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  1. #1

    Supply & Return for Open-Concept House

    I have a client needing to install supply and return lines for their Air-Exchanger/Heat-recovery unit. The house is 3 floors with cathedral roof. The two upper floors are "lofts" and open on both sides. To get an idea, make a right triangle 35' at the base. First floor is 1000sqft, second 600sqft, and third 200sqft, all centered. This is new construction so they are very flexible. The house is heated with radiant floors and convection wall heaters. They want to install the A/E in the insulated ceiling in the third floor. I need to install a supply coming from a basement intake (their decision, they want fresh air from a lower level) to the A/E on the third floor then supply the house back down all three floors along the corner of the cathedral ceiling. Returns will be on the opposite side of the supply. If I supply to 3 registers down along the ceiling, is it best to supply through 3 individual lines to avoid all the fresh air exiting the first register or will one duct be sufficient to get the air all the way down? Also, not too sure of the sizing for these.

    First time encountering this scenario, any advice is appreciated.

  2. #2
    Join Date
    Aug 2003
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    Fort Worth, TX
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    Draw and then upload a diagram of this house and your duct design to give the responders here a better idea what you're saying. If I'm picturing this house as you've described it, the structure will require a bit more design/engineering than typical residential construction.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Quote Originally Posted by Shophound View Post
    Draw and then upload a diagram of this house and your duct design to give the responders here a better idea what you're saying. If I'm picturing this house as you've described it, the structure will require a bit more design/engineering than typical residential construction.
    Here is a quick sketch...
    Name:  House Ventilation.jpg
Views: 382
Size:  37.1 KB

  4. #4
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    Thanks for the sketch. To get my bearings, the air exchanger/heat recovery unit is for ventilation purposes only, correct? Until you uploaded the drawing I was not sure exactly what you meant, but now it appears we're dealing with fresh air exchange for the house, and not cooling or dehumidification. Is this correct?

    I did see where you stated the floors are heated with a radiant system and wall convection heaters are elsewhere. So we're not speaking about air circulation in terms of effective heating or cooling of the structure, but more from effective distribution of fresh air. Is this also correct?

    Where is this house located, roughly?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
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    Quote Originally Posted by Neo1 View Post
    Returns will be on the opposite side of the supply. If I supply to 3 registers down along the ceiling, is it best to supply through 3 individual lines to avoid all the fresh air exiting the first register or will one duct be sufficient to get the air all the way down? Also, not too sure of the sizing for these.
    You need to know the output of the air exchanger to size the supply duct run properly. And what static pressure its fan can work against. If you size and install the supply duct correctly, you should be able to use one duct run for all three levels. Offhand I'd say since this air is primarily ventilation air and not heating or cooling air, the supply outlets should be configured and placed so as to not cause drafts on occupants in each level.

    For the returns I'd be more inclined to specify one duct per level, as it can be harder to get multiple return inlets on the same duct to draw evenly. Alternately you could place dampers in the return inlets for a single duct return trunk, and then adjust the dampers so each return inlet draws what it should.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
    Join Date
    May 2012
    Location
    San Diego, CA
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    352
    Quote Originally Posted by Neo1 View Post
    I have a client needing to install supply and return lines for their Air-Exchanger/Heat-recovery unit. The house is 3 floors with cathedral roof. The two upper floors are "lofts" and open on both sides. To get an idea, make a right triangle 35' at the base. First floor is 1000sqft, second 600sqft, and third 200sqft, all centered. This is new construction so they are very flexible. The house is heated with radiant floors and convection wall heaters.
    To confirm, no cooling just heating?

    They want to install the A/E in the insulated ceiling in the third floor. I need to install a supply coming from a basement intake (their decision, they want fresh air from a lower level) to the A/E on the third floor then supply the house back down all three floors along the corner of the cathedral ceiling.
    I am a bit confused about the basement intake. It seems like you have an air intake in your supply? From what you said, it sounds like your have the intake in a separate duct..

    Returns will be on the opposite side of the supply. If I supply to 3 registers down along the ceiling, is it best to supply through 3 individual lines to avoid all the fresh air exiting the first register or will one duct be sufficient to get the air all the way down? Also, not too sure of the sizing for these.
    IMO the spaces are all open so 1 supply and one return line is fine, when it comes to air delivery the simpler the less static pressure loss. You can have adjustable dampers on your registers if you are really worried. You could refer to ACCA Manual D for duct sizing if you want to really get into it. Otherwise you can get a ductulator and just size your ducts to 0.1 in w.g. per 100 ft based on your CFM.

    First time encountering this scenario, any advice is appreciated.
    This is for ventilation only correct? Will the homeowner just flip a switch, is the ventilation continuous, or it everything on thermostat? If it is just for ventilation, you won't need as much air as if it was for cooling.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  7. #7
    The only purpose is ventilation. I convinced them to have 1 supply and 1 return in the third floor. The A/E will be supplied with fresh air from the basement intake as per their wishes. I am going to install a 24KBTU AC with heat exchanger on the third floor.

    Thanks for all the help.

  8. #8
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    Quote Originally Posted by Neo1 View Post
    The only purpose is ventilation. I convinced them to have 1 supply and 1 return in the third floor. The A/E will be supplied with fresh air from the basement intake as per their wishes. I am going to install a 24KBTU AC with heat exchanger on the third floor.

    Thanks for all the help.
    My confusion is why the supply is in-taking air from the basement? Unless your "supply line" means as supplying your A/E (which is actually the return for the levels).
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  9. #9
    It is in-taking fresh-air from a basement intake (to outside). The homeowners insisted they want that fresh air from the garden and greens in the rear of the house (no contaminants to worry about).

  10. #10
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    Quote Originally Posted by Neo1 View Post
    It is in-taking fresh-air from a basement intake (to outside). The homeowners insisted they want that fresh air from the garden and greens in the rear of the house (no contaminants to worry about).
    I understand that part, my question is whether there is an independent duct to the A/E, then from the A/E to the supply registers. The current drawing shows air coming from the basement and out the 1st, 2nd, 3rd, and then the A/E or it shows the air being sucked in from the intake in the basement and all the levels.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  11. #11
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    Quote Originally Posted by Neo1 View Post
    It is in-taking fresh-air from a basement intake (to outside). The homeowners insisted they want that fresh air from the garden and greens in the rear of the house (no contaminants to worry about).
    Lots of bugs to clog up the air filter quicker.
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  12. #12
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    That is a great point.
    An answer without a question is meaningless.
    Information without understanding is useless.
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  13. #13
    Quote Originally Posted by hcong View Post
    I understand that part, my question is whether there is an independent duct to the A/E, then from the A/E to the supply registers. The current drawing shows air coming from the basement and out the 1st, 2nd, 3rd, and then the A/E or it shows the air being sucked in from the intake in the basement and all the levels.
    There is a single duct from the intake to the AE. Then the AE supplies the house starting from the third floor. The drawing was preliminary... now it is changed to 1 supply and 1 return on the third floor. Their logic makes sense i.e. the fresh oxygen off the greens, it just needed some tweaking to make it work efficiently. They have to live in the house so I work with (within reason) what they want.

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