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Can I vastly undersize a condenser on a multi-head mini-split system

rszimm | Posted in Mechanicals on

So I’m building a guest house.  I’m doing a 4 zone mini-split system.  I’m in Tucson so the cooling calcs are more relevant than the heating calcs.  The zones as well as the manualJ cooling calcs (in parens) and the selected mini-split system unit as follows:
Zone 1: Walkout basement / shop area. (10k)   –  12k Wall Mount
Zone 2: BR1 (4k) – 7k Wall Mount
Zone 3: BR2 (4k) – 7k Wall Mount
Zone 4: Great Room/Bath – (13k) – 18k Ducted

So here’s my question.  When I add all the individual units together I get 44k.  However, when I add in the actual manualJ loads, I only get 31k.  Fujitsu has a 4 zone condenser that’s 36k and one thats 45k.  I’m thinking the 36k unit would save me cost, provide sufficient cooling/heating, and be more efficient than the 45k.  Thoughts?

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Replies

  1. Kyle R | | #1

    If you are building a place, can you run ducts?

  2. rszimm | | #2

    So the 18k ducted unit has ducts, but it supplies the main room and bathroom. It’s mounted in a drop ceiling over the bathroom. Everywhere else is a flat roof with ceiling mounted to the roof joists so no not really room for ducts without really giving up headroom.

  3. Kyle R | | #3

    If you go to this page (https://www.johnstonesupply.com/product-view?pID=B76-516) and download the technical manual you will see the capacity of different combinations on page 255.

    If you can find a way to fit ducts in your basement and have one ducted unit, I think you will have overall greater comfort and save some money.

    1. Jon R | | #4

      From the page itself: "Max. Connectable Capacity: 39,000" which is less than your proposed 44K.

      1. Kyle R | | #5

        Right, might be able to get by with a 7-7-9-15

        1. rszimm | | #7

          Fujitsu doesn't make a ducted 15. I would have to drop down to the 12, which by my manualJ will be a bit undersized.

          So it looks like the 45k condenser will have to be. That's unfortunate when the manualJ states my real load will only be 31k.

          1. Kyle R | | #8

            Are the two bedroom next to each other? The ducted units can install in the vertical position so one can take up relatively small amount of closet space if there is a shared wall or you can run a soffit above the bedroom doors to hide the duct.

        2. rszimm | | #10

          Kyle, the bedrooms are together. The closets are not. The master closet already has the HRV installed in it. It's possible I can maybe drop some soffits around and get this done with a ducted unit, but that wouldn't be my preference unless strictly necessary. The two bedrooms also are on different sides of the house so one gets lots of morning Arizona sun, and the other gets lots of evening sun, so having the separate zones was a nice bonus. Plus, it's unlikely that we'll have guests in both that often, so being able to zone one down to zero when not occupied was also a bonus. Finally, we have some family members that like their bedroom temp vastly different (like artic tundra / Serengeti different). Again...a plus for the zoned system. I will re-look at your suggestion though...

    2. rszimm | | #6

      slab on grade unfortunately under where the guest house living space is. The walkout basement is actually only toward the front of the house (I'm on a slope here). Thanks for the link.

  4. rszimm | | #9

    So it seems the Fujitsu has a max connectable capacity of 39k, so I'll have to go with the bigger condenser to make the Fujitsu work. Mitsubishi's 36k condenser has a 42k max capacity and their smallest wall units are 6k, so that gets us to a 6-6-12-18 = 42.

    So, given this and my manualJ of 32k required, would you go with a 6-6-12-18 Mitsubishi with a 36k condenser, or a Fujitsu 7-7-12-18 with a 45k consenser?

    1. Kyle R | | #15

      I would look at the minimum output of each. It looks like the 3 ton Fujitsu and Mitsubishi can turn down to about 12k btu/hr cooling, while the 4 ton Fujitsu can only turn down to ~20k btu/hr. Double check with actual model numbers you are considering. In this case I would go with the Mitsubishi.

  5. Jon R | | #11

    Note that the match between design load and max capacity (say < 50% over-sized) is usually not the most important factor when it comes to efficiency or comfort. It's far more complex than that.

    1. rszimm | | #12

      Thanks Jon. Maybe you can steer me toward a better resource. It's been my neophite view that I'll maximize efficiency by sizing the system so it's just barely satisfying the load during some of the hottest days of the year. Comfort goes along with that so you're not thrashing between too hot and too cold in a too large system.

      Here in AZ, the "professionals" tend to just throw a much too large system in. No one really cares much for building science so houses are built with inefficient fenestrations, very little envelope sealing, code minimums on insulation, etc. We did things differently with ICF construction and trying to follow the best practices for efficiency. It's funny, even the big HVAC contractors down here had not really had any experience with HRVs. For "tight" homes they just wire a bath fan on all the time and pull outside air directly into the return system.

      Long story shorter, I gave my plans to a local HVAC contractor and they sized in a 48k condenser, a 24k ducted, 9k units in each bedroom and a 12k in the basement/shop. So a MUCH larger system. I'm just trying to scale that back to perhaps improve on efficiency and comfort.

      1. Jon R | | #14

        SEER, COP vs load, turn down ratio, thermostats, CFM/ton vs load, need for dehumidification, zone dampers, etc all have an effect.

        Unfortunately, I've never seen any modeling software for such things. But I'd take a 50% over-sized Daikin Quaternity mini-split over a perfectly sized multi-split.

  6. James Howison | | #13

    You are basically giving up all the advantages of VRF by having these heads oversized for the rooms and having the outside compressor so oversized. Do some searching on temperature overshooting with mini-splits. It is often written about as a cold climate problem but we had similar issues in our hot climate. See discussion here: https://www.greenbuildingadvisor.com/question/ductless-mini-splits-in-my-bedrooms-a-terrible-idea

    The biggest issue is that the outside compressor can't modulate down much because the minimum rate is so close to your actual max demand. So when a unit calls for cooling (or heat) the other units get unwanted refridgerant and the always on blowers overcool/heat. You can think of it like a central ducted system but you have very limited control over the size of the ducts and no dampers to rebalance the system.

    If there is any way to do these ducted you'll have a much better time, but at least consider doing these as 1-1 systems. I would work hard to match the compressor max load to your design loads, and if 1-1 is not possible then consider all combinations (ie maybe 2 1-1 and a single multi-head compressor serving two very similar zones).

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