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Community and Q&A

Upgrading an HVAC System in an Old Farmhouse

tamahime | Posted in Mechanicals on

We live in a 115-year old house in the north suburbs of Chicago (climate 5A)  which presents many… unique… challenges. This year it’s replacing the HVAC system and I’m spinning from trying to figure this out.

The house was obviously not built with ductwork but at some point some previous owner retrofitted it for central air and heat. The spaghetti ducts work OK for the first floor, but the ones that go to the second are a couple of inches tall and have no returns, so basically they’re useless.

We’re not working with anything near what I generally see here as far as energy efficiency – it’s embarrassing to think how inefficient our house is, but it’s important to me to try to make it better. Whenever I replace something or work on something I always try to update it to the greenest/most efficient option that makes sense. That said, it’s still a stick frame farmhouse from 1908 with dormers and a nearly flat roof and a ton of windows. Roughly 850 sq. ft. per floor.

Anyway, our AC condenser was installed in 1984 and leaks freon, so I’m thinking I might as well splurge and install a system that actually heats and cools the whole house and is energy-efficient. I called at least 10 different contractors. Six of those actually called me back to schedule estimates. Of the six that came out, only three of them ever sent me a written estimate. Of those three only one answered any follow-up questions. This whole process took more than a month. I only mention this because suggestions like “Make sure the installer does a Manual J” seem laughable based on my experience. It was all guesswork.

The winner-by-default proposes this:

1. Replace the existing AC compressor with a 13 SEER conventional AC.
2. Keep our existing 80% NG furnace (7 years old)
3. Install a Mitsubishi Hyper-Heat 24,000 BTU outdoor unit for the second floor
4. Install three 9k heads, one for each bedroom (according to the installer, that’s the smallest size they have/install).

Is this the best option, under the circumstances? Trying to put in new ductwork is impossible without basically just tearing the house down. There’s no attic. The upstairs rooms are oddly shaped and often closed off from each other when people are up there. The basement is finished and without the residual heat from the inefficient furnace would be frigid in the winter (and is also oddly segmented). The estimate I got for going all mini-split was truly staggering ($30,000!). An option to use a ducted air handler for the first floor and basement was not much better.

Any suggestions or advice (or reassurance) would be appreciated. I’m limited by my wife’s dwindling patience and the apparent lack of local HVAC contractors who have capacity. Are there any other configurations or options I should consider? Thank you for your time.

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  1. kyle_r | | #1

    The HVAC industry (and many others) is seeing a huge spike in demand. My advice would be to just replace your current AC, assuming you can’t make it through the rest of the summer without.

    Then take the next 9 months to find a firm that will properly design an HVAC system for your home (Manual J, S, and D). You can then take this design and ask multiple companies to bid on the installation. Hopefully by then demand will be back to some sort of normal and more companies will be interested in bidding and pricing is more competitive.

    Mitsubishi makes 6k heads. Even these are too big for a bedroom and I would stay away from a multi split. You would be spending tens of thousands of dollars on a system that is very likely to not be comfortable or efficient.

    A new HVAC system that provides comfort and efficiency is going to be expensive. Spend the time and money up front to make sure it’s designed properly.

  2. Expert Member
    Dana Dorsett | | #2

    >" 1908 with dormers and a nearly flat roof and a ton of windows. Roughly 850 sq. ft. per floor."
    >"1. Replace the existing AC compressor with a 13 SEER conventional AC.
    2. Keep our existing 80% NG furnace (7 years old)
    3. Install a Mitsubishi Hyper-Heat 24,000 BTU outdoor unit for the second floor
    4. Install three 9k heads, one for each bedroom (according to the installer, that’s the smallest size they have/install)."

    Start by using the fuel use from last winter's gas bills to estimate your 99% design heating load, using the 80% gas furnace as the measuring instrument. The methodology for this can be found here:

    In your area the 99% outside design temp is going to be within a few degrees of 0F, so if you're not sure, use 0F, not your all time low temperature, or last winter's low temperature. The idea is to start with the 99% design load, and upsize appropriately from there. See:

    Then run a room by room load calculation using a Manual-J-ish type tool. The two online freebie/cheapie tools I usually recommend are LoadCalc(dot net) and CoolCalc (dot com). Be aggressive rather than conservative on estimated air tightness and R-values- both of those tools tend to overestimate (by a lot if you're being conservative). Even with aggressive inputs those tools usually overshoot a fuel-use measured heat load by 15-35%, but it'll put you in the ball-park. For heat pump applications (such as the ductless Mitsubishi), use the BetterBuiltNW load tool ( ) which uses more appropriately aggressive U-factors. (You have to create a free account, but it's free, and pretty easy to use.)

    A seven year old gas furnace still has decades of lifecyle left, and will be running at it's nameplate efficiency even if ludicrously oversized. Read the BTU-in and BTU-out on the nameplate, sometimes found inside a panel on the unit, but if you know the exact model number you can look it up online. But if it's truly ludicrously oversized (3x the fuel-use measured 99% heat load) what it WON'T do is provide optimal comfort. ASHRAE recommends a 1.4x oversize factor, which has a high enough duty cycle to provide comfort and heat rooms relatively evenly when it's cold out (1/1.4= 71% duty cycle at your 99% outside design temp), but at 3x+ the run times are short, often too short to fully heat rooms at the ends of the duct runs even when the duct design is adequate.

    Bottom line- if way oversized you may still want to consider replacing the furnace with a right-sized 2 stage condensing gas furnace.

    There are several ways to add returns to the upstairs rooms without adding ducts or ripping apart walls/floors, etc. For doored-off rooms with supply registers consider using a transom transfer grilles or a partition wall stud bays as a "jump duct". See:

    When using framing bays as jump ducts it's important to install well sealed air barriers at the top & bottom of the section being used as the jump duct, especially in homes that might have used balloon framing with no top plates to partition walls. Cut'n'cobbled plywood, 1x lumber etc (even corrugated cardboard) foamed in place with 1 part expanding can foam or sealed at the seams with polyurethane caulk will do.

    It's EXTREMELY unlikely that the heating & cooling loads of a bedroom for even a completely uninsulated second floor antique house rise to the output of a 9K ductless head married to a 24K hyper heating compressor. At typical FH09 pr FH09 head married to any MXZ compressor will deliver 9000 BTU/hr cooling, and nearly 11,000 BTU/hr heating (until compressor capacity craps out).

    Even a single FH09 (6000- BTU/hr nominal cooling) is usually sub-optimally oversized for bedroom loads. Take a look at your room by room Manual-J numbers- if the calculated whole house load numbers (add up all the rooms) exceeds the fuel use numbers by more than 10%, scale the room load numbers down appropriately:

    scaled room load = room load x (fuel use whole house load/Manual-J whole house load)

    If the scaled bedroom load is calculated to be under 5K (heating or cooling) you absolutely don't want to be blasting it with a 9K head. If there is a closet space or enough headroom in the hall to install a lowered ceiling a single 9K or 12K slim duct cassette (with jump ducts to a common return to a ceiling grille in the hall) or maybe even a 15K or 18K (unlikely- probably oversized) is likely to provide more comfort and higher efficiency, since it'll actually modulate a bit when it's rather than cycle on/off all year. (Yes this is a bigger deal installation than 3 ductless heads.) eg: A 12000 BTU/hr (rated cooling) NAXDKS12A112AA compact duct cassette married to the hyper heating SUZ-KA12NAHZ compressor delivers 15,000 BTU/hr @ +5F, (which is probably close to your true load for an 850' somewhat insulated tightned up second floor), yet modulates down to under 8K @ 47F.


    If you run all the load numbers and report back I can probably dig out an even more appropriate solution.

    If there is little to no ceiling insulation in the upstairs, it may be better to re-roof and install 4-6" of polyiso above the roof deck, then dense-pack cellulose into the rafter bays. There are a few vendors of reclaimed or factory seconds roofing foam at steep discounts (50-75% less than virgin stock perfect retail) in northern IL to take the sting out of the material cost. Run these searches every week or so- several will pop up, probably one within easy driving distance of you:

    Even with full-dimension 2x6 rafter bays stuffed with cellulose (R22) as little as 3" of roofing polyiso (R17) is sufficient for dew point control at the roof deck. As long as at least 40% of the total R is on the exterior you're good to go.

    1. tamahime | | #4

      Hi Dana,

      I see you updated your response, so in addition to my post down below a couple of other replies to your thoughts:

      I tried using the tool you linked to for minisplit sizing but I quickly ran into so many "I don't knows," number fudging, or outright not understanding the tool that, I'm sorry to say, I abandoned it.

      You are absolutely correct that what we mainly run into is "lack of comfort" in both the summer and the winter. The thermostat is on the main floor, which is also where the ducts are large and direct (and where the only return is). Even on this floor (Which is basically completely open) the middle of the room can be warm while the areas near the windows/doors is much cooler. To this point what we end up doing is closing most of the vents on the first floor to try to force some air to the second which helps but still leaves the upper floor ~10 degrees cooler/warmer than the lower floor during temperature extremes (say, > 80 or < 30). I'm assuming this just results in inefficient cycling because only so much air can get through those very small ducts.

      I looked at those links you posted about ways to add returns but they don't apply in my case, I don't think. All of the supply ducts to the second story are in the floor - they are a flat duct underneath the hardwood floors that is no more than 2-3" tall. Moving air between the upstairs rooms isn't really an issue, it's getting it back to the furnace/AC in the basement.

      There are some weird closets upstairs but none of them have access to all the rooms, and again we don't have any attic space to run SDHV tubes or anything like that. In addition, the upstairs ceilings are very low (7'). (It is a weird house. Every contractor we've ever had in here do do anything says something along the lines of "I've never seen that before.")

      Roof is also new, as I point out below. And I have no idea what the permitting process or construction costs right now would be to try to raise the roof - I imagine it would well exceed our ~$15,000 budget for fixing this heating/cooling problem, and still leave us with the issue of "how do we get conditioned air in/stale are out of the the second floor"?

      In addition to the numbers below, is there anything else I can provide that would help?

  3. tamahime | | #3

    Thank you for the detailed responses. I ran some of the numbers based on your post, Dana:

    Current furnace: 88,000 in / 71,000 out

    65 BTU calc: 12465.7409
    60 BTU calc: 13971.50764

    According to the installers (two of whom were Mitsubishi Diamond contractors, for whatever that's worth) they could not install a ducted cassette or ceiling cassette because of the lack of attic; there's not enough clearance to put it in the ceiling.

    I'm trying to get out of the window unit game - been doing that since we moved in and it's a pain to take them in and out and store them every year. They are proposing to replace the condenser and coil and of course use current refrigerant.

    We had the roof and siding redone right after we moved in - we actually didn't have a choice, it was damaged in a huge hailstorm the week after we closed! We asked for extra insulation in the roof and the roofer told us they put whatever they could in the limited space. What rating it is or whether they were telling the truth I don't know - we were new homeowners blindsided by this huge problem and just trying to get it done. But it's a membrane roof 7 years old, it's not ready to be replaced.

    My problem is basically I don't know how we can actually approach sealing it: it has older (20 years) vinyl double-paned windows. They were gapping and had some other issues, so I paid the window manufacturer to come out and repair them and all replace the panes where the seal had gone. As I said we already redid the siding and roof; even if it's not what we would have chosen now I can't see justifying the waste to do it again, and assume that the contractor sealed what he could during the install. It has some weird things going on like an enclosed back porch that isn't conditioned or well-insulated space but is open to the house (at some point a previous owner basically enclosed and finished off a back stairwell and deck into living space, it appears). Similar situation with a front sun porch. I've done some DIY things to improve the air transfer between these places - every GC I've ever asked about it just threw up their hands - but I'll never get it anything like "tight" without major construction.

    I am curious about the bedroom situation - I lived in Japan for a year during college and stayed with a host family. My room (MUCH smaller than any of the rooms we're talking about here) had its own mini-split head that worked great for heating and cooling, and this was 20 years ago. Shouldn't they have heads that are viable for small rooms in the US by now?

  4. Expert Member
    Akos | | #5

    Depending on your local energy costs, especially if your electricity is cheap, a good start would be to replace your furnace and AC with a correctly sized cold climate heat pump.

    You can look at Carrier Greenspeed / Mitsubishi Zuba /Daikin SkyAir. I believe the Greenspeed units can also be paired with a NG furnace if you want to keep that as main heat in the near term.

    Unfortunately this does not fix your 2nd floor issues. In older houses with limited access the best solution tends to be a high velocity setup (ie Spacepak) as the ducts can be fished inside walls. Spacepak also makes a heat pump unit so you can use this for both heating and cooling your 2nd floor. Not cheap though.

    Proper fix as Dana suggested above does mean doing proper ducting, which is hard to do outside of a major renovation.

    If you just want to improve cooling performance, a budget option is a single wall mount unit in the hallway upstairs. This might take enough of the cooling load off the existing system to allow the undersized ducts to keep the bedrooms reasonably cold.

    Wall mount in every bedroom is a terrible idea for comfort (I have this at home). Way too much airflow for comfort and the oversized heads do a terrible job of dehumidification. I think the reason these setups work in Asia is most buildings have no insulation and leaky single pane windows.

    1. tamahime | | #6

      Thanks for the comment. I looked at SDHV as an option but it was quickly eliminated due to the aforementioned lack of an attic - there's both no place to install the unit itself and no place to run the ducts.

      The problems with a single unit upstairs are:

      1. The "hallway" (such as it is) doesn't have an exterior wall.

      2. The time when the additional cooling/heating is most needed is at night, when the doors to all the bedrooms are closed.

      I'm feeling super bummed out now because it feels more and more like I've scheduled an expensive solution that still won't work very well but there's not even really another viable alternative. Other than, I guess, replace the existing condenser and keep the status quo (which sucks). They're supposed to install next Wednesday. I was excited but now this feels like just another one of this house's "problems with no solution."

      1. Expert Member
        Akos | | #7

        Sketch up a rough floor plan including any knee wall spaces and closets. A couple of pictures would also help.

        There should be some way to get something in there that will work better than you existing setup.

        1. tamahime | | #8

          Here's a floorplan I drew up of the upstairs. (Can also do this for the first floor and basement if that's useful.) I marked the things I thought would be helpful. X's are existing floor supplies as described above. Measurements are rough, just to give some idea. Ceilings as stated above are around 7' (they vary from room to room).

          Let me know what pictures might be helpful. Thank you so much for your help!

          1. Expert Member
            Akos | | #9

            Something like this should work.

            I would mount a slim ducted air handler up in the ceiling above the staircase since you have plenty of height there. Set the unit up so that it has the intake on the bottom (most can be changed by swapping some covers).

            The supply runs can be simple runs in a couple of small bulkheads above the doors/corners. Generally any bulkhead behind but above a door is not too noticeable.

            Something like this is probably in the ballpark, it should cool your upstairs without issues and also provide plenty of heat in the winter time if needed:


            There is also a Carrier branded version of the same unit that might be more readily available:

            The unit itself is under $1500, so not a huge cost but install does tend to be pricey.

            Generally I find the best is to only get the HVAC company to quote the install of the unit only, that is:
            -outdoor unit install
            -indoor unit mounting
            -line set and condensate drain
            -leak test and fill
            -operating test

            Have a GC quote you on running the ducts through the house and making the bulkheads as needed. The bulkheads needed tend to be pretty small for this type of setup, I've attached a picture of one.

          2. tamahime | | #10

            Thanks for the idea. Here's where a picture will help, though.

            As I mentioned, the ceilings are very low. As you can see in the photo (taken in the hallway area, bathroom on the left) there is basically no "above the door," no place to run a bulkhead like you describe.

  5. Expert Member
    Akos | | #11

    7' is on the high side of some of the basements. Most are even lower than that and when finishing them as living space and you still have to make room for ducts. Tight but can be made to work.

    6" and 8" oval ducts are your friend as you can fit them into low profile bulkheads. With a bit of care you can squeeze them into bulkheads that only drop 4" from the ceiling. Any ducting inside conditioned space does not need to be insulated which also saves on size.

    If there is enough height for a ceiling fan, there is space for ducting.

    Sometimes when tight on space, running a bulkhead along the bottom of outside walls works. The space above them can be used for shelving or storage.

    To get proper functioning HVAC, you'll have to give up some space somewhere.

    1. tamahime | | #12

      Oh, for sure. Just trying to figure out where to do that in this application. There's literally 3" of space between the top of the door trim and the ceiling in those pictures (and, in the case of the bathroom, zero.) So while putting an air handler in the stair area makes sense from the standpoint of accommodating the cassette, there's really no way to run ducts from it (except to the front bedroom).

      Funny you should mention ceiling fans - the fans in these rooms, like the one you see in that picture, I can't walk directly underneath because I'll hit my head on the lights! Our basement is even lower - my head almost touches the ceiling (I'm 6'3").

      1. Expert Member
        Akos | | #15

        Sometimes even 3" can work (3 1/4" is better though). You can go with 3 1/4x10 or 3 1/4x14 rectangular duct, this can even be left exposed and painted to match the ceiling if you are very tight on space.

        You can see the 3 1/4x10 duct running above the door to the next bedroom. Height was not an issue in this install so the duct was boxed in and drywalled. Unless pointed out, nobody notices that there is a duct there at all.

        The unit I linked to can also be mounted in the vertical direction, another option is to install the unit inside the linen closet at the top of the stairs. This would let you feed two of the bedrooms directly with almost no ducting/bulkheads and you only have to run one feed to the bedroom with the ceiling fan.

        1. tamahime | | #16

          I think those are the kind of ducts we have now, just in the floors. I'm assuming (hoping?) that with the air handler the airflow would be better than what we get now from central heat/air? Because right now they provide a trickle of conditioned air at best. Some questions:

          1. If I mount the air handler in the closet, will it really get enough air intake?

          2. Would I then need some kind of transom vents to allow the air in the rooms to cycle back with closed doors?

          3. Will this style of duct perform well with the several 90* turns needed to bring it to the office?

          1. Expert Member
            Akos | | #17

            Lot of times in older houses the ducting has to do a lot of jogging to make it up to the top floor. I've also seen where the installer was not careful the ducts came apart over time. Sometimes even ducts get disconnected and used for another part of the house for no reason. Old houses...

            1. Simplest is to replace the closet door with a louvered door. Intake is pretty easy as there is no return ducting, just a filter mounted right to the unit. You can also cut a large grill into the door if you want to keep the existing one. Either option should work.

            2. You will need transfer ducts or door undercuts to return the air. Door undercuts are the simplest.

            3. The hard bends in 3 1/4x10 tend to be very restrictive, you might have enough pressure for two 90 bends, 3 would be pushing it. With a lot of bends I would go with oval ducts in that case. 8" oval would probably be in the ballpark (2x 6" oval is definitely enough) for the office even with some 90s along the way.

  6. Expert Member
    Dana Dorsett | | #13

    >"Current furnace: 88,000 in / 71,000 out

    65 BTU calc: 12465.7409
    60 BTU calc: 13971.50764"
    I'm not sure what those last two numbers are. Is that the heat load using base 65F vs. 60F? Either of those would be on the very LOW side even for a brand new code-min house that size.

    If you are willing to post your ZIP code (for more precise weather data) and your gas meter reading dates & amounts (in therms or ccf or whatever energy units the gas is billed in, not dollars) from November through February I can do the arithmetic for you here on the forum.

    You may be able to use floor joist bays as jump ducts with a floor grille in each room with a supply, and a ceiling grill at some convenient location on the first floor spaces below (preferably some distance down the joist bay for light mitigation & sound privacy.)

    1. tamahime | | #14

      That was the result of me following the instructions in your "Out With the Old, In With the New" link. But it's very possible I screwed something up. ZIP code is 60077. Here's the numbers I plugged into my spreadsheet:

      Input BTU 88,000
      Output BTU 71,000
      Efficiency 0.806818182

      99% temp -1

      Bill dates: 1/20/2021 2/18/2021

      Jan Therms 88.98
      Feb Therms 126.06
      Total 215.04

      Heat Delivery 173.4981818
      Delivery BTU 17349818.18

      HDD 65 1391.8
      HDD 60 1241.8

      65 BTU/Degree 12465.7409
      60 BTU/Degree 13971.50764

      Here's the Nov - Feb meter data:

      11/18/2020 - 74.47
      12/18/2020 - 114.23
      01/20/2021 - 179.98
      02/18/2021 - 215.04
      03/19/2021 - 115.28

      Something funny I just noticed - our gas company has a feature to plot our usage against other data. If I plot it against HDD, the usage almost *exactly* matches the HDD trendline.

      1. Expert Member
        Dana Dorsett | | #18

        The math (using only base 65F data since it's at best a 2x4 house, nowhere near current IRC code):

        17,349,818 BTU/1391.8HDD= 12465 BTU per degree-DAY.

        With 24 hours in a day that would be 12465 BTU/24 hours = 519 BTU per degree-HOUR.

        With a presumed 65F heating /cooling balance point and a presumed 0F design temp that would be 65F-0F= 65 heating-degrees. So the furnace-measured design load is:

        65 degrees x 519 BTU/degree-hour = 33,735 BTU /hour

        As a rule-of-thumb type sanity check for an 850' + 850' = 1700 ' house that's a ratio of 33,735 /1700'= 19.8 BTU/hr per square foot of conditioned space @ 0F, which is credible for an insulated 2 story 2x4 framed house with clear glass double panes (or storms over single-panes) over an unconditioned 850' basement with no foundation insulation. With insulated foundation walls and some focused air sealing & spot insulation it could hit the 15 BTU/hr per square foot range ( ~ 25-26,000 BTU/hr).

        With no dedicated return paths for the upstairs it's likely that more than 10% of that is air handler driven infiltration & duct losses due the higher static pressures generated when the room doors are closed. It could easily be a ~30K load + 3-4K of parasitics due to the lack of returns.

        With a furnace output of 71,000 BTU/hr and a design load of 33,735 BTU /hour you your oversize factor is (71,000/33,735 =) 2.1x. While that's not great, it's also not a disaster.

        If it's really a 30K load it's a (still credible) design load/area of about 17-18 BTU/ft, and a (71K/30K=) ~2.4x oversize factor.

        As long as a decent sized path can be ensured from each doored off room to a return grille the parasitic losses will go away, and the rooms will get more flow. A 2-2.5x oversize factor is far from the idea 1.4x, but with even semi-adequate returns the temperature differences will even out a bit. With the parasitic losses mitigated the duty cycle will still only be (1/2.4 =) 42%, or 25 minutes out of every hour when it's 0F outside, but that's still better than what happens at 3x+ oversizing. At the ASHRAE recommended 1.4x oversize factor the furnace would be running a (1/1.4x=) 71% duty cycle @ 0F, or 43 minutes out of every hour, which is enough to do quite a bit of temperature-leveling room to room.

        If there is no way to use floor joist bays or partition walls for returns, and you are willing to cut in to the bottom of the doors (not the nicest thing to do to a nice antique, but usually OK for replacement doors) Tamarack has a solution that works, for about $40 per door:

        (I considered those for my antique sloped ceiling head-banger guest suite, but opted to utilize the cramped attic spaces behind the kneewalls instead, after insulating at the roof deck.)

        It's basically the same thing as a transom grille with sound & light baffles, only built on to the door rather than the wall.

        Most 3 ton cold climate heat pumps can cover a 30-33K load @ 0F, with a bit of heat strip to cover the Polar Vortex events. Even a (not-so-cold-climate, but better than some) 4 ton Carrier Infinity w/ Greenspeed could cover that:!/product/29548

        A mid-static 3 ton Fujitsu "compact ducted" version too:!/product/25352 (Fujitsus can be mounted in an upflow configuration, and can probably be set up in place of your existing setup)

        So can a 3 ton Midea (also possible to mount vertically in an upflow configuration) pretty much could too:!/product/36735

        Most of the full-sized air handler modulating heat pumps these days have a 2: 1 turn down (or more) and would be running a 100% duty cycle during cold-ish weather if right-sized, but in the examples above you'll see that the turn-down of the compact ducted units is usually 4:1 or better, which means they will run super-long near 100% duty cycles even during the shoulder seasons.

        But in non super-insulated 2-story houses it's usually better to zone the system by floor. With a decent Manual-J you could figure out the floor by floor loads and install two (ideally separate) ducted mini-splits to get decent temperature control floor-to-floor both summer and winter.

        1. tamahime | | #19

          Thank you! That was all super helpful. I will definitely look into those under-door grilles, they seem ideal for my situation. The doors are nothing special so it's fine as far as I'm concerned to cut them.

          I think the 2 floor approach is best. Opening things up in this house is historically asking for trouble. I'd rather not go exploring in the ceilings, who knows what may have been done in there.

          I will have to do some thinking and discussion with my wife based on your and Akos' posts. She would not like giving up her linen closet for a piece of equipment. And I can't think of any other place to put a ducted unit. But based on this data is it certain that the ductless option with a head in each room would be bad? I know she was excited about the "room by room" controls; she likes her sleep temperatures much more extreme than I do.

          1. Expert Member
            Dana Dorsett | | #20

            >" But based on this data is it certain that the ductless option with a head in each room would be bad?"

            Yes, pretty much bad. Three 9K heads can pretty much cover you ENTIRE home's heat load.

            A 24K multi-split won't modulate below 7-7500BTU/hr, which is probably half your upstairs 99% load. Three SEPARATE 9K heads could modulate down to a collective 4800 BTU/hr.

            Given your not super-tall ceiling height a high-wall coil is going to be a head-bonker even for not-so-tall people, whereas a 3" x whatever you like hard duct in a soffit below the ceiling would eat up less than 5" total head room, even with some insulation.

            >"She would not like giving up her linen closet for a piece of equipment."

            A vertically mounted low static or mid-static 1 - 1.5 ton Fujitsu or Midea can be fitted into as little as 10 square feet of floor area in a "micro mechanical room" type cabinet/ closet, if there's enough space at the top of the stairs. This is a 1.5 ton Fujitsu mounted in an entry hall:


            This is before they built the cabinet around it. The big grille is the common return & filter. The box in the middle is the cassette itself. Above that is a very short plenum for tying in the flex. With rectangular hard duct you might be able to squeeze it even tighter.

            The Fujitsus can all throttle back to 3100 BTU/hr @ 47F eg:



            Midea's minimums vary by model & size:





        2. tamahime | | #21

          Ok, so I got my wife to agree in theory on the ducted cassette idea. Here's what I'm running into now.

          First, the installer that I'm using only installs Mitsubishi. The Mitsubishi slim ducted cassettes are only horizontal mount, which actually isn't a problem. The problem is the minimum outputs (which are MADDENINGLY hard to find). From what I can tell:

          SEZ-KD12NA: Cooling Min - 3800 BTU/hr, Heating Min: 4800 BTU/hr
          MUZ-FH12NA: Cooling Min - 2500 BTU/hr, Heating Min: 3700 BTU/hr

          SEZ-KD18NA: Cooling Min - 3800 BTU/hr, Heating Min: 4800 BTU/hr
          MUZ-FH18NA: Cooling Min - 6450 BTU/hr, Heating Min: 5150 BTU/hr

          So the cassettes seem to have the same mins - sounds fine. But the outdoor unit takes a MASSIVE jump for the 18k. My fear is that the 12K one would be undersized based on our heat load calculations, and the 18k one isn't really that far off from the issue I'd face with the multi-split - min of 7200 (how big a deal is 2000 BTU/hr?), and at that point I might as well go for the solution that allows room by room control and easier install?

          The other issue is the installer (probably no surprise) doesn't seem keen on the ducted idea. In fact, he wants me to go up to 12k floor heads because he can't get ahold of any 9ks and, in his words, it is "a very small difference".

          As an additional data point, I checked the window ACs we have been using for years now (1 in each room), and they are 6000 BTU. They have always done a fine job of keeping the rooms cool, they're just expensive, loud, don't heat, and are a PITA to take in and out. Since they obviously don't modulate, is that a data point in favor of needing 18k of capacity? If I were to insist on waiting for 3x 6k wall heads, wouldn't that be nearly the same as the 18k ducted but with additional benefits? Wall heads are not great for two of the rooms but not impossible to find a place for, just have to move some artwork...

          1. Expert Member
            Akos | | #22

            In a leaky two story structure, lot of your heating will come from the main floor unit, a right sized unit will keep up without issues, there is no need to oversize. There is also a 15000BTU unit that has a pretty decent modulation range.

            The problem with the 3 zone multi split is the minimum capacity of the outdoor unit. If you look at either MXZ-3C24 or MXZ-3C30 they only have a 2:1 range, this means neither unit can run a 6000BTU (or even a 9000) head without cycling or bypassing refrigerant. Since the heads would already be oversized for both cooling and heating, the chances this will happen a lot during operation outside of polar vortex days. This type of operation is pretty bad for energy use, it is what I struggle with at home with my own multi split.

            Just because you can keep a room cool with a 6k window shaker, it doesn't mean the cooling load is 6k. You can get a closer estimate by measuring the compressor on/off time on a hot day. Except for the office, it also doesn't matter if the AC can barely keep up in a bedroom during the early afternoon heat, so you can always get away with a bit of undersizing.

            Window shakers also add a fair bit to the cooling load as there are always a lot of air leaks around them even with a careful install.

          2. tamahime | | #23

            I checked out the 15k outdoor unit submittal data, and it has the same minimum specs as the 18k - 6450 BTU/hr cooling, 5150 BTU/hr heating. So it seems like whatever they did to the unit to beef it up above 12k applies to the 15k as well.

            So if the MXZ-3C24 has a minimum output of 7200 BTU/hr, that means with all three heads turned on they would each put out 2400, which is slightly above their individual minimums. I'm guessing you're referring to a situation where only one of the heads is in use?

            But, under the assumption that the 12k ducted is not sufficient, the minimum the 18k ducted could output would be 5150 (heating mode), which is ~1720 BTU/hr per room. Not really all that different from the 3x 6k right?

            It looks like to me that the ducted solution is better if the 12k unit is appropriate to the needs of the entire floor. If not, 3x 6k or 9k isn't going to be all that different because it can't modulate much lower than the multi-split anyway. Or am I missing something?

  7. Expert Member
    Akos | | #24

    Besides the limited modulation range on the multi splits, the other problem is the indoor units don't modulate (my LG ones definitely don't, some VRF might). The outdoor unit modulates to match the capacity of the indoor units calling for heat/cool but the heads only run either off or at rated capacity.

    If you are going to use it for cooling only it can be made to work, but for heating, it will create efficiency issues.

    1. tamahime | | #25

      Ahhh... *now* I understand what you've been saying. I had been assuming that the indoor units would modulate. Now I also realize why it's been so hard to find the capacity ranges for the indoor heads - the minimums are included when paired with a single zone compressor but not with a multi-zone.

      To make matters more confusing, though, both Mitsubishi and my contractor say that the indoor heads *will* modulate on a multi-split, but aren't able to say the exact range. I mean, it does make sense - if I have 3x 9k heads, I shouldn't expect that any time I have all three of them turned on it's delivering 27k of heating/cooling, right? Unless they are just outright lying in their marcom materials about the way the system functions. Why even have a minimum outdoor capacity if it always delivers the full capacity of the indoor heads that are calling? Technically, if that were the case, the MXZ-3C24 minimum would be 12,000, the rated capacity for 2x 6k indoor heads, right?

      I guess this is the key point the whole thing hinges on. I see in some older posts here and other forums where Dana seems to agree with you that the indoor heads will not modulate. But in this thread I've gotten the impression he agrees with Mitsubishi that they do? If the indoor heads won't modulate in a multi-split it's a slam dunk for the ducted single zone (or 3x single zone wall heads). But otherwise it's not as clear (to me) that there's much of a tradeoff...

      1. Expert Member
        Dana Dorsett | | #26

        Individual Mitsubishi heads WILL modulate on a MXZ compressor, but the range is limited, still constrained to by the minimum output of the compressor, and how many heads are calling at the same time. When the heads & system are sub-optimally oversized, most o the season only one head will be calling at any given point in time, delivering 100% of the compressor output to that one zone. Only when given enough load that the compressor can modulate will the cycle times be long enough for the zones to overlap.

        So while a 12K head won't be blasting the full 12K (or more) EVERY time it's running, it might only rarely be able to deliver less than the minimum output of the 3C24, which 15,500 BTU/hr at 47F, probably well above the whole house heat load @ +47F:!/product/29025

        If the three individual zone loads don't exceed the minimum output of the compressor the heads might still modulate down a bit (nowhere near the minimum when on a dedicated single zone system), but it'll be cycling the compressor on/off, which cuts into efficiency.

  8. Expert Member
    Akos | | #27

    My folks have a MXZ-2C20 with a 12k wall mount+12k ducted. I was over there and decided to put a current clamp on the outdoor unit (these have a 0.90+ power factor, so it gives reasonable power estimate).

    Not very scientific test but it the modulation range seems even better than the spec sheet calls for. With only ducted zone calling for cooling, the outdoor unit was consuming 340W, bellow the 530W min from the NEEP database. Ramping the setpoint down to min slowly brought the power consumption up to a steady state max of 890W.

    For some reason the wall mount had a higher min load. The min load before compressor shut down was 500W with a max of 810W. Assuming a COP of 3.5, that gives a range of 6000BTU to 9500BTU.

    So it does seem the Mitsubishi unit does modulate the indoor heads but it is still significantly less than most one to one setups.

    While playing around with it, I did notice one big weakness of hitting the min limit. To keep the compressor from cycling too much, it won't turn it back on till the temp increased by about 2F. Might not sound like a big swing but during this time you also get no dehumidification (plus re-evaporation from the coil) and the place can get uncomfortable quickly enough for one to reach for the remote and bump the setpoint down. This is what would be very annoying in a bedroom.

    1. tamahime | | #28

      Thanks for doing this and posting, that's interesting data!

      I looked it up and it seems the compressor your folks have is subject to the same minimum outputs that the 3C24 is - 6000 cooling, 7200 heating. So it seems like the wall heads might be able to modulate lower but you were bumping against the compressor min? Not sure why the ducted was able to go below that, then...

      I am assuming that, if Dana is correct and when the rooms are near the setpoint that often only one head would be calling at a time, I'd still run into the cycling problem you describe, plus wear on the system. It still bugs me that there is such a big jump to the 18k single zone compressor minimums. But I guess the advantage there is that the minimum output is spread across 3 rooms, meaning theoretically the unit should run longer and modulate more. Correct?

  9. tamahime | | #29

    Wanted to post a follow-up to let you all know what we ended up doing and why. Maybe it will help someone else in a similar situation.

    First I really tried to engage an HVAC design pro. I reached out to the ones commonly recommended around here - Allison ( did not respond to my request at all. Isaac ( was responsive at first and then suddenly said he couldn't take the project and referred me to Tabetha ( who also communicated at first and then disappeared. Very frustrating; I understand that their time is limited and that if faced with bigger/more profitable jobs they would choose those. But it kinda sucks in the "green" space to only be able to find people to work with if you have some giant McMansion!

    Anyway, before the handoff Isaac did offer a kernel of advice based on what I'd been able to share so far - he suggested I go with the 3x 1-to-1 mini-split option.

    Meanwhile, the installer came and replaced the old conventional condenser. Based on Dana's advice I convinced them to size it down - they were open to it, and considered going as low as 1.5 tons but ultimately went with a 2 ton. This has so far worked quite well for the first floor.

    I was a huge pain and had them quote a bunch of different options for the upper floor based on all the advice. To sum up:

    1. 3-head multi-split - original option, ~$10k
    2. Single ducted cassette - $11.5k (+ additional cost for finishing)
    3. 3x 1-to-1 splits - $13.9k
    4. 1x 1-to-1 (master BR), 1x ducted cassette (office/2nd br) - $14.5k

    As you can see, each thing I asked him to quote got more expensive than the last; not sure if it was just because he was tired of quoting them or if that was really the situation but either way it's what we had to choose from. Ultimately we narrowed it to option 2 or option 3. From there, knowing that for basically the same cost we could have room by room control, no messy construction inside for ducts and returns and all that, and no repairs/finishing to do, as well as Isaac's endorsement, the decision seemed obvious.

    Installation took a few days and was not without its headaches, as is typical for this house. One side of the house has 2 units bolted to it and line hide snaking up over the gutter to each room. Although neatly done it's not the slickest thing but honestly I don't spend much time looking at the side of the house so I don't really care about that. The third unit is mounted on a stand near the regular AC.

    The heads are big - in the small rooms they certainly stood out at first but now I'm used to them. Only one of them is even really "visible" during any normal usage of the room. 1x 6k wall head, 2x 9k floor heads. The outdoor units I'd say were also a little bigger than I expected them to be but again I don't care much what the side of my house facing the neighbor's driveway looks like.

    So far the functionality has been fabulous. I was seriously impressed by how quiet they are. I was standing next to the 2 outdoor units talking to the installer and he said I should go inside and check out how they were working. I said "oh, I can turn them on?" He laughed and said "they're on now!" Couldn't hear the compressor standing a foot away!

    I did a test where I left one room off (the master) and all the doors open and the other two units running. The hallway/shared space cooled off but the bedroom with the unit turned off was noticeably warmer than the others. Due to this I definitely think any solution that didn't have supply and return to all rooms would not have solved the problem.

    We're heading into the season where I won't need them as much, and there's still the winter to find out how well they do on the heating side. But so far I'm happy. Is it the ideal solution if all resources/time/money were on the table? Undoubtedly not. Did it cost too much? How much is comfort when sleeping and working worth?

    Thanks again to all who offered insight and advice.

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