Return Air Locations

     When using ACCA's Manual T for register selection, there are several basic rules that are followed regarding location. Most guidelines revolve around room air circulation and stagnant air, as well as the equipment application (heating only, cooling only, or combination). Application is the largest determining factor when it comes to return air locations.
   

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8 comments:

  1. This comment has been removed by a blog administrator.

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  2. I just stumbled across your blog as I was searching for information on duct design. I appreciate your informative writing style that really seems to open ones eyes to basic principles regarding HVAC design. I have just had the furnace replaced in my house (attic install that services the 2nd story) and am fairly certain that my duct work could use some work. We have a couple of room upstairs that do not cool well in summer. I am researching the duct design, but am now also concerned that my return is not effective. It is a single 20x20 in the hallway. I am certainly intrigued by thought of installing a return for each room, but it may not be practical in a retrofit situation.

    My head is swimming with possibilities, but I am concerned that proceeding with any of these ideas might be ill advised without first having proper analysis done on my current system. The companies that I had come out to quote a new furnace didn't speak of Man. D or J. It was only afterwards that I became familiar with such things. However, I may call the company who installed my furnace and inquire about the Man D/J just to ease my mind.

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  3. Chris, thank you for this blog - it's very informative not only to professionals but to consumers like me. I'm learning about HVAC system design and important considerations before I hire a contractor to install my new system. I am looking for a quality system and don't want to pay big bucks for a big mistake.

    Regarding air return location, I've heard that there is a reason to have a low return (closer to floor) in a primarily cooling situation (climate zone 2 - hot, humid). All other considerations being equal or notwithstanding, some contractors tell me that a low return is better because it is easier to cool already cooler air versus a high return, where the unit would be constantly trying to cool somewhat hotter air. This sounds reasonable to me.

    Now, it also makes sense to me that we would want to go after the hotter air with a high return, as does a split level return (with baffles that close the high or low based on heat or cool thermostat switch). And proper supply design and air mixing should reduce the stagnant temp differences, but they still happen.

    So, what is your thinking on the "easier to cool cooler air, thus a low return location" position?

    Thanks!
    (sorry, I know, old thread)

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  4. Thanks for your comment and question!
    Personally, I would want to get the warmest air back to the air handler as this air will have more enthalpy (heat content) and latent heat. Moisture can only be removed at the evaporator coil, and we are more likely to feel comfortable by getting the humidity between 50-55% than worrying about the sensible temperature on the thermostat. So, for this very specific reason, and not to throw any contractor that may misunderstand this theory "under the bus", but "easier to cool the cooler air" is just about a load of crap! If you cannot mix the air in the room, get the moisture removed from the air at the coil, then you will never feel comfortable! The return location can play a part in this, but more of a result of the supply location...
    Hope this helps!

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    Replies
    1. Thanks Chris! I really agree with the points you make about getting at the humidity content within the hot/humid air. The "high return" always did make sense to me. The "system doesn't work as hard to make cool air cooler" seems reasonable, but for me, it's not enough. I want to attack the hot/humid air.

      Since it's a new install, I expect proper load and duct calculations. I am also getting closed cell spray foam resulting in closed envelope and unvented/conditioned attic (and yes, I understand IAQ and venting issues). In the end, I expect stratification (upper layer of hot/humid air) to be minimal. (hope I'm using the right terms)

      I have one more question on the topic, if you don't mind. Given that I will have high return (in ceiling), does it matter where in the house (one story) that return is located? I vote for somewhat centralized location, but one hvac contractor said that it could be located it in the corner of the house (still in ceiling) because "it's all the same air". Now, while I certainly plan to have transfer grilles or jump ducts between rooms, I still think that "farther from the return" rooms will have less air changes per hour than the room containing the return. With IAQ being a concern, I think air changes are important. Sorry, don't think my wallet can handle return in every room.

      So what do you think? Centralized return location is best, right?

      Thanks again Chris!
      Steve

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  5. Thanks Chris! I really agree with the points you make about getting at the humidity content within the hot/humid air. The "high return" always did make sense to me. The "system doesn't work as hard to make cool air cooler" seems reasonable, but for me, it's not enough. I want to attack the hot/humid air.

    Since it's a new install, I expect proper load and duct calculations. I am also getting closed cell spray foam resulting in closed envelope and unvented/conditioned attic (and yes, I understand IAQ and venting issues). In the end, I expect stratification (upper layer of hot/humid air) to be minimal. (hope I'm using the right terms)

    I have one more question on the topic, if you don't mind. Given that I will have high return (in ceiling), does it matter where in the house (one story) that return is located? I vote for somewhat centralized location, but one hvac contractor said that it could be located it in the corner of the house (still in ceiling) because "it's all the same air". Now, while I certainly plan to have transfer grilles or jump ducts between rooms, I still think that "farther from the return" rooms will have less air changes per hour than the room containing the return. With IAQ being a concern, I think air changes are important. Sorry, don't think my wallet can handle return in every room.

    So what do you think? Centralized return location is best, right?

    Thanks again Chris!
    Steve

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  6. Hi Chris,
    In new home construction -- I am placing "pure air" cleaning filters on forced air system. I have heard that placing outlet registers higher - and return air registers lower -- will drive more of unwanted pollution and dirt in the air into the filter. Also will require less dusting in the home. do you agree with this? Larry

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  7. I think this is an informative post and it is very useful and knowledgeable. therefore, I would like to thank you for the efforts you have made in writing this article. ac repair services

    ReplyDelete