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Best way to balance a new system?

brian_wiley | Posted in Mechanicals on

Hi all,

I just installed a new ASHP last week and redesigned/embiggened (thanks, Dr. Bailes, for the blog posts) the return ducts, but the supply ducts were left alone for now. I’ve been using ASHRAE-111 as reference, but have had some difficulty in parsing the best procedure for balancing the system. 

My Manual D calls for 664 cfm supply and return. I have 1060 cfm of supply, and 750–800 cfm return. Currently I have the fan speed on medium, and plan on setting that to low which should get me to about 940 cfm or so according to the manual. It is not an ECM, fwiw.

Should installing dampers (none currently exist) on the supply takeoffs and adjusting them to match the Manual D’s room-by-room requirements be the first step?

And this might be a silly question, but if the supply ducts are supplying the stated values in the Manual D for a total of 664 cfm, is it problematic that the returns can take a total of 750–800 cfm? Or do I need to make sure that the supply and returns are equal in cfm?

Finally, if there are any good resources on this process that I may have overlooked I’d love to hear about them.

Thanks for the help!


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  1. walta100 | | #1

    Are you going to buy an Anemometer?

    Are you going to measure the speed of the air entering each room given the size of the duct you could calculate the CFM at the register.

    This type of measurement is generally only done on very large project like hundreds of thousands of square feet of offices.


    1. brian_wiley | | #2


      I've been using an Alnor 731 Balometer/capture hood to measure the CFM, which is how I arrived at the numbers above.

      However, I have an anemometer, and can certainly measure the air speed at each register to calculate the cfm. I'd planned on doing this for subsequent measurements as I don't personally own the balometer; it's something that I can check out for work so I don't always have access.

      Pragmatically, I'm a bit worried about using dampers to go from 940 cfm to the 664 cfm that's called for via the Manual D. Is a 30% reduction in air flow okay by way of closing dampers, assuming that I stay within the equipment's rated static pressure?



  2. Expert Member
    NICK KEENAN | | #3

    I'd be more worried about static pressure than CFM. Static pressure is what affects performance.

    I think you're at the point where you should just run it for a while and see how it feels. Modeling is just that, modeling. You probably won't know until the coldest day of the winter or the hottest day in the summer how the performance is.

    1. brian_wiley | | #4

      My static pressure is pretty good. The ESP is at .238 iwc, and the media filter is at .1 iwc. The system allows for up to .4 iwc, so it's in certainly within range.

      After spending a couple of weeks in a 32" tall crawlspace on this project, I'm more than happy to take a wait and see approach. For some reason I had it in my mind that the supply side and return side had to be as close to equal as possible, which is what ultimately prompted the question. If that isn't the case, and I can fine tune it over time based on comfort, then that sounds like a plan to me.

      1. Expert Member
        NICK KEENAN | | #5

        Unless you're pumping air out of the house supply and return will equalize.

  3. Expert Member
    BILL WICHERS | | #6

    Is there any way to easily add a bit more return? Sometimes you can just cut a register into the side of a duct in a basement, maybe use a lower restriction grate on a wall somewhere. It would be good to get the supply and return closer.

    I wouldn't want to use dampers to try to bring the flow down this much, since you'll increase the static pressure in the system. You're better off lowering the blower speed, but you need to make sure you keep within your system limits so that you don't have issues with excessive heat exchanger temp when running a heating cycle, or freezing the evaporator coil in the cooling cycle. As long as you're within the limits you should be fine, but adding some extra return side CFM is a better option.

    Remember that as your filter gradually clogs up, static pressure seen by the blower will increase. The usual rule of thumb is "replace a filter when it doubles in back pressure", but that would put you up at 0.438 inwc, up over your system limit. You need to allow some wiggle room with the filter numbers since those are a bit variable as the filter gradually fills up with gunk. If you have no other options, I'd at least watch the pressure across the filter for 6 months to a year or so until you have a good idea of when you need to replace filters to keep within your system limits.


    1. brian_wiley | | #7

      Thanks for the reply, Bill. I'm not sure how easy it'd be to add a return; would a conditioned unvented crawlspace that's regularly about 8–10 degrees cooler than the living space be as good as a basement? If so, then it'd be a piece of cake. I can certainly add a return vent in one of the bedrooms on the floor if that's what it'd take, but it'd be a bit more work.

      The changing of the fan speed would only be within the prescribed limits of the system. The fan speeds are handled via dip switches on the board as opposed to a re-wiring, so all the settings match their tolerances if I'm understanding it correctly. I know it won't be much, but I'm assuming I'll see a slight reduction in static pressure by lowering the speed, so hopefully that works in my favor.

      After you detailed the procedure on watching the filter on that previous post, I ordered a manometer and pitot tube set (I'd previously been using the one on the balometer) so that I can keep a closer eye on the filter so as not to create too much static pressure. My longterm plan is tackle the supply side ducting which is pretty restricted. Once I get that straightened out I think I'll be able to hold on to the filters until they double in pressure, but until I'll make sure to keep the current one within system limits.

      1. Expert Member
        BILL WICHERS | | #9

        The usual way to put in measurement "ports" is to drill some holes in the ducts that are a snug fit for the manometer's tubing. Put the holes in good spots to be on either side of whatever you're monitoring. Order some small rubber stoppers to plug the holes when you're not taking measurements.

        If you plan to leave the manometer connected all the time (I don't recommend doing this), then put some rubber grommets in the holes and use those to hold the tubing in place by friction.


        1. brian_wiley | | #11

          I hadn't planned on leaving the manometer in. So far I've just been using a small square to cover the test holes that I drilled in between the times I've taken a measurement.

          Do you have any thoughts on the idea of an extra return in the conditioned crawl versus creating a new return in the living area (likely a bedroom)? The crawlspace would be much easier, but don't want to do it if it'll cause another issue that I'm unaware of.

  4. Expert Member
    Akos | | #8

    If your supply and return flows don't match you either have a measurement error or leaky ducts.

    Flow measurement is prone to a lot of error, but I would expect it to be closer than 200CFM off but would also not be surprised. If your ducts look tight, I wouldn't worry about it.

    As for balancing, make sure your most restrictive run gets sufficient flow, this generally means closing the balancing dampers on the less restrictive runs. Usually this gets you into the ballpark, tweak the rest plus the blower static pressure setting as needed to maintain design flow. The goal is to have the blower running at the lowest setpoint possible to deliver the required flow rate.

    If you have a humid shoulder season or have a hard time with humidity control in the summer time, it is helpful to reduce the blower static pressure setting a bit to decrease air flow rate closer to 350CFM/ton. This is less efficient, gets you less cooling but much better humidity removal. For heating set it back to normal flow rate for best efficiency.

    1. Deleted | | #12


  5. Jon_R | | #10

    > best procedure for balancing the system

    Once constructed, you need to wait for colder weather (I assume) and then measure temperature in each room. Then add/adjust dampers to the rooms that are too warm when the thermostat is set to keep the coldest rooms warm enough. Final balancing is about comfort, not design flows.

    Note that proper balance isn't fixed - you may need some zoned dampers.

    > do I need to make sure that the supply and returns are equal in cfm?

    Pressure differentials across closed interior doors should be less than 2.5 pascals This will be true when duct supply and return flow in each room are reasonably equal.

    1. brian_wiley | | #13

      Thanks, Jon, for outlining the comfort-based balancing procedure. We got down to 30 last night, so I don't have to wait long for chillier weather.

      And I probably should have included this information from the get go, but it's a single zone system with two main returns; one in the front of the house, one in the back. There are no returns in individual rooms, unfortunately. We sleep with the doors open 99% of the time though.

      I've attached a Manual D, and a screenshot of our layout for reference relative to how the front of the house is almost completely open whereas the back of the house is a series of small rooms.

  6. brian_wiley | | #14

    Thanks for all the advice, everyone. I think my plan going forward will look a bit like this:

    1) Lower the blower speed to the next lowest setting.

    2) Make sure that all the return duct work is tight and seal any leaks that I may find. Once that's complete, I'll retest to see if the supply/return cfm values are within 10% of one another. If they are, I'll stop worrying about it.

    If they're still off by a larger margin, then I'll add a return as per Bill's advice. The location will either be in the conditioned crawlspace or one of the bedrooms pending Bill's response.

    4) Once the supply/return volume is relatively balanced, I'll move to balancing the individual spaces based on comfort and by testing the rooms' temperatures relative to one another. I'll make sure to keep an eye on static pressure and keep it within equipment limits to make sure any of the dampers don't put too much of a strain on the air handler unit. If there's too much disparity in comfort between the two sides of the house, I'll look into a zone damper as Jon suggests.

    If that seems off I would love to hear it. Otherwise, I'll report back when I run through the tests in hopes that it'll help someone else running through the process.

  7. walta100 | | #15

    Brian you need to rethink your crawlspace strategy.

    Vented crawlspaces have proven to work.

    Conditioned crawlspaces have proven to work.

    “Encapsulated” /unvented crawlspaces are playing a game of chicken with mold and rot. When the temp in the crawlspace fall more than a few degrees below the temp in the house some spot or spots in the crawlspace will fall below the dew point and condense water from time to time. If the spots get wet enough and stay wet long enough mold will grow and wood will rot.


    1. brian_wiley | | #16

      Hi Walt,

      I probably wasn’t super clear above in my response to Bill. In my haste I called it an “unvented” crawlspace; in a subsequent response I called it conditioned. It’s definitely the latter.

      Today the living space is 69 degrees with 50% rh; the crawl is 65.5 degrees with 42% rh. The biggest temperature disparity is in the summer when the living space is at about 75 degrees and the crawl is still around 66, but we have much lower humidity in the summer as well. That’s why I mentioned the 8-10 degree disparity to Bill.


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