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Thread: Position ductless mini split head unit

  1. #1

    Position ductless mini split head unit

    I live in the desert so am mostly worried about cooling. I plan on using a ductless mini split for the HVAC and would appreciate advise regarding placement of the head units. The shop, under construction, has one room of 40'x25' and then a second garage section 25'x25' with a wall separating them.

    In my minds eye I like the idea of the head units being more like a directed wind blast as opposed to keeping the entire space at a specific temperature. I have no experience with ductless though. So I could place the units in the area where I will spend most of the time or try and put them so they would more centrally located. Thoughts, thanks ahead of time?

  2. #2
    In the desert, assuming decent construction I'd go with a 36kbtu condensing unit and two 18kbtu evaporators. I would place them kitty-corner to each other on opposite long walls about 12' away from the corners. That will deliver the best circulation. In cooling mode the units blow the air across the ceiling anyway so you should not be directly in the air stream. The 18's have 4 way programmable louvers to assist in directing. I would put a separate 15 or 18 in the 25x25.

  3. #3
    Join Date
    Mar 2003
    SE PA - Central Bucks County
    My experience with my unit is that it works pretty darn well for air circulation, particularly with the moving shutters that can oscillate. I agree with Jack that by placing things opposing, you can set up a very nice air flow pattern that will keep things even throughout that larger space. You'll likely want to mount the indoor units up reasonably high since your primary need is for cooling and cool air sinks. Be sure that your building interior height is taken into consideration when it comes to sizing of your system(s), too. That air volume needs to be accounted for!

    The most expensive tool is the one you buy "cheaply" and often...

  4. #4
    Join Date
    Mar 2016
    Quote Originally Posted by Jack Frederick View Post
    In the desert, assuming decent construction I'd go with a 36kbtu condensing unit and two 18kbtu evaporators. I would place them kitty-corner to each other on opposite long walls about 12' away from the corners. That will deliver the best circulation. In cooling mode the units blow the air across the ceiling anyway so you should not be directly in the air stream. The 18's have 4 way programmable louvers to assist in directing. I would put a separate 15 or 18 in the 25x25.
    Based on experience with my 18k in 20x22 space I completely agree with this recommendation.

  5. #5
    10' ceiling in the workshop, 12' in the garage section. The 3 header layout was what I was looking at, but was originally thinking smaller than the 36kbtu - will definitely go larger thanks to your feedback. The construction quality is equivalent to residential because for resale value I want to be able to convert the workshop space to a 2 bedroom/2 bath/living room structure - 2x6 walls, well insulated. So I am framing, plumbing etc to make the conversion as easy as possible.

    I see the logic of having the units kitty-corner, the logistics of that might get interesting. Thanks again.

  6. #6
    Join Date
    Mar 2016
    The HVAC guy will calculate your needs as less most likely but I don’t think you’ll regret going oversized. They said I only needed 9k btu for my space. I let them install it under agreement they would replace at their cost and I only pay equipment difference if it couldn’t keep up. I’m in a Florida and have a double garage door and the 9k was just not up to the task. Even with full insulation with foam in walls, ceiling blown insulation matching the house and foil radiant barrier in the rafters. Big door insulated too.

    I dont find that that the mini splits dehumidify at all either unless they are run in that mode which basically just runs them at full chill. Mine puts the room down to like 55 degrees if I put it on that mode. I just run a small dehumidifier full time with a drain line and keep the shop around 74 degrees full time. Works great.

    You may may not have the same humidity concerns but for sheer heat the extra umph of the twin 18k will keep it under control easily. That said twin 18k will be overkill if you subdivide the place up later. May be better to think about smaller units. Not sure if they have one with 3 or 4 9,000 btu inside units.
    Last edited by Greg Parrish; 08-11-2018 at 4:08 PM.

  7. #7
    Join Date
    Sep 2016
    Modesto, CA, USA
    Jack, you do not run swamp coolers and ac at the same time.
    Depending you may want to have heat pump effect for winter from the mini splits. That feature will only add a few hundred to the units cost. It will take a lot of heating years to make up the cost of installing a separate NG heater even if NG is cheapest heat source.
    Bill D.
    Last edited by Bill Dufour; 08-11-2018 at 7:19 PM.

  8. #8
    Join Date
    Sep 2012
    Tampa Bay area
    An A/C system over sized for a given space is a major cause of the system not dehumidifying the space properly. The over sized system brings the temperature down to the thermostat setting too quickly for the for the evaporator to properly dry the air. Less of a concern in the desert than it is in a humid climate.

  9. #9
    Join Date
    Oct 2005
    Helensburgh, Australia
    If you suffer circulation problems consider fitting an overhead fan.

    Everything I like is either illegal, immoral or fattening

  10. #10
    I agree that for 1000 sq ft that a 3 ton unit is oversized. I almost always say undersize a mini-split as they seem to exceed peoples expectations. It is a large open space and keep in mind that with 2 18's you may run only one most of the time. Joe says "desert" and "residential spec for value", so, Joe, you have decisions to make on the lay-out. The same 36 can handle 4 nines or you could put 2-12's in the big space and a single 12 in the other. I don't have a lot of desert climate control experience. Another thought, in my home when I pulled all the ducting and central system and took it to the dump I put in a dual unit for the two upstairs bedrooms. That was a 16 seer 24k btu unit. I have in 7 years never run the heat upstairs and use it only for Fan or cooling in the summers. Downstairs I run a single 25 seer 12. When we re-modelled I added a 15 21 seer unit in the kitchen area. It is an open floor plan now so we rarely use both units. Point is multi's are less efficient than single units. Don't be constrained by having to have only one unit. Be aware that the big jump price wise is from 24 to 36. Having a 16 seer 24 and a 25 seer 12 came out to almost the same price as a 36 with 3 evaps. Look at your space and look at how you live in it before making decisions. Simple, huh?
    Jim, don't mount a wall unit to high. The actual sensing element for the built-in thermostat is on the evap coil. Putting the unit to high on the wall in cooling season throws off your set-point. Yes you can buy optional stats, but I think you are just spending more money. I wouldn't mount a high wall unit over 7'6" to 8'. Cold air is heavier than warm. If I have a 12' ceiling I don't care how hot it is from 8-12'. In the heating mode the same actually still applies, well, other than the physics. The high wall evap in heat mode will blow the air down at the floor. It will then do its thing in the space.
    Bill, not following you on the evap cooler and NG heater. I do not have a lot of evap cooler experience. I have a vast amount of experience with Ng equipment.
    For anyone interested in mini-splits I would suggest that you spend some time at I suggest this as there is an enormous amount of information on the Fujitsu units. I am most familiar with Fujitsu as I represented them in the New England States for 14 years. I no longer have any affiliation with them, nor do I profit from anything related to Fujitsu. Look at the min/max capacities by model. Do you need deep heating? Check out the amperage draw and the line set lengths. Honestly the brochure is so detailed I kinda meet myself coming around the corner, but it is a very good reference for comparison shopping. I will say I've run Fujistu's for 15 yrs in my homes and my girls homes and have not had a problem.
    Mini-splits do need to be cleaned. It is the old, "If you do not schedule maintenance on your equipment, it will schedule it for you." There was a recent article in Fine Woodworking where Christian V. said he had a unit in his shop for 3 years with no cleaning but the easy to access filters. They published my response that the fan wheel and the coils in and out need to be cleaned. The manuf suggests annually. If you have a mini-split know how to remove the entire cover so you can inspect the coil and fan. They are a pitn to clean, but they work well and effectively. There is a glass of wine calling me. See ya!

  11. #11
    Jack and others, yet again thanks for the feedback. Good to figure this out now, framing starts next week so plenty of time to plan. I live in Las Vegas, so low humidity. I am strongly leaning towards a single compressor because I have limited amperage and the specs I see say a single compressor needs 25/30 amp while each single is 15 amp. I don't know how much pushback the inspector will give when looking at all the electrical layout.

    As I mentioned I am planning for an eventual conversion to living space so its tough to balance what makes sense now Vs then. Mini splits are uncommon here so getting quality info is tough.

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