or diffusers chosen by the contractor. Register design and diffuser design strongly affects air distribution and occupant comfort in a situation like yours.

  2. Eric Schroeder | | #2

    Well...since I cant use closed cell spray foam since I already have the tyvek and 1" xps on the exterior I guess Im going to have a leaky house. I was trying to avoid this and thought this was the right way to go in the begining. Now I guess im stuck. What diffuser design do you think would work the best?

  3. GBA Editor
    Martin Holladay | | #3

    1. Installing spray foam does not guarantee a tight house, and many homes without spray foam are extraordinarily airtight.

    2. Hopefully, your contractor is taking all the necessary steps to detail your envelope in a way that reduces air leakage to a bare minimum. This is usually confirmed and measured with a blower door test. I hope your contractor plans to conduct a blower door test, and knows how to interpret the results.

    3. If the idea of air sealing and blower door testing is news to you, ask your contractor. If these ideas are news to your contractor, you should probably hire an energy consultant before it's too late.

  4. Riversong | | #4


    If your HVAC system is used primarily for heating, then floor supply and return registers are best for even heat distribution and avoidance of temperature stratification. With AC, it's just the opposite. And with retrofits, often compromises have to be made.

    I will respond to your quandary about spray foam on your other thread.

  5. Michael Chandler | | #5

    As to diffuser design look for a curved fin design.

  6. Eric Schroeder | | #6

    Not to beat a dead horse, but if my house is tight enough the location will matter less? Given that the returns will be low no matter what for a $$ more (it's always more isnt it) I can have the supply registers low too, but they will be shooting across the floors from from the exterior walls not in front of and up the windows and or doors. The contracter stated that since they wont be optimally located in the floor firing up in front of the heat loss locations (windows/doors) on the exterior it would be better for them to be in the ceiling with proper grills blowing down on the heat loss locations, instead of across the floor and low. Sorry if I sound like im repeating, but this is a one shot deal.

  7. GBA Editor
    Martin Holladay | | #7

    Yes, if you achieve a high degree of air tightness, and install above-code levels of insulation and high-performance windows, then the location of your registers or diffusers matters much less.

    Envelope first!

  8. Riversong | | #8

    I don't know, Martin. They say in real estate it's all about location.

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