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mini-split equipment selection

reimhagen | Posted in Mechanicals on

I got a load calculation professionally done for my house.  I have a question about equipment selection.  If my calculation says, for example, that I need 1,978 heating BTUs and 1,070 cooling BTUs for one of my zones (i.e. one mini-split unit), should I be looking into the technical specs of whichever equipment I’m looking at to see the actual BTUs generated at the design temperature of my region?  I’m in the Seattle area, so I would want to check how many BTUs the unit can produce at 28 F, right?  I’m guessing that simply going by the advertised BTU on the product overview would not suffice since that is usually at a higher temperature.

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Replies

  1. Jon R | | #1

    Yes, output at design conditions. Although if your design temp is 28F and you buy equipment that shuts off at 27F, you aren't going to be happy.

  2. Expert Member
    Dana Dorsett | | #2

    The advertised "nominal" or "rated" heating and cooling is the modulation rate at which the thing was tested for efficiency, not necessarily it's max capacity at either the +17F & 47F test temperatures. If you pull the "Submittal" sheet it will spec the AHRI tested output.

    Your main problem will be not having enough LOAD to run super-efficiently. Many mini-splits won't go that low.

    A good resource would be to search NEEP's product listings, which publish the efficiency at both min & max capacity at +5F, +17F, and +47F in heating mode, and at both 82F and 95F in cooling mode:

    https://ashp.neep.org/#!/product_list/

    eg, this 3/4 ton LG can throttle down to less than 1000 BTU/hr @ +17F (not all will go that low) albeit at a very LOUSY COP 0.88 (= less efficient than a space heater when running that sl0wly!), at that minimum:

    https://ashp.neep.org/#!/product/31912

    Whereas it's cousin does QUITE well at a COP of nearly 3, even at very low modulation levels:

    https://ashp.neep.org/#!/product/25817

    So the one unit is likely to use 3x as much power as the other in your tiny load application.

    You can also look at multi-split systems serving multiple zones off a single compressor, but those compressors have even higher minimum-modulation levels and would run at truly abbysmal efficiency serving just one tiny load zone.

    1. reimhagen | | #3

      Hi Dana,

      Thanks for the information. I also felt that my load seems to be very low. Do you think the load calculation itself is suspect then? The numbers I wrote was the combined for one of my designed "zones" that I meant to serve with a ducted unit -- a bathroom, a laundry room, and a storage room (which itself has a heat pump water heater, so that will bring up the heat requirement a bit). The total square footage of the basement area served by this unit would be approx. 420 sf.

      I am refinishing the bathroom using the technique outlined in the "stay dry no mold finished basement" article, with 1.5" of XPS on the floor and 2" of XPS on the walls. I am considering also adding in mineral wool into the stud bays as well, but with the load calculation coming up as low as it is, I don't know if it's worth it.

      The person that performed the load calcs also designed a duct system for the three rooms too.

      1. BFW577 | | #5

        That LG has some impressive cold performance with a COP of 2.88 at 5 degrees. I recently read something about thermodynamics that it is theoreticaly possible to achieve much higher coefficient of performances than what we currently have. Like a COP of 10 is possible at extremely low temperatures. The technology will certainly advance in the next few years as their is plenty of room for improvement.

  3. reimhagen | | #4

    Bumping this thread up to the top as I still am a bit confused.

    Based on Dana's response, I'm questing the accuracy of my load calculation. Could someone take a quick look at the plan I received (this is not the full report, but a portion of it) that shows the zoning plan as well as the calculated loads for each zone, and tell me if it sounds reasonable?

    The entire basement area is about 1,500s.f., and the calculation was done assuming that I am putting R-10 continuous on the walls and R-7.5 on the floor. (As mentioned earlier, I'm following the method outlined in the "stay dry no-mold finished basement" article to re-do my basement).

    If the numbers are accurate, it seems that the available equipment is all too big to serve my basement! I somehow find that a bit hard to believe.

    Thanks in advance.

  4. Expert Member
    Akos | | #6

    It looks you have a bad case of micro-zoning. Be careful is this can be contagious and carry onto the rest of the floors, you might end up with a commercial VRF system with BACnet controls.

    You have a basement with 8000BTU heating load and you are installing 30000BTU of equipment. There is no practical way of making that work efficiently, never mind the cost of all that hardware. Also keep in mind that each wall mount is a maintenance item, need regular filter cleaning and blower wheel cleaning every couple of years.

    Zoning is a great thing. Usually you want to zone when areas have very different cooling and heating loads throughout the day. For example, if you have a large living room with excessive west facing windows, it would have much higher cooling loads than the rest of the house, zoning is the only way to keep it from overheating in the afternoons.

    A basement generally has pretty even HVAC loads, zoning inside there makes very little sense. You can run your entire basement off a 9k slim ducted unit. In your climate you can probably get away with a non-hyperheat unit as well.

    If you must have zoning for an area, you can do a simple setup by adding zone damper to duct feeding the bedroom. What you do is set up the flow to be about 120% of what the area needs, then set the zone valves so that in closed position it supplies about 70% of the flow.

    A separate thermostat in this area now drives this zone damper. You don't have to tie it into the mini split or need any special controls, all units will easily modulate around the small change of air flow.

    1. reimhagen | | #7

      Hi Akos,

      Thanks for the reply. It's interesting that my zones are too small; in Japan i often see wall units serving a single bedroom & and their houses overall are much smaller there.

      Anyway, back to my house; the zoning that we came up with was driven primarily by a desire to avoid soffits for ductwork. The ducts shown run parallel with the joists so they're able to be tucked into the joist cavities. I would still like to avoid having to put in soffits if possible.

      Given that my loads are so low, would it make more sense then to use electric baseboard heating in my basement? I don't think I'll ever need cooling down there and the heating loads are so low. Perhaps the family room can get a mini split unit, but the other rooms can probably simply use baseboard heating.

      I'll still use minisplits on my main level as the loads are higher there...

      Thanks

      1. Expert Member
        Akos | | #8

        There are a lot of threads about folks sizing multi splits for houses, read through some of those first. Unless you go full VRF, it is hard to get the head per room setup to work properly, creates a lot of comfort and efficiency issues. I really hope this isn't your plan for the main floor. The setup that works reasonably well is a single wall/floor/ceiling mount unit for an open concept living space with the rest of the bedrooms and bathroom connected to a single ducted unit.

        As for the basement, going all resistance or mostly resistance heat plus a single wall mount is a much better option.

        Bulkheads and ducting in basements is the norm around me.

        If you do a good job of the layout, there will be only a couple of bulkheads along one side, the rest of the runs are in the floor joist bay. This lets you place the air handler in the basement and run ducting from there to each room upstairs. Depending on the layout of the place, you can have the whole house on one or two ducted units. Much less cost and generally way more efficient than a head in room setup plus you can now have a proper intake filter. It does require a bit of design work upfront.

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