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Mini-split options for cooling a small home?

KurtGranroth | Posted in Green Building Techniques on

Building a small home (700 sq ft) in Phoenix AZ (Zone 2) where cooling is paramount and heating is practically an after thought.  I’m aiming for R20+ walls and R40+ attic.  Few windows but big glass doors everywhere.  Patios and overhangs everywhere, too, so tons of shade.

House has one main area incl. kitchen and living room + one bedroom + a bathroom.

I did a quick ersatz Manual J on loadcalc.net and came up with a maximum cooling load of just under 12,000 Btu for the whole house (worst case scenarios with my assumptions).  Mini splits look to be the best fit for such a  small space and (relatively) small load.  But how?

Option 1: One 12,000 Btu (maybe one notch bigger) head unit in the main living space.
From a pure square footage and load perspective, this seems like it would work. But I’m concerned about the temperature in the two rooms with doors — the bedroom and bathroom.  I’ve seen references to the bedroom being 5 degrees off the main temp overnight… but those are invariably referring to heating modes.  I can find very little concrete info on how a single head would work in a closed-door bedroom when extensive cooling is at play.

Option 2: Dual zone; one unit in the main living space and one in the bedroom
Is this actually an option?  I’m not sure they even make a unit small enough to service one small (12×12) bedroom?  Adds a bit to the cost, if it is possible.

Option 3: Ducted mini-split with low static ducts
Ducted air handler in the attic with a “pair of pants” duct coming off splitting into the main area and bedroom (and maybe even bathroom).  Maybe the return is also split into the various rooms.  This would almost surely work to keep a super consistent temperature.  But… it’s not as efficient as a head unit; costs more; and would almost surely require having a conditioned attic since all installation examples suggest that the unit and ducts can’t be buried in insulation.  An R40+ conditioned attic would be notably more expensive than a vented attic with just a lot of blown in cellulose.

Am I missing anything obvious?  What are y’all thoughts on these options, with specific focus on cooling?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Dual zone mini-splits don't modulate with load the way single zone mini-splits do.

    When running a Manual-J you should be aggressive on assumptions, not "...worst case scenarios ..." For yuks run it again using every factor you can that could reasonably be expected to lower the load, the way the instructions in the Manual prescribe.

    S0, run it room-by-room. A room with a peak cooling load less than 3000 BTU/hr won't hit the efficiency numbers with a 3/4 tonner than can't modulate down to half that or less.

    A conditioned attic is expensive- hard to really rationalize just for ducts & air handlers, but a plenum truss for the attic for running the ducts can work just fine, much cheaper than a conditioned attic, but requires a bit of planning. With long-throw ceiling registers located near the interior of the house directed toward the exterior walls of the rooms can work well with very short duct runs to a central mini duct cassette.

    https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic

    https://www.finehomebuilding.com/membership/pdf/72033/021233086.pdf

    Fujitsu's mini-duct cassettes can be mounted vertically, so it's possible to make a 7-10 square foot "utility closet" or where it's easily accessed from floor level. This is a 1.5 ton version, bigger than what you'd need:

    https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg

    https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg

    The big grille at floor level is the common return- doored off rooms would need a jump-duct plan, which could be built in to a transom over the door, or a partition wall cavity, with a grille near floor level on one side, near ceiling level on the other side of the wall to deal with privacy & light leakage issues.

    1. KurtGranroth | | #3

      > When running a Manual-J you should be aggressive on assumptions, not "...worst case scenarios ..." For yuks run it again using every factor you can that could reasonably be expected to lower the load, the way the instructions in the Manual prescribe.

      Oh, interesting! Wouldn't that create a conservative load estimate that could result in an under-powered mini-split? I thought that the goal for mini-splits, in general, is to mildly (150% max) oversize them.

      And unfortunately, I can't run the load calculation from loadcalc.net on a room-by-room basis as it just does whole-house. I consider it just one tiny step better than a "rule of thumb" calculation.

      > .. but a plenum truss for the attic for running the ducts can work just fine, much cheaper than a conditioned attic

      Very cool. I had thought of something similar but in my mind was referring to it as a "conditioned box". Knowing that this is a real thing and it's called a "plenum truss" helps tons. In general, just knowing what words to use in a search is often critically important to finding any useful information!

  2. Aaron Beckworth | | #2

    Kurt,

    Have you considered mounting a single indoor mini split head in the bedroom? This may be an option if the bedroom is left open to the main living space during the day. Then your bedroom could be nice and cold at night!

    If the house is air tight and insulated well, how much could the open living area warm during the night? Should be easy enough to cool it back down again in the morning.

    Good luck,
    Aaron

    1. KurtGranroth | | #4

      I hadn't but that's a very interesting idea. Let's think this through a bit.

      I wonder how effective keeping an open door is at maintaining an event temperature. If the thermostat was in the bedroom, then would it mostly be working as if it was conditioning a much smaller space since that room would hit the temp faster than the main area? Or would the open door create enough air flow naturally that it would work as a larger area?

      Maybe then the tactic is to put a remote thermostat in the main area and then the head unit will have to work at a sufficient level to get that down... but would that result in a super cold bedroom? And it would have to turn off at night or else the closed door would mean that the head unit would never hit the desired temp.

      Hmm... the idea definitely has merit, but it does seem to raise just as many questions and has just as many unknowns as having a single head unit in the main area.

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