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Community and Q&A

Balancing Air Temperature

ben87 | Posted in Mechanicals on

adjustable 2-story air balancing

hi, I’ve done a large addition on a 2-story duplex in Central VA and am replacing the whole HVAC & duct system (heat pump + gas furnace).

Previously the HVAC system was not well designed and the upstairs was always much hotter than the downstairs. Obviously hoping to remedy that but stuck working within confines of very limited space for mechanicals & ductwork, 8′ ceilings, and an unconditioned attic with blown insulation on the floor.

To avoid attic ductwork, I’m planning to run all ducts off a main trunk along ceiling of 1st floor with takeoffs running through the 2×8 floor joists to supply at the perimeter (ceilings of 1st floor and floors of 2nd floor).

I used this software Load-Calc ( to estimate heating and cooling loads for each room and am working out duct sizing based on that. (side question: doesn’t seem like this takes stack effect into account. Anyone have any recommendations on how to account for that?)

The issue of course is that heating loads are higher downstairs and cooling loads higher upstairs. So if I size my ductwork for one, it will be off for the other. I’ve looked into zoning as one potential solution, but seems like there are some downsides to that. As a potentially simpler, more affordable, and better for my situation approach I have the following idea:

1. Balance the ductwork to meet cooling requirements based on the aforementioned load calculations. This should make it comfortable in the summer but too hot upstairs in the winter.
2. Get bedroom registers with dampers (all bedrooms are upstairs–plus a hall and 2 baths). Then occupants can throttle airflow down if it’s too warm.
3. Upsize all of the supply branches so that I don’t put too much static pressure on the system when the bedroom registers are throttled back

Some potential drawbacks I can think of would be:
– noise at registers from partially closing dampers
– occupants having to manually adjust things
– velocity getting too low in supply ducts (but is this a problem since they’re in a conditioned space?)
–  getting too little throw & mixing at registers (but wouldn’t this be a problem for variable speed equipment anyway?)

Very curious to hear others’ thoughts or warnings about potential pitfalls since I haven’t heard of others doing this.

Thank you!

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  1. jberks | | #1

    Traditional zoning wouldn't be possible with your trunk configuration. But yes, balancing the system by each register is the way to go about it.

    To avoid the drawback of this, try looking into a Flair automated duct damper system. It essentially is a room-by-room zoning system. I haven't tried these, but its a neat idea for a traditional central hvac system. Although the price can add up.

    For noise transder, basically you'll have a straight shot for sound waves from the 2nd floor to the first through the trunk. Not sure if thats a major concern for you. You can try using flex for the branchlines to dampen the sound transfer.

    I don't think you have to worry about low velocity. I don't think you're putting in 20" spiral duct for your trunk. traditional low velocity systems with window registers have always had poor mixing, the system just runs longer.


    1. ben87 | | #2

      wow, the Flair devices look really slick. Thanks so much for pointing me to those.

      And wasn't too concerned about noise transfer. But that's good to be cognizant of, so thanks for that. And good, glad lower velocity shouldn't pose a problem.

      Also, if you don't mind me asking you another question, do you think it would be advantageous to install a central return for the upstairs? The furnace will sit in a first floor closet near the center of the duplex shared wall and there will be a central return beneath it. My friend who does HVAC thinks I need dedicated returns in every bedroom, but that would require ductwork in the attic, and I like the idea of baffled return air grilles above bedroom doors to create return air pathways better. I've been planning to add a central return high on the wall at the top of the stairs (with the duct dropping down behind the furnace below) so that I'd have a central return on each level. Intuitively, this will let the upper return pull the hottest air in the house during the summer and the lower return pull the coolest air in the winter, and if they are sized sufficiently I could even add dampers or close one of them off seasonally to optimize where the return air is drawn from. But I've read others arguing that return air placement is pretty unimportant, and if bedroom air is being drawn out from above the doors, I'm already getting the benefit of any mixing from a high return there. Also, the stairway and downstairs are all open floorplan, so air can flow to the downstairs return no problem.
      And, space is really tight and I need to run vents & plumbing & electrical for furnace, water heater, and washer/dryer all crammed in and around that space. So adding the upstairs return is doable but far from easy.

      So given that you seemed knowledgeable, what do you think? Or if anyone else would like to chime in, that would be great too. Sorry to be so long winded, but thanks for reading this and taking the time to help!


  2. Expert Member
    Akos | | #3

    "do you think it would be advantageous to install a central return for the upstairs?"

    This is an important part for summer time cooling performance. In the summer stack effect works in reverse and air leaks make it into your house through the 2nd floor ceiling. You need a nice snorkel there to pull that hot air in and cool it down. Generally just adding in this return and a bit air ceiling air sealing fixes most summer comfort issues in older homes.

    Make sure this return is large and should be near the ceiling. No need for a return in each room, a central hallway return works just fine with undercuts on the doors.

    Generally you will still need some seasonal adjustment, if you are doing new ducting, I would split the main and 2nd floor onto two separate trunks and have a balancing damper right by the hvac unit. You then mark out the position of each for summer and winter and adjust them once per season. Also easy to add zoning down the road this way.

    1. ben87 | | #4

      Akos, thanks so much for answering. I will take your advice on the high return and find a way to add that.

      I've also considered doing two trunks like you suggested. It's hard to explain concisely, but the main problem is that space is so tight that I'm not sure if I can make the two trunks work. But I've been doing more research on the Flair vents, and I see some downsides, so 2 trunks would probably be a more reliable solution.

    2. ben87 | | #8

      Do you think adding a ceiling fan in the hallway at the top of the stairs would mix the air well enough to negate any stratification and allow having a single return downstairs to work OK?

      Asking b/c routing the ductwork for an upstairs return is creating a host of problems...

  3. walta100 | | #5

    If you want good control in an older poorly insulated and air sealed home each floor needs its own system.

    Consider doing the math for the dual fuel system. It almost never works and certainly not in a mild climate.

    How many hours a year would it be less expensive to run the gas furnace than the HP?

    How many hours a year would it be so cold that the HP could not cover the load?

    From a dollar a cents point of view cutting the gas pipe off and saving $200 a year in connection fees seems very appealing.

    My variable speed heat pump heats my house down to 7°F with the strips locked out.

    If this place is as poorly insulated and air sealed as I am imaging consider spending some money on the thermal envelope.

    Is this a rental?
    If so, do the tenants pay the utilities?


    1. ben87 | | #6

      hi Walta, thanks for the input. If I was starting over, I might reconsider the gas & would definitely do the math you're suggesting. But I already have the equipment. This project has been a first and a learning experience. Also, I'm in a city where natural gas is relatively cheap (electricity is around 5x more per unit energy), and I have a gas range and will probably go with a gas tankless water heater. But in the future I'd love to build a house new, and all electric has its appeal.

      And it's a house I live in but rent out the other bedrooms, so we split utilities.

      As for the overall insulation and air sealing, the addition wraps a good portion of the house, so that much should be pretty tight. I'm sure the front of the house and attic floor are quite leaky though. I plan to do some air sealing in the attic.


  4. nrosdal | | #7

    in my house currently under construction we had separate trunks for up and down and put manual dampers in each (they could be motorized at a later date if wanted but cost did not seem worth it). This way after we have a winter and summer under our belts we will have a setpoint for each damper for each season to keep things uniform. I am assuming that in winter we will have the basement a little more open and up partially closed and the opposite in the summer as we have lots of windows and heat gain upstairs will call for more cooling in warmer months. This seemed like the best way to handle it to me (i did hvac for about 8 years many year ago but am surely no expert).

  5. Expert Member
    DCcontrarian | | #9

    Let's say in the winter you need 1000 CFM on the first floor and 500 CFM on the second floor to be comfortable. Let's postulate further that in the summer you need 1000 CFM on the second floor and 500 CFM on the first floor.

    You need an air delivery system capable of delivering 1000 CFM on each floor. Full stop. You also need a return on each floor capable of returning 1000 CFM.

    You also need some way of modulating the output on each floor. It could be as simple as balancing dampers as Akos suggests. It could be two separate units. I could be automated dampers.

    What's probably not going to work is asking a system capable of 1500 CFM to throttle down to 500 CFM.

    1. ben87 | | #10

      Yes, thanks. Totally agree on not throttling down the system from 1500 to 500, and the need for the 1000cfm capacity on each floor (using the numbers from your example).

      Separating it into 2 trunk lines that are each capable of handling 1000cfm, and supplying the upstairs with one and the downstairs with one would be a good solution. The problem is that I either wind up with a supply trunk in the unconditioned attic or else two trunks running in parallel along the first floor ceiling. The problem with the latter option is that both trunks need to be sized to flow the 1000cfm and I wind up with a very large soffit which will not look good architecturally (and I don't want to drop my whole ceiling as they're only 8'). And to further complicate, the furnace is in a closet where there's a lot going on structurally (and I already need to make some structural changes to get one adequately sized trunk out of there), so to get 2 supply trunks out of there may not be doable (and I can't just split it later b/c there's also the stairs going down the middle of the house so I'm very limited in the space I have to run ducts to the side of the house on either side of the stair).

      So that's what's pushed me to explore options that would allow me to make seasonal adjustments with just 1 trunk. Obviously that means adjusting at the individual takeoffs, and my idea is basically to size the main trunk normally (1500cfm in your example), but upsize the takeoff ducts (so the takeoffs headed downstairs could flow 1000cfm total and the ones going upstairs could flow 1000cfm total). Then when I make the summer/winter adjustments the system can still flow the 1500cfm total, but different proportions are headed upstairs vs downstairs. I suppose this adjustment could be done by leaving access panels and adjusting manual dampers at the takeoffs, but doing 10 of them would be a pain, and that's a lot of access panels. So just doing this at registers would seem preferable to me.

      However, my idea goes 1 step further, in that it tries to eliminate the need to adjust dampers on the downstairs level at all. This requires upsizing all of the takeoff ducts headed upstairs one step further. This way, even when downstairs registers are fully open, the upstairs would still get 1000cfm and downstairs only 500. But as bedroom registers are throttled down, the proportion of airflow would gradually shift so that more and more air is going downstairs, and I would know that the downstairs ductwork is already sized to handle that. Also, since the thermostat would be downstairs, I'm assuming that space get to the desired temperature either way. It's just a question of whether upstairs bedrooms get too hot (and they can be independently throttled back to comfortable levels by the occupant).

      If helpful, here's a spreadsheet I worked out based on the Manual J calculations:

      My main questions at this point are as follows:
      1. Is there a reason this wouldn't work?
      2. What kind of floor registers should I use? I'm thinking the kind with opposed-blade dampers that are adjustable with a screw would give the best control
      3. Is an upstairs return necessary? Akos mentioned the need for a "snorkel." I'm wondering if a ceiling fan run in reverse could achieve the same purpose by destratifying the air. There would be transfer grilles above bedroom doors and an open return path down the stairs.

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