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Is this the right system?

alcoprop | Posted in General Questions on

I had a Manual J done by my local HVAC supplier. And of course I believe it is not accurate and oversized. Another GBA user (Matt F) commented on another one of my posts and agreed with me. He did a quick calc and came up with numbers a lot lower. 

The house is roughly 1600sqft upstairs, 8 ft ceilings and half finished basement with the other half conditioned crawl. The wall assembly is 2×6 framing, r23 mineral cavities, 7/16 zip, r12 mineral exterior, siding and R60 blown cellulose in the attic. Not for sure what we are doing in the basement and crawls yet. Probably closed cell in rim joists. Windows are Marvin casement with a U-factor of .28. This is a remodel so duct work is already in place and would like to keep it or should I start over? Ceiling is getting drywalled in a week so I can finish air sealing and do my blower door test. My goal is 1ach50 or lower and I won’t stop until I reach that! 

Attached are plans of the house and the original Manual J. Please give me recommendations on what system you might use. Matt suggested the Fujitsu concealed line with a 1 ton upstairs and a 3/4 ton for the downstairs. I am curious to know what other pros think.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Shaun.

    This is not my area of expertise, but there are plenty of GBA members who are likely to have thoughts for you, so I'm giving your post a bump.

    1. alcoprop | | #2

      Thanks Brian!

  2. user-5618764 | | #3

    A couple mistakes that you can see in the reports
    1.) leakage is set at "semi-tight". If you know you are going to get to 1ACH50 then they should use the "detailed infiltration inputs" where you put in leakage in ACH. This will affect the heat load considerably not not too much on the cooling.
    2.) Window u-value say .48 in the reports. This is a big difference than what you speced. The SHGC is likely off as well but I can't tell from this.
    3.) They put 5 occupants in the house which will drive up the cooling load a little.
    4.) There is something funny with the sqft and volume. They have the whole footprint x2 as the sqft served by the system and the volume indicates a 5.7ft average height. It looks like they modeled the crawlspaces as conditioned space maybe. I am not sure if this would affect the load. It might just be a different way of inputting things.
    5.) They don't have anything in for ventilation. I hope you are going to put in ventilation if you are building to 1 ACH
    6.) The wall u-values are around .06 which is about R15 whole assembly. That is not what you stated above.
    You might have the contractor correct these issues and see what comes out.

  3. Expert Member
    Dana Dorsett | | #4

    Remember, a Manual-J is to determine the heat load, not an equipment spec. An oversize factor of 1.2x from the 99% load is usually enough with modulating equipment, so equipment that can deliver 20% more than the load at the design spec is a good starting point. An oversize factor of 1.4x is pretty much always enough even for Polar Vortex disturbance cold snaps without a need for auxiliary heat, but will in some cases take a slight toll on average efficiency- it depends on the equipment.

    So, the first obvious thumb on the scale I see is using +1F as the design temperature for Xenia OH, when the 99% outside design temp for Dayton is +7F, and in Columbus it's +9F:

    That lower- than 99% design temp adds 10% or more to the heating load bottom line.

    Using +74F for an inside design temp (rather than a 68F code min) is adding another 10%.

    So even if the rest if the analysis is perfect, the stated number is already at the 1.2x oversize factor.

    The Marvin's published U0.28 window U-factor is about half the U-0.47 used in the contractor's Manual-J. Whatever the stated window losses were, first multiply by 28/47, (~0.6) then multiply by 0.8 to deal with the design temp selections, call it half.

    The cavity insulation in the Manual-J was R21, not the R23 that is actually used (at least they didn't enter R19 :-) ), but they didn't credit ANYTHING for the R12 continuous insulation(oops!). Your actual wall losses would be about half at the stated design temps, about 0.4x if using +7F outside/+68F inside.

    So basically your real whole-house numbers are bout half the HVAC contractor's Manual-J, which was 28,756 BTU/hr for the whole house. Call it 15,000 BTU/hr. An oversize factor of 1.4x would put you at 21,000 BTU/hr.

    According the the extended temperature capacity tables the 3/4 ton Fujitsu slim ducted AOU/ARU-9RLFCD puts out 14,000 BTU/hr @ -5F outside/+70F inside, which is almost as as much as the 15,000 BTU/hr delivered by the 1 ton.

    At +5F (closer to your real design temp) the 3/4 tonner is good for 15,400 BTU/hr to the 1-tonners 16,800 BTU/hr.

    Go over it room by room, but there doesn't seem to be any point to using a 1 ton for either floor. At -5F outside load is about 1.25x the 99% design load, so expect it to be about 19,000 BTU/hr for the whole house. The capacity of a PAIR of 3/ 4 tonner is 28,000 BTU/hr, well above the whole house load. Using a 3/4 tonner for the upper floor will probably show more than sufficient margin even when moving the outdoor design temp to -5F.

    To assess the existing ducts, look at the room by room loads, and apportion the cfm of the air handler to each room, then look at the cross sectional area of the ducts to each room to estimate the duct velocity. If the duct velocity is under 400 feet per minute everywhere the static pressures induced by the ducts is pretty low, if it's under 300 feet per minute it's barely even worth estimating the static duct pressure.

    It's important to ensure that there is a proper return path for every doored off room with a supply duct to keep the door from introducing static pressures that would limit heat/cool at the max speed. Ideally there is a return register for every supply register behind a closed door. If not, there are a half-dozen or so ways of creating low impedance "jump duct" return paths, but what makes sense for a particular room will vary.

    1. alcoprop | | #5

      Dana you literally just blew my mind!! Thank you for all of that valuable info. The current duct system does have a return in each room except for the bathrooms and laundry. Any info on how to estimate the duct velocity. I would be willing to pay for your time if you have any to guide me through this or help design a new duct system. My client has spent a ton of money on this remodel and I want to make sure she is comfortable with zero complaints.

      1. Expert Member
        Dana Dorsett | | #7

        >"Any info on how to estimate the duct velocity."

        The first cut estimate is simple arithmetic.

        Take a look at the cfm spec for the air handler at it's maximum speed. The ARU9RLF can deliver as much as 353 cfm at a static pressure of 0.5" w.c., so assume 353 cfm.

        Then take a look at the cross sectional area of the plenum/trunk duct. For the sake of argument, let's say it's a 14" x 10", which is 140 square inches. A square foot is 144 square inches. If you're shoving 353 cfm into 140 square inches the velocity is going to be (144/140) x 353= 363 feet per minute.

        Under 400 fpm? Yes!

        Moving on...

        Take the room by room heat loads and figure out the fraction of the total. The cfm get apportioned in direct proportion to the load fraction. eg, say your whole floor's design load is 11,555 BTU/hr, and the room's load is 1590 BTU/hr. That room's "share" of the 353 cfm is then (1590/11,555 ) x 353 cfm= 49 cfm. Say it's being fed by a 6" round supply duct, a cross sectional area of 28 square inches. The design velocity in that duct is (144 / 28) x 49= 303 cfm.

        This is just a crude quickie analysis, but as long as the numbers are all <400 cfm you don't usually need a more rigorous approach. Very slow velocities aren't a problem as long as the ducts are inside of conditioned space, and it's a modulating air handler that runs very long duty cycles. You'll probably have to use balancing vanes on the branches to get the actual delivered cfm/temperature balances right for each room, since the range of proportion of duct size to the cfm requirements will vary, and even at low velocity there is still SOME static pressure induced by length & turns.

        For low load homes some duct designers will just punt, install a very short plenum just long enough to accommodate enough 6" round flex connectors, with home runs to each room.

        1. alcoprop | | #8

          I did measure the duct size today and it is a 16x8 and then reduced down to 12x8. So just quick math from your response and my 16x8 is just under 400 cfm but my 12x8 will be 529 cfm. Sounds like I need to redesign the duct?

          Thanks for taking the time to teach! If I am using your assumed whole floor load compared to what my actual contractor gave me in the manual J how do I calculate each room load? Also does it matter if the room has multiple supplies. Example my LR, DR, KI is all one wide open room of about 660sqft and I know there are at least 5-6" round supplies in this area.

          1. Expert Member
            Dana Dorsett | | #9

            >"So just quick math from your response and my 16x8 is just under 400 cfm but my 12x8 will be 529 cfm."

            I think you mean 400 fpm and 529 fpm, not cfm.

            Does it tap off to any room ducts before the reduction to 12 x 8 ?

            If yes, subtract out the cfm that went to the other room or rooms, and only the remaining cfm to the reduced size trunk. On a better duct systems they often neck down after branches to keep the velocities roughly the same, since the trunk is moving less air in the portion after the branches.

            With multiple supply ducts & register to the same room or open area, for purposes of estimating duct velocity the cfm allotment to the room gets split among that collection of ducts. Five 6" rounds adds up to a cross section of about 140 square inches, so the average duct velocity in that group will be lower than in the 16" x 8" (128 square inch cross section) trunk feeding it.

            If the reduction to 12 x 8 comes before any branches it just means you have to pay closer attention to the lengths & numbers of turns there are before it hits a branch (after which the remaining cfm in the trunk is reduced.) 529 fpm isn't a show-stopper by any means for the 9RLF- it is can still push some impedance, but if the whole system were north of 500 fpm it may need a more rigorous assessment of the friction rates, equivalent lenghts, etc need to stay under the 0.5" w.c. budget with the 9RLS.

            At sub-400 fpm that all pretty much falls under "who cares?" since most of the static pressures on low velocity systems like that are induced by the registers & filters. Even 529 fpm isn't going to induce very much static pressure for shorter duct runs with few turns. Bigger air handler AC and hot air systems are often driving duct velocities of 1000-1500 fpm or more.

            You may want to skim through Allison Bailes' duct design primer series here:


            ...or jump straight to the duct sizing part (which you are working at in reverse)


  4. alcoprop | | #6

    Bump for the day!

  5. alcoprop | | #10

    Dana yes there are multiple taps before the reduction. I will do some calculations and see what I can come up with! Thanks again.

  6. MattJF | | #11

    Dana is right about the design temp. I had quickly looked it up and grabbed data from the wrong column (4d instead of 3b)

    I had even stared at your location on the US map wondering how it was so cold where you are.

    Adjusting my prior calculation gives
    12,216btu/h @ 7f/70f.

    I am also guilty of upsizing to the 1 ton, knowing the 3/4 would work. We are in the weeds at this point, but the 3/4 ton should work and with even less cfm, should work on all but the worst existing ducts.

    As far as existing ducts, is there an air handler still installed? If there is, take a static pressure measurement using a manometer. Passable cheap ones are available on Amazon.

    Use that measurement to look up the cfm from the air handler manual. Then use the following formula cfm2^2/cfm1^2 * static1 = static2, where cfm1 and static1 are for the existing system and cfm2 (ie 353cfm) is for the new system. The standard Fujitsu slim duct systems have .35 inh2o static available. I think Dana has a rare typo and said .5”h20 above.

    If the basement won’t see much use, a standard ductless head might make sense and save some money. I don’t know the ductless as well as the ducted units as there are so many options.

    1. alcoprop | | #12

      Unfortunately the air handler is already removed. I am going to do a sketch of the current duct layout today and I will upload it along with some pics today on here to get some opinions. I need to make a decision in the next week on what I am going to do b/c I need to finish rough ins. I can say that I have for sure learned a lot on this.

      1. Expert Member
        Dana Dorsett | | #13

        To the extent possible, air seal as much of the ductwork as possible (with duct mastic) while it's open, and run a fiber-optic inspection looking for damage or big leaks in sections that might not be accessible without more demolition/rebuild. Even though the static pressures are low, stray cfm going somewhere unintended makes tweaking the room to room temperature balances more difficult. In a ~Y2K survey of existing homes in California an average of more than 25% of the air was going somewhere else.

        Don't forget to assess the return ducts as well.

        1. alcoprop | | #14

          Dana the great thing about this remodel is it’s all in the basement and crawl and I have access to everything. I will be sealing all joints!

  7. Deleted | | #15


  8. alcoprop | | #16

    Here is a rough drawing of the current hvac installed

  9. MattJF | | #17

    The 16x8 at 353cfm of the 3/4 ton Fujitsu comes in right 401f/min. So that looks good. My guess is that you are totally fine for the 3/4 ton unit to serve upstairs with regard to static pressure.

    You will need to plan out how to orient and tie in the new unit. You can probably mount it vertically under the supply and make up or have made up a plenum T. Ideal is for the plenum T to have sweeping elbows. You can use the same T to hook up the returns and have a return plenum/filter box on the floor.

    You should be able to get away with a central return as well and abandon the ducted return. The existing ducted return could also possibly be turned in the a supply duct for the basement.

    1. alcoprop | | #18

      Matt you mean not have a return in each room like there is now? Each bedroom and the main living space has a dedicated return duct.

      1. MattJF | | #19

        With the lower CFM of these units a central return can work well if rooms with a closed door need less than 80cfm. See:

        You need a room by room manual J to figure out how many CFM to put in each room.

        Go with the existing return if everything fits, but it would be an option for fitting another system in there.

        A ductless head could probably cover the basement. The office off the side would be only concern. If you can put a bit of insulation down on the floor at least there, you should be good.

        1. Expert Member
          Dana Dorsett | | #20

          >"A ductless head could probably cover the basement."

          A ductless head would be OVERKILL for the basement if insulated as well as the rest of the house.

          What does the adjusted/corrected Manual-J show for the basement load?

          1. alcoprop | | #21

            I haven't been able to get an adjusted/corrected Manual-J. The HVAC supplier wont do it and of course says his manual J is not oversized and if I go with what you guys are suggesting it will be undersized. So I am trying to find someone that can do it but have had no luck.

  10. MattJF | | #22

    EnergyVanguard is one of the go to providers that post on this site.

    Dana, are you thinking the right approach is to do one duct Ed system for both floors or something else?

    1. alcoprop | | #23

      I reached out to EnergyVanguard and they are only taking on jobs with full architectural prints and are running like 4-6 week turn around time.

    2. Expert Member
      Dana Dorsett | | #24

      >"..are you thinking the right approach is to do one duct Ed system for both floors or something else?..."

      Probably, with seasonal adjustments or a thermostatically controlled register or zone damper to keep overheating/overcooling the basement when there is significant load on the main floor, assuming the peak basement loads are low enough.

      The heating and cooling loads of basements don't change nearly as much with outdoor temperature as the first floor- there's no way to "set and forget" a proportional flow to the basement the way it can be done room-to-room on the first floor.

  11. alcoprop | | #25

    So I talked the HVAC supplier into making some changes to the Manual J. Only thing that I see he didn't change was infiltration as he left it semi-tight. What do you guys think of it now?

  12. alcoprop | | #26


  13. alcoprop | | #27

    According to their corrected Manual J they have an entire load of 20,240. Looks like the main level comes in at 12,872 and room by room cfm totals 708cfm. The Fujitsu unit is 353cfm so are their calculations for room by room cfm numbers still too high or?? See attached pdf a couple posts above.

    1. Expert Member
      Dana Dorsett | | #29

      >"Looks like the main level comes in at 12,872 and room by room cfm totals 708cfm. The Fujitsu unit is 353cfm so are their calculations for room by room cfm numbers still too high or??"

      Their cfm numbers are predicated on some paradigm heating/cooling unit that they SHOULD have specified if designing the rest of it. What they did specify was:

      Air flow factor 0.037 cfm/Btuh (heating)

      Air flow factor 0.063 cfm/Btuh (cooling)

      Clearly Fujitsu xxRLF units have different air flow factors than their presumptive model, but it doesn't really matter. The AOU/ARU 9RLFCD can deliver 15,400 BTU/hr @+ 5F at a max flow of 353 cfm, an air flow factor of (353/15,400= ) 0.023 cfm/Btuh

      That is about 60% more heat per cfm than what was assumed in the Manual-J.

      1. alcoprop | | #30

        Dana so with these numbers and with the basement do you suggest I go with one unit and find some thermostat controlled registers? And if so do you believe I stick with a 3/4 ton or should I bump up. Matt even said to go with a 1.5 ton. Want to pull the trigger this week and get the equipment here. Also I have planned to do a Fantech SER150 for ventilation.

        1. Expert Member
          Dana Dorsett | | #31

          >"...with the basement do you suggest I go with one unit and find some thermostat controlled registers?"

          I didn't see an easy breakdown of the loads floor-by-floor (and I'm not going to try to figure that out on paper from the room loads), but basically yes, the basement load is likely to be small enough to just piggy back on the first floor's mini-split.

          >"And if so do you believe I stick with a 3/4 ton or should I bump up. Matt even said to go with a 1.5 ton."

          A 1 ton only buys you another 1000 BTU/hr @ -5F above what the 3/4 tonner delivers at that temp, and only 1400 BTU/hr more at +5F (closer to your design temperature.) Assume the 3/4 tonner is good for 15,500 BTU/hr @ +7 (your actual design temp). If the load for either the top floor system or the combined first floor + basement adds up to less than (15,500/1.2 =) ~13,000 BTU/hr don't bother upsizing.

          If you want the full ASHRAE recommended oversize factor draw the line at a Manual J for the zone at (15,500/1.4=) 11,000 BTU/hr or more before upsizing. But note that Manual-J's generally overstate the load by at least 10%, and upsizing by 1.4x from a Manual-J isn't really required. Upsizing by 1.4x may even oversize if the Manual-J had a pinky on the scale for any of the inputs. The fact that they went with a "simplified" infiltration model rated only "semi-tight" rather than "tight" the 3888 BTU/hr of infiltration loss is probably overstating the loads of each zone by more than 1000 BTU/hr.

          Upsizing to the 1.5 tonner can deliver 18,400 BTU/hr @ -5F, and 19,7oo BTU/hr @ +5F (which is practically the entire whole house load if the calculated ventilation load is much reduced due to the Fantech HRV.)

          Bottom line, I suspect you're still looking at a pair of 3/4 tonners.

          1. alcoprop | | #32

            Sorry for all the questions. So put a 3/4 tonner upstairs and a separate 3/4 tonner downstairs on its own ductwork. A separate unit downstairs wont be oversized for the small load of the basement/crawlspace?

  14. MattJF | | #28

    Definitely progress. I don’t have time to go through everything, but the infiltration and ventilation numbers are still very high. Whatever the tightest setting in the software should be used. Has the ERV been spec’d? They should use the efficiency from that (should be 70%-85%) and use the code spec for cfm.

    If also looks like there is only cavity insulation in the basement walls, you will want at least 1” of foam against the concrete.

    That should get you to just under 20kbtu. A single 1.5 to fujitsu will cover the load at that point. The challenge there is the seasonal damper aspect Dana brought up for the basement. I don’t know what the options are for thermostat controlled dampers that would work for this. I think there are commercial vav boxes that will modulate a damper from say 50% to 120% of the design Cfm. It is on my to do list to figure out the best way to do such a damper, but I haven’t gotten there.
    For a few hundred dollars you can put a single force electric wall heater as backup to make everyone feel good, but it should never be needed.

    I didn’t comb through the room by room cfm. The cfm per room is just percent of the load times the total cfm.

  15. alcoprop | | #33

    Well I have officially been trumped by the home owner. Her new husband (like within the last week) knows the HVAC contractor that put in the unit that was in the house when she bought this property. He basically said I am an idiot and the unit that is installed is exactly what she needs. It is a Westinghouse 80,000 2 stage with a 2.5ton heat pump. Funny thing is he said he installed the unit 7 years ago which might be correct but the tag on the unit says manufactured in 2008. So she has told me that it needs to stay and also is not letting me put in the ERV b/c he said it is a waste of money. He knows of the remodel that I am doing and how tight the property will be.

    I know she will have humidity issues along with other things in the house. And I know she will have caulk crack and drywall cracks b/c there will be so much movement from humidity fluctuations. I am going to make her sign something after I am done with the house and she signs off that she approves of the quality so I am not responsible for the cracks that will occur. I also assume there is a good chance that they will experience headaches since they will not have an ERV in the house now. Any other recommendations?

  16. alcoprop | | #34


  17. MattJF | | #35

    You win some and you loose some I guess. I don't know that much about traditional heat pump systems. It won't be nearly as comfortable as a fully modulating system. If you have the HSPF, you should be able to calculate the operating cost difference based on the load estimate and heating degree days.

    On the ERV, ventilation is required by code. Given the significant enclosure modifications, you should provide some form of ventilation. This could exhaust only ventilation with a continuously operating bath fan.

    1. alcoprop | | #36

      Yeah that is what I am going to resort to with the ventilation unfortunately.

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