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Community and Q&A

DIY minisplit advice

azad_lassiter | Posted in Mechanicals on

I Currently Live in a 2700 sf Log cabin in Madison WI. (Zone 6 if I recall)
We have a basement, 1st floor and 2nd story each about 900 sf. Our current heat system is Electric Baseboard in various rooms with a cheap wood stove for supplemental heat.

I have gone through a bunch of options as what I should do to get more energy efficient and more convenient for this house. (we have no A/C either)

I want to install a Ductless Mini Split Heat Pump DIY(DIY units I am looking at are precharged. so I wouldn’t need an HVAC installer to charge it) (You guys have been talking about it enough and sold me on them!) and get it done for under $2,000 (I will be hiring an electrician to run the supply) My plan is to have one unit to start on the first floor (2nd floor is adequately covered by baseboard and window units for now) to supply Heat and A/C

I am looking at these two units and was wondering if anyone had any advice as to which is better or if I can get another better system elsewhere (remember the budget) I am also using home depot because I can get really great discounts b/c the company I work for is a managed account. I plan to get up to 20% off the sticker price here after talking to my account rep.

I was also wondering about these units as far as heating in the winter? will I need to keep my baseboard heaters for when it gets really cold?

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  1. Expert Member
    Akos | | #1

    Only the MRCOOL DIY is the one you can install without special equipment. The other one is pre-charged, but the lineset and head unit is not, you would need and hvac tech to vacuum the line set.

    Neither unit is hyperheat and their specs are pretty slim, most likely you would loose 1/2 the rated capacity bellow -20C.

    24000 btu head unit moves a lot of air, I would be careful with location as it would be uncomfortable to sit near it.

  2. Expert Member
    Dana Dorsett | | #2

    The 2-ton DuctlessAire unit is good for ~14,000BTU/hr @ -5F and 68F indoors, interpolating from the extended temperature capacity charts:

    Madison's 99% outside design temp is -6F, so if 14K covers the heat load for that floor, it'll be enough, but it also might be too much. If the basement walls are insulated I'd expect the 900' of first floor to have a heat load of about that much, but it could be higher or lower, depending on the thickness of the logs, the air tightness, the U-factors of the windows, etc.

    Despite claiming 25K nominal heating output (which normally means it can deliver ate least that much at +17F under AHRI conditions, 70F indoors) according the capacity tables heating output at +17F is a fairly wimpy (for a 2-tonner) ~18.5K , which strongly indicates that it's not a cold-climate heat pump compressor design, even if they tested it down to -22F.

    The MrCool 2-tonner is rated at a nominal 25K (and it's AHRI certified, so it probably really does deliver that at +17F) but there aren't any extended temperature capacity charts for the MrCool.

    In an zone 6 location it would be important for any mini-split to have a pan heater for clearing defrost ice from the outdoor unit, but I don't see indications of either of those units being equipped with a pan heater. When ice builds up in the bottom of the pan thick enough to interfere with the fan it's pretty much done-for.

    Home Depot carries some Gree products, but I"m not sure they carry all product lines or whether they can special order them. The 1.5 ton Gree Crown 18HP230V1A puts out over 17KBTU/hr at -4F, which means it's a true cold-climate mini-split too, and a better choice that the 2 ton HD picks (assuming they carry the Crown series.)

    A couple of years ago via web-forum I coached a guy in Wisconsin through a mostly-DIY installation of a Fujitsu 9RLS3H for his newly winterized ~600' former summer cabin. All but the final refrigerant charge adjustment and commissioning testing was done by the homeowner. IIRC the refrigerant tech billed something like $400-500 for the service call. That 3/4 Fujitsu is good for 13,500 BTU/hr @ -5F (almost as much as the 2-ton DuctlessAire) and is fully designed & equipped for cold climate operation, tests at HSPF 14.0, and is all around the better choice. It's priced around the same as the 2 ton MrCool at web-stores, but it's a much more appropriate pick.

    1. charles3 | | #3

      Somewhere I read that the high SEER ratings of these ductless mini splits takes into account that there will be no duct losses, so isn't just a reflection of the efficiency of the equipment itself. True or False? Another question is whether the higher the SEER, the more complex the equipment and therefore the less reliable it is?

  3. Expert Member
    Dana Dorsett | | #4

    SEER is only about the equipment performance, not duct performance.

    A high SEER doesn't necessarily require more complex equipment or necessarily lead to lower reliability. A high SEER is usually correlated to a higher sensible heat ratio (less dehumidification) but that's not a hard rule.

    1. charles3 | | #5

      Well, that's a bit confusing. The Fujitsu 9RLS3H you mentioned has a very high SEER but also has a dry mode. Are you saying that when it runs in dry mode the SEER drops a lot?

      1. Expert Member
        Dana Dorsett | | #6

        Yup- the as-used SEER isn't the same in DRY mode as it is in normal cooling mode, which is just one of several reasons why that mode is only used when needed.

        To improve latent cooling the evaporator coil has to operate at a lower temperature, which means there is a bigger temperature difference to maintain between the indoor and outdoor coil temperatures. That bigger temperature difference takes marginally more compressor power than cruising along at a lower difference that's still low enough to maintain the indoor air temperature setpoint, so it's marginally lower efficiency.

        For an AHRI rated SEER the unit tested under a specified parameters of indoor and outdoor humidity & temperatures, but that doesn't mean YOUR indoor and outdoor temperatures & humidity conditions will always fall within test conditions

        If the humidity rises indoors to where it doesn't keep up at the sensible heat ratio it has in normal cooling mode, it's not an energy disaster to take the efficiency hit and use DRY mode. The sensible heat ratio of most mini-splits is never zero- it's always delivering some sensible cooling when in DRY mode, but it's removing a lot more moisture per degree of sensible temperature drop than in normal cooling mode. That sensible heat of vaporization is still real heat that needs to be pumped out of the house during the cooling season, and it's still a pretty efficient way to dehumidify while cooling.

        Unlike most mini-splits Daikin Quaternity series does a bit of super-chill with reheat using clever coil design & additional valving to continuously adjust the sensible heat ratio and is alleged to be capable of simultaneously operating to both a humidity and temperature setpoint, and can dehumidify without sensible cooling.

        1. charles3 | | #7

          Does anyone rank the various minisplit brands for reliability? Of course the quality of the installation is more important than the equipment. What equipment would you recommend for installation in the humid Deep South? Conversely, what equipment would you avoid installing in the Deep South? Is it bad, for example, to install the Fujitsu 9RLS3H you mentioned because it's meant for colder climates?

  4. Expert Member
    Dana Dorsett | | #8

    Cold climate mini-splits work great in warmer climates, but there are some features you don't really need. There are no problems with installing the 9RLS3H south of the Mason-Dixon, but I'd save a coupla Frankins and go with the 9RLS3 (no -H)

    The Fujitsu 9RLS3 is identical in almost every respect to the "-H" version except that it doesn't come fitted with a pan heater in the outdoor unit for clearing defrost ice build-up. Pan heaters are only necessary in areas where daily temps don't go above freezing for weeks on end.

    The -H...

    v.s. not-H...

    Cold climate mini-splits typically have more modulation range and higher efficiency due to the vapor-injection compressors used, and they are definitely worth considering for use in the southern US. On another forum I convinced a guy in Raleigh North Carolina to go with a Mitsubishi FH09NA cold-climate minisplit to cool & heat his small sunroom addition after running some online load calculations. (There were contractors proposing over sized 1-tonners , even a ridiculously oversized 1.5 ton mini-split for that space.) The higher efficiency and lower minimum modulation seemed to convince him that the additional ~$450 for the cold climate version was "worth it".

    When his heating system crapped out the following winter during a cold snap the high heating output capacity at single-digit outdoor temps was a real benefit!

    The particulars on that live here:

  5. natesc | | #9

    I just finished installing a 1.5 ton mitsubishi hyper heat on my house. It came out very well, it passed an overnight pressure test @550PSI and held vacuum at 100 microns (!) for 2 hours.

    LG also does a cold weather mini split.

    If you want to DIY you will need to spend in the neighborhood of $1000 on tools, you will also void the warranty. I still came out ahead a several thousand dollars.

    R 410A refrigerant operates at a much higher pressure than the old systems, and for that reason there is a lot of misinformation on the internet. Everything does need to be very precise - that includes a torque spec for everything with threads, special R410A flares, the ability to perform a high pressure nitrogen test, and triple evacuate/nitrogen purge down to a deep vacuum. You have to do your homework but it is completely doable.

    Also you are really supposed to be "Section 608" certified per the EPA.

  6. charles3 | | #10

    Suppose you were selecting equipment today for a house that will eventually undergo a deep energy retrofit that will approximately halve its current heating and cooling loads. Would that eventuality influence your choice of equipment? If so, how?

    1. Expert Member
      Dana Dorsett | | #11

      If the deep energy retrofit will be completed in under 5 years, size the equipment for the "after upgrades" version of the house. Size increments come in pretty big chunks- the equipment that covers the "after" loads will usually cover well over half the current loads. Run the Manual-J aggressively on both the "before" and "after" picture to gain some confidence.

      If the DER won't even get started for another 5 years and could be a "maybe never" scenario, size it so that it covers the load of the current version of house in an aggressive Manual-J using an extremely low air infiltration presumption. Air sealing is pretty cheap and effective, and in most cases is extremely cost effective even in the short term.

      With modulating heat pumps like minisplits, look for minimum modulated output numbers, not just the max capacity. This can make a huge difference in as-used efficiency in the "after upgrades" picture, even if 2x oversized for the eventual loads.

      1. charles3 | | #12

        What happens to the COP. or SEER or HSPF, at minimum modulated output?

  7. paultschida | | #13

    Yes , will need backup heat. My Mr Cool 2 ton shuts off at about 15 below.

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