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.
      For heating only applications, a low return duct is ideal to bring back the coldest air otherwise stagnant in the room. This is true regardless of the supply register location, floor or ceiling. I knew this at a very early age, lying next to the return (after fighting their dog for it!) at my grandparent's house was the coolest part in the home, even though they were just using the wood stove. As we know: "hot air rises", most house as a system folk can argue the true meaning of this, but either way in the heating heating season the cold air remains low and relatively stagnant. So, most of those floor registers in basement systems work well - in heating only applications.
      In cooling only systems, we want to return that warm, stagnant air near the ceiling first. This works well for attic systems due to the ease of installation. Adding a return duct to every room keeps the temperature difference between conditioned spaces low. Remember, in the cooling season the warm stagnant air tends to remain high in rooms, so why do contractors still think under-cutting a door is an acceptable return air path? This method might have worked well in the heating only applications with low returns, not at all ideal for an air-conditioner. In New England, a considerable amount of installations I've seen continue to use central hallway returns for attic installations. Since this provides for poor air circulation in the surrounding rooms, both heating and cooling operation tends to see poor performance.
      For combination heating and cooling duct systems, we have to decide a location knowing that the performance of heat or cool will suffer. That is, unless you install a high/low combination return duct in a wall. The problem with this configuration is that contractors were not ducting in the return, just using the open bay in walls and floors. Since this is not a sealed return air path, it is likely that very little air came from the conditioned space. So, most make a choice of either a high or low return. Is it any wonder why you get the uncomfortable customer calling you the following season?
      Since new replacement furnaces tend to have higher efficiencies, and in turn require higher air flows, I have found the stagnant zone in most rooms tend to shrink during the winter. The lower tempered, higher volume of air tends to create more velocity at registers and mixes well in the rooms. This results in a more comfortable customer in the heating season. But, what happens when we switch to cooling, try to force the air through poorly designed airways, and cannot get the stagnant humid air back to the evaporator? An unhappy, frustrated customer that will always insist the system never worked correctly from the start. Technicians need to open their eyes sometimes, use the information the customers gives you, and "think outside the box" - look at the distribution system for once!

6 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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