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Ducted Split — 4 zones

josephny | Posted in General Questions on

I’m real close to making a decision but, as always, the more I learn the more questions I have.

The zone 6 house has 3 floors, 1540 sq-ft per floor.

The HVAC contractor proposes an LG Red 36KBTU (LUU360HHV) for the 1st floor, AHU (LVN361HV4) and ductwork in the basement with floor registers to the 1st floor.  And, an ERV tied into this ductwork.

He proposed an LG Red 24KBTU (LUU240HHV) feeding 2 AHU’s (LDN127HV4 & LDN187HV4) for the 2nd floor (one for front half of floor, one for rear half).

And an LG Red 18KBTU (LUU180HHV) for the 3rd floor with an AHU (LDN187HV4).

Then I started to figure out the proposed return solution.  If I’m understanding this correctly, each floor’s system will be disconnect from each other floor’s system (i.e., operating completely independently). 

The first floor will have the ERV for fresh air exchange.

The 2nd and 3rd floors don’t seem to have any means to get fresh air.

Am I over-thinking this?

Then there’s the question of an HRV or ERV.  The way we changed from HRV to ERV is I mentioned that heat in the Winter can really dry out the air inside the house.  He said that an ERV will take some of the outdoor humidity and let it in (yes, Winter humidity is lower, but the thinking is that it will be substantially higher than inside a tight, heated house — yes?).

Would very much appreciate thoughts on this.


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  1. Expert Member
    Akos | | #1

    Zone 6 means around 0F design temperature. The main floor LUU360HHV is rated for 40000BTU at 5F, say it de-rates a bit at 0F to 38000BTU. With a well sealed and insulated house as you were discussing earlier, you can probably heat your entire house with that, there is no need for extra air handlers.

    I would re-run the number and size the units for the actual load.

    Assuming your roof deck is insulated and if your attic space has knee walls, you can probably combine the 2nd and 3rd floor onto a single air handler and run the ducting in the knee walls for both areas.

    Generally even in cold climates you want an ERV (unless you have a very wet basement that is dumping humidity into the house) to maintain indoor humidity in the wintertime. The fresh air feed from this ERV should be split and connected to the return of each air handler. This way all parts of the house get fresh air, this is especially important for bedrooms where there will be people sleeping with the door closed.

  2. josephny | | #2

    I've asked this repeatedly of the HVAC installer, who hired TL Phillips to do a (what looks to me like a very complete) Manual J/D/S.

    The answer I keep getting is that yes, on a strictly BTU/Hr basis, I'd be fine, but the air circulation (CFM) would be inadequate. Does this make sense? I can post the TL Phillips report if it would help.

    Roof deck is very well insulated (R49).

    The design now is to have 1 AHU behind knee wall A and another AHU behind knee wall B, each feeding half of the 2nd floor. And another AHU in the dropped ceiling for the 3rd floor.

    Here, too, the response has been that the air flow from a single AHU wouldn't be sufficient.

    Splitting the ERV air makes perfect sense, but I don't think that was the plan.


    1. Expert Member
      Akos | | #3

      When it comes to mechanical sizing it goes like this:

      House construction and climate gives you your heat load
      Use this heat load to size your equipment.
      The equipment selected gives you the CFM/ton you need to design for.
      Size ducting to each room based on its heat/cooling load and the above CFM/ton
      Once ducts are sized, size the registers based on the velocity to get good mixing in the room

      If you look at that design process, CFM is an output not an input to equipment sizing. There is no magical "circulation" you need extra CFM for.

      A 3 story house will have some stratification but that is best handled by having a return on both lowest and highest level. You already have that if you have a main floor air handler and one in the attic. You don't need more units.

      If the equipment is sized correctly and the right amount of CFM is fed to a room, the register are well placed for good mixing, the room will be perfectly comfortable.

      Installing 6.5 tons of equipment (rated to around 7.5tons of heat at 5F) into a house that size is beyond silly. You are spending a lot of money for mechanicals you don't need and also some pretty big ducting to be able to connect them up.

      For a 3 story structure like that I would go with 2 air handlers at the most. You can even go with a single unit but you need a lot of ducting and this can eat up a lot of sqft.

      1. charlie_sullivan | | #4

        Just to underscore the point about CFM of circulation not being a fundamental requirement, lots of houses, mine included, have hydronic heating systems, with 0 CFM of air circulation in the HVAC system.

        1. josephny | | #6

          Makes perfect sense.

          I don't have a deep enough understanding for a air/ducted system of the relationship between CFM/pressure and meeting the heating/cooling load as determined by the man-J.

      2. josephny | | #5

        Do I understand correctly that the Manual J should be done prior to equipment selection?

        And that equipment selection may be limited by the results of the man-j, it is not dictated by it or affect the man-j numbers?

        Is the problem that we started out with the equipment selection -- LG Red based on a ducted heat pump for zone 6 that would allow at least one zone for each of the 3 floors?

        Attached is what the Man-J shows.

        Thank you!

        1. Expert Member
          Akos | | #7

          Yup. Man J first. I've only worked with LGs non-hyper heat units, they worked good so far, I can't comment on the red series, on paper the numbers look pretty good.

          Make sure your man J is correct. Ventillation losses seem high, that amount of BTU works out to about 90CFM/floor, which seems a bit on the high side.

          Your main floor losses also seem high compared to the 2nd floor for a simple box shaped house, they should be pretty close.

          Assuming each floor has 1500sqft wall with 20% double pane low-E windows (~r4). Walls about R20 wall assembly (3.5" cc SPF + 2" batts). 0F design temp, so 70F delta to indoor.

          Walls 1500 * 80% * 70 / 20=4200 BTU
          Windows 1500 * 20% * 70 / 4=5250 BTU

          Add in some air leaks, take away heat generated from equipment/lighting/people, you are nowhere near 20000BTU.

          Splitting the 2nd floor up makes only sense if you have a lot of glazing on one side and there is significantly different heat/cooling load throughout the day, otherwise I would stick to one zone.

          You can definitely put each floor on its own zone, just make sure the equipment is sized for it. Based on the rough ballpark above, you are looking at 1 ton hyper heat per floor.

          LG only makes the red ducted unit down to a 1.5 ton size, the modulation on it is reasonable, but min cooling is a bit on the high side, so you don't want to connect up one to a couple of small rooms by itself. You might need to reduce your zoning to get it to closer match your capacity.

          One option is to use a non-hyperheat unit for the 3rd floor. Usually that gets plenty of heat from the other floors, but needs more cooling in the summer time, a non-hyper heat such as this can work:

          1. josephny | | #8

            Ventilation in the current analysis is by a LifeBreath Max 155 -- not sure if that helps explain the high ventilation loss.

            As you very closely assumed, the 1st floor has 330 sq-ft of window glass and 11' ceilings -- does that explain the higher loss compared to the 2nd floor which has 280 sq-ft of glass and 10' ceilings?

            But, the 1st floor total wall (include the windows/doors) is 1738 sq-ft (45x34x11). The 2nd floor has 1580 sq-ft (45x34x10). The 3rd floor has dormers on each side, so I would approximate the wall area (vertical area) at under 1000 sq-ft.

            That breaks down to :

            1st flr: 1408 wall; 330 glass
            2nd flr: 1300 wall; 280 glass
            3rd flr: 880 wall; 120 glass

            The second floor is not split such that one side is South and the other North -- it is split East-West. I think this would eliminate the explanation of glazing. I think the explanation is that the AHU can't move enough air for the entire floor.

            The response from the contractor is simply that he deals with LG and Mitsubishi, so we are going with LG, and that the units modulate, and that my electric supplier (NYSEG) has limited equipment to get my rebates.


          2. charlie_sullivan | | #9

            The high ventilation loss could be explained if someone put in the ventilation rate of the LifeBreath but didn't account for the fact that it recovers most of the heat--84%. So your actual loss should be maybe 16% of that.

            The "AHU can't move enough air for the entire floor" your contractor told you doesn't make sense at all.

            Any more contractors you can try calling?

  3. Expert Member
    Akos | | #10

    The Max 155 is around 70% efficient, this means at 150CFM you loose 3500BTU when it is 0F outside. 150CFM is only needed for a house with a lot of bedrooms, something with 4 to 6 bedrooms would only need to run the unit at 100CFM, so 2500BTU.

    Your actual wall sqft is not that far off, although the one of the 3rd floor is a bit trickey since there is a lot of roof area as well. Either way, I doubt you are in the 50000BTU range for the whole house, especially with SPF in the walls/roof and new low-e windows. I'm assuming your basement is insulated, if not, that can add a fair bit of heat load to your house.

    Ducts have no problem delivering air to places far away, they just have to be sized correctly. Since you can heat an entire 2.5 story house with a single air handler in the basement, there is no way you can't make a single low static unit heat a single floor.

    I would have an HVAC engineer run a real as built manual J for your place and sized based on that. It is very easy to fudge the inputs to a manual J and have it show any heat load the contractor is looking for. The current size is pretty close to the usual rule of thumb of 500sqft/ton, which is WAY off.

    There is nothing wrong with either LG or Mitsubishi. Both make great equipment. As long as they are sized correctly and installed with care, you should have no issues.

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