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Ducted is likely better, but ductless is more affordable: which to do?

torreyho | Posted in Mechanicals on

Hi all
We are in the process of re-configuring our house, turning the 2nd floor into 4 bedrooms from the current 3. In Colorado(5b) it has started to get much hotter in the summer so I would like to take the opportunity to add conditioned air while the spaces are open. Heating is currently supplied by a gas fired boiler with hydronic baseboard and does a good job but the summers have gotten too extreme for comfort. 
I calculated the BTU heat load for the past winter(which was pretty heinous) at ~16K BTU/h load, which is pretty low. The HVAC contractors are specing around 48K BTU systems, despite me telling them the actual heat load we have had. The have agreed to “allow” a 24K BTU system, without a “guarantee”. 

My issue is comfort. I would like minisplits to be installed in the second floor(and the ground floor too), to allow some cooling in the summer and some heat in the shoulder seasons. The quote for a ducted system for upstairs, which I believe would deliver a more comfortable experience, comes in at around $13K while a ductless system comes in at $9K. These bids include 1 24K compressor and a 12K BTU ductless head downstairs and then another 12K BTU upstairs. I marked on the plans where the ductless is supposed to go. We also plan on sealing the house with Aerobarrier and adding an ERV.

So my question is…given the western heat(master bedroom is on the west side and gets plenty hot come hot come August 8PM), is there really no other option then to spend the extra money now and go ducted? Or will that one wall unit throw enough cold air to the west to cool off that bedroom? Will the expensive/time of an independent HVAC contractor be worth it?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Chris.

    We have some GBA members with extensive knowledge of and experience with ducted and ductless minisplits. I'm sure you'll get some helpful advice soon. In the meantime, did you read this article? A Pretty Good House in the Sierra Nevada

    In it, the author and architect describes his recent experience with a ductless system on a second floor. To make a long story short, when bedroom doors are closed in the summer, some bedrooms overheat a bit. Of course, this is a very tight and efficient house. Still, while he reports that the ductless system does an excellent job on the open plan first floor, he'd consider a ducted system for the second floor if he were to do it again.

  2. gusfhb | | #2

    I so wish the ducted units had been ready for prime time 10 years ago when I did my current house.

    I realize budgets are real, but my reaction to reading your post was, 'that's not that bad'
    ducted is a whole bunch more complex than ductless

    Go ducted, you will not regret the better room to room comfort I am sure.

    Get more detailed about the install, make sure they can be balanced, that you can get the air to that western room, and that sound transmission is thought about

  3. Expert Member
    Dana Dorsett | | #3

    Take a step back, and run a room-by-room Manual-J for both the upstairs and downstairs. If using a not-great-but OK freebie online tools such as LoadCalc or CoolCal be extra-aggressive on air tightness & R values and it'll be close enough for equipment sizing.

    The bedrooms 1-3 are all too small to warrant a ductess head. Bedroom #2 with just the one window could probably coast along comfortably most of the time with no heating at all, but bedrooms 1 & 3 being corner rooms with more exterior wall area plus 2-3x the window area of #2 will need something.

    Don't forget to derate the equipment capacity at your 99% outside design temperature appropriately for altitude. If your design temp is +5F and the 2-tonner such as the Fujitsu AOU/ARU24RGLX is good for 22,220 BTU/hr @ +5F at sea level, at 7000' it's only good for (0.77 x 22,220=) 17,109 BTU/hr, which isn't much margin on a 16K load.!/product/25350

    For a single zone mini-split with a 5:1 or greater turn down ratio an oversize factor up to 1.5x at the 99% outside design temp usually won't negatively impact efficiency.

    Most multi-zone units run about a 3:1 turn-down at +47F (some only squeak a 2:1). For multi-split units try to hold the line at a 1.2x oversize factor @ the 99% design temp, or consider using some amount of auxiliary resistance heating for managing cold-snap coolth. Most vendors' full sized air hanlder units have built-in aux-heat options. Fujitsu's mini-duct units have a control signal output that can be used for auxiliary heat, but I haven't looked into those options extensively (with Fujitsu, or any other mini-duct vendor.)

  4. torreyho | | #4


    I used LoadCalc and despite being very aggressive with my current/future insulation, it came up with a load of 50K BTU for heating. Which seems very different from my calculated 16K usage (using the method that you wrote about
    Which brings me to the bigger issue. While we will be installing multi-splits, we still have the gas fired boiler with hydronic baseboards. These do a great job keeping the house comfortable and while I would like to go all electric eventually, we have a relatively new gas fired HWH that I don't feel like replacing yet. And with gas so cheap, it is hard to not use the hydronics. The boiler will stay until it's bitter end
    Really, the multsplits are for cooling and some shoulder season heating. Maybe they would do well enough to heat year round, but that isn't the reason to install them. We had an awful fire season last year and having to seal up the house on summer nights was brutal. Hopefully the fires go away(don't expect that to happen) but with the lid open it seems best to get us comfortable now.
    The HVAC contractor doesn't like the idea of an undersized system, but it the goal is to take a 80F bedroom to 74F, in August, then I think we have succeeded. At least at this time.

    1. Expert Member
      Dana Dorsett | | #7

      Some houses get a lot of solar gain error when using fuel-use based methods. Loadcalc isn't a really great tool, but it's free, and usually at least hits the right ballpark.

      Have you totaled up how much baseboard you have? If it's keeping the place warm, the water temperature settings on the boiler and the total baseboard length would establish a firm upper bound on the load. (Mind you, I see lots of houses with boilers set to 180F with 2x the amount of baseboard needed to actually heat the place with 180F water.)

      A more realistic Manual-J calculation is in order. The free/cheap CoolCalc tool is at least (unlike LoadCalc) ACCA-approved. But like anything else garbage-in = garbage-out. I've never used it, but have seen both releastic, and bizarre freakishly off-base results from that tool used in others' hands, particularly on non-standard construction. (Can't really tell if the oddball cases were user competence issues or problems with the tool.)

      >"...the goal is to take a 80F bedroom to 74F, in August..."

      If it's primarily the master bedroom that has the comfort issues, a single 3/4 ton Mitsubishi FH09 or half-ton FH06 would probably fill the bill for both cooling and heating. I'm something of an an anti-fan of multi-splits due to the design and installation limitations they impose. Single right sized ducted systems are usually a better solution when individual room loads are low.

      What does LoadCalc come up with for heating and cooling loads on just the MBR?

  5. johns3km | | #5

    I have a much more open second floor landing/staircase with 1 head serving 2 bedrooms, a bath, and handles most of the cooling for the first floor as well. It's a pretty optimal layout and the west facing bedroom will be 3-4 degrees warmer during heatwave stretches/afternoon sun. We keep the door 1/2 open and a ceiling fan running with blackout blinds. We also have a 6k in the master, and would have been disappointed without it as the air just can't find it's way over there.

    With your split staircase, narrow hallway, and lots of rooms, I think you'll be disappointed with the one head aimed towards the master. It'll overcool that wallway space and turn off.

    It's also relative- I want the house 71 and can't tolerate more than 1-2 degrees variance. Some people are fine with 76 on the hottest days. Knocking down the humidity is good for feeling a couple degrees cooler as well.


  6. Jon_R | | #6

    > the west side and gets plenty hot come hot come August 8PM

    Systems are typically designed as if the ratio of loads in different rooms is fixed. But the reality is that it varies a lot over short (and long) periods. To the point where one will probably suffer more discomfort with ducted than if they put a 3x over-sized (rated capacity:design load) dedicated mini-split in the room. Only expect to match the superior dedicated mini-split comfort if you have a thermostatically zoned ducted system (unlikely).

    > The boiler will stay until it's bitter end

    Colorado currently uses enough coal that it's better for the environment to use nat gas when it's cold out (ie, heat pump COP is low). You can size your heat pumps for cooling only. Open interior doors work better with the low loads of AC (vs higher loads of heating).

  7. torreyho | | #8

    I finally had some time to use CoolCalc and got about the same numbers, 50K BTU for heating. Which is what the HVAC contractor got too, so makes me think they were using these programs.

    I'll have to see how this plays out, using a ducted mini split mostly for cooling. And maybe some shoulder season heating.

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