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Community and Q&A

DIY Install of Ducted Minisplit System

waltyl | Posted in General Questions on

Hello there,

I am building my own house in Washington State on the Olympic Peninsula and doing most everything myself with minimal sub contracting. I am to the point of needing to finalize some HVAC decisions. In short, I need to decide if I will tackle the install of a ducted mini split / heat pump myself. I have received quotes from local HVAC companies and have been shocked by the cost of professional install and while I would prefer a a licensed specialist to perform the install for warranties sake and perhaps sanity sake, I am feeling like it is to the point of being cost prohibitive. I am building a small (850 sqft) 2 bedroom home with a cathedral ceiling (i.e. no attic). I have a portion of one loft within the building envelope designated for a ducted minisplit and the install seems very simple in my minds-eye. There will need to be a plenum with three supply registers, but the runs from plenum to each register will be very short (<3 ft), which is to say that I am >95% confident that I am within the static pressure limits of the machine. My plan is to have return registers from each room back into the enclosed area of the loft containing the handler (i.e. there would be no return ductwork… The negative pressure generated by the handler would bring air into the enclosed area from each return register, or that’s my thought at least. I have been looking at the Mitsubishi 15k M series Condenser + Mitsubishi 141000 BTU handler (SEZ-KD15NA4k1). I can purchase the necessary items, including lineset, disconnect, whip, wired programmable thermostat, etc. on eComfort for well under 50% of the installed bids I have been receiving. I have been given quotes of 11,000 and 12,000 dollars for 12k BTU slim duct units- this feels outrageous for single head systems, that are going to be installed in an easily accessible loft space with very minimal duct work. Am I crazy to try to install a unit myself? I understand that I would need to find someone to pressure test the system, purge the lineset and add any necessary refrigerant, but as far as roughing in the system, pouring a condenser pad, mounting the handler and installing the ductwork, am I crazy to try it myself? Knock some sense into me please!

Also, there are some prefab plenums that I found for the aforementioned Mitsubishi unit that make the duct work all that much easier. And while I do have some outstanding duct design questions, like sizing each run, etc. I believe I would be able to figure it out with more research.

I should mention that I very much appreciate the expertise and skill that HVAC professionals bring to the table, I am just a young man trying to build his own home in a pretty challenging economy already and these bids are throwing me.

Perhaps I should also mention that I have already installed a woodstove in the main living area and plan to use that regularly as well, and this adds to my feelings about the cost of the minisplit/heatpump.

Thanks in advance for any thoughts,


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  1. Expert Member
    DCcontrarian | | #1

    Congratulations on your build!

    Have you done any energy modeling or a Manual J? The most common problem with installations is improper sizing.

  2. walta100 | | #2

    Have you seen the MR Cool brand DIY units? They have a leakproof connector for the refrigerant tubing that is plug and play no license required. It looks like Amazon will ship the only real down side is you will likely have some extra tubing coiled up behind the outdoor unit.

    If you want to make your own connections as a DIY option the first hurdle is a legal one in that to work with refringent legally you will need a 603 license from the EPA. The test is not a big deal most of the question are to put you on the record that you understand it is illegal to vent refrigerant to the atmosphere and that you need an approved recovery machine onsite to work legally and what the fines could be if you chouse to break the laws.

    The second hurdle is none of the local supply house are likely to sell to you.

    The third hurdle is you will need to buy some expensive tool the you are unlikely to use very much.


    1. Expert Member
      DCcontrarian | | #4

      OP wrote "I understand that I would need to find someone to pressure test the system, purge the lineset and add any necessary refrigerant."

  3. 5Stud | | #3

    As Walta says, the refrigerant is the problem so you need to source a DIY friendly system.
    You are right that you can (and should!) do the research to size your ducts properly.

    I installed one of these in a two bedroom cottage two years ago.
    I built my own 4 supply plenum very easily. I brought my R/A's back to a proper filter because these units have laughable filters. Yes it takes up some precious space but well worth it imo.

  4. Expert Member
    Akos | | #5

    If you managed to build a house by yourself, running some ducts and mounting a ducted unit to some unitstrut doesn't seem like a big stretch.

    Make sure the HVAC tech does a proper leak check of your install, that is pressure test with nitrogen and vacuum decay with a micron gauge. Getting the proper leak test done seems to be an impossible ask for a lot of techs. Getting your own micron gauge, vacuum pump, gauge set and pressure regulator might be worth it just to make sure you have a leak free install.

    You can simplify your ducting by home running everything to a duct board.

    I would look at some other ducted units as well, the offerings from Midea/LG/Samsuning tend to be better priced and most will do a bit more static pressure than the SEZ-KD. The Midea units can also be mounted in vertical orientation. I've ducted the SEZ-KD without issues, you do have to watch duct sizing and bends though.

    I don't think you can use the service cavity of the unit as a return. Usually code requires returns to be lined with something, so it can't be exposed framing. The simplest return is a door undercut and a central return with a big filter in the hallway.

    For a simple return what I have done is convert the ducted unit to bottom intake and seal a larger filter grill against the bottom of the unit. You can see the ducted unit with before the service panel and the filter grill goes on in the picture.

  5. kyle_r | | #6

    I installed a ducted 12k Fujitsu unit. No, it’s not that difficult. I had an HVAC tech pressure check and vacuum the line sets and commission (open the refrigerant valve on the outdoor unit and turn it on).

    Make sure you have the duct design and return filter properly specd. Other than that the hardest part for me was making sure not to kink the line set. This product helped, just make sure to keep them clean.

  6. Deleted | | #7


  7. waltyl | | #8

    Wow, thanks everyone for your thoughts!

    DC Contrarian- I have not performed a manual J of my own, but I also don't really think the three HVAC contractors I received bids from did so either. They barely looked at the install location and while I did send them each a plan set, their equipment choice did not seem coupled to any real calculations. My house is a hybrid timber frame, the ceiling is insulated with rigid panels @ R-52, the wall system is a 2x6 wall system which will have blown in fiberglass, and the floor system will be insulated with fiberglass batts in the crawl. There are A LOT of windows (24 to be exact, but most are very small with 18x36 is the most common window size), and three exterior doors. If I run a very basic calc based on # of occupants, # of windows and doors, and sq footage it comes out to a much higher load than the equipment spec'd by said HVAC contractors would cover. But, the house is also positioned/designed for a good amount of solar gain in the winter and we have a lot of operable windows to utilize in the summer. And again, there is the wood stove for those cold snaps in the winter... So, short of doing a full manual j that would take into account more details than the simple one, am I off base with just using a 12-15k unit? The HVAC companies all spec'd 12k, but 15k makes me more comfortable given the number of windows/doors. Short of paying for HVAC engineering from an online company I am not sure how to make a more educated decision.
    Walta- I have looked into the MrCool options. Initially, I was concerned about equipment quality, but it appears MrCool is a division of Midea, which makes me feel better. As for the equipment you link to, their central ducted system seems a bit extreme for my situation, but they do have a concealed slim duct unit in their olympus line that I was looking at ( This particular unit does not seem nearly as DIY friendly as their wall mount units/kits, which made me think: Why wouldn't I just go with a slim duct unit from a more reputable brand?
    5stud- thanks for the link to the Senville unit. That is a very competitive price for handler, lineset, and condenser. What are your thoughts on this system after those two years? Did you build a sheetmetal plenum or did you use ductboard? How did you size your four ducts for this design/equipment? I would have three or four supply ducts leaving a plenum as well, so am very curious about the diameter of your flex duct and register size.
    Akos- Thanks for the critique on return duct. I suppose it was hopeful for me to think I could use the service cavity. That said, the unit will be placed directly above a hallway and I could change the return air to the bottom of the machine and have a straight, short return duct to the unit. I am a little concerned about noise though, so I may opt to run it out the back and 90 down so the duct is a little bit longer. Do you have thoughts on the length of return duct and noise and positive pressure in rooms without direct returns?
    As for the supply ducting, I do plan on home running everything from a plenum. Each home run will be <10ft with one longsweep 90 or less. I am hoping 4-6" flex duct will cut it. The supplies will be high in each room, which I understand is not ideal if you're heating dominant, but getting them low would introduce a lot more static.

    Kyle- thanks for the response- do you have any resources on duct design?

    1. kyle_r | | #10

      Here is how I did it. Start with your Manual J to get your room by room load. Then use the percentage of each room's load vs the total and multiply that by the maximum CFM your chosen air handler outputs. This gives you the CFM needed per room. I then used the chart here to determine duct size ( and tried to keep the velocity around 400 fpm.

      Rigid duct is best. If you use flex duct, still use rigid fittings (elbows, etc.) and make sure the flex duct is drawn tight to reduce pressure drop. Try to limit the number of fittings (elbows) per branch to 2-3. The fittings far out weight the pressure drop of straight duct. Make sure to use foil tape or mastic to seal all ductwork.

      I went with a 20"x20" central return and used a 2" deep filter. The deeper filters typically have a lower pressure drop. I used Flanders Pre Pleat 40 LPD 20"x20"2" filters. I found a 20"x20" filter return grill on Home Depot's website that accepts a 2" thick filter.

      A central return works nicely with these units as the total CFM output is typically low at 300 -400 cfm. With typical door undercuts, I don't have any pressure imbalance issues.

      Akos may chime in again. His comments on other threads helped a lot with my design.

      1. waltyl | | #15

        Kyle, thank you! This is exceedingly helpful. I just did as you said according to my Manual J from coolcalc. Here's what I got:

        Master bed: heat load 3384btu = ~22% of total x 383 cfm for 12k fujistsu = ~90cfm = ~4" duct @ 400fpm

        Living/kitchen: heat load 8192 btu = ~54% total x 383 cfm for 12k fujitsu = ~200cfm = ~9" duct @ 400fpm or perhaps two 6" ducts.

        Second bed: heat load 3222 btu = ~21% total x 383 cfm for 12k fujitsu = ~90cfm = ~4" duct @ 400fpm

        Does that check out with you? Obviously you're not looking at my manual j, but as far as a gut check goes? I am tempted to break from this design a little and just do 4 6" ducts w/ 31/4 x 10 supply registers, one to each bedroom and two to the living room. This would make the living room under resourced perhaps, but then again we have the wood stove there and it gets the most solar gain (bedrooms are both on the north side of the house) and if the controller was in a bedroom, i think that would work out nicely.

        Thanks for your notes on the return duct / filter as well!

        Lastly, when it comes to plenum design, any resources on that?

        Thanks again!

        1. kyle_r | | #18

          At 90 cfm you should be at a 6” duct for 400 fpm. I would go with the four 6” duct design. I would also put Manual dampers on each trunk so you can adjust flow to each room so you can balance the system.

          For the plenum, I would just get a piece of rectangular duct big enough to mount to your air handler and fit the four take off collars. I would do home runs from there.

          I would also think about where you can position the air handler so you have the option to gravity drain the condensate instead of using the built in pump. One less thing to break.

    2. 5Stud | | #12


      All sheet metal. Easy to work with. Get some decent snips!
      My plenum is just an 8x8 half stack (2 L's fit together) and two end caps.
      I used 6 inch ducts for supply and returns. Returns should be bigger but that is what I had at the time!
      3.25 x 10 for boots.
      Been running well but I had some minor issues with the controller but the tech support was fast and helpful.

      1. waltyl | | #16

        Thank you! I'm going to seriously consider the unit you mentioned.

  8. jameshowison | | #9

    Check out for your Manual J. Doing it properly (room by room) is probably only a few hours work (and, unlike building a house, pairs well with a whisky).

    If you have any concerns on static pressure at all, don't forget to consider the "multi-position air handler" options as well. The smaller ones are really not much bigger, although they are more square (17" x 22") than the slim-duct (8" x 28"). That can match up better with filter boxes and generally available ducting supplies. Also, if you have easier access from the side than from underneath, they are good options. And they have substantially more static pressure (although it's all ECM motors so they don't use what they don't need).

  9. Expert Member
    Akos | | #11

    A couple of things there. Sounds like an SIP roof, make sure you read about ridge rot and take all precautions. The seams taped on the warm side with a quality tape is a must. Permeable roofing or even better a top vented roof is a good idea.

    With a crawlspace it is best to insulate the stem walls not the floors. This uses less insulation, easier to do and you get warmer floors in the end. The bonus is that you can now put your air handler and ERV down there. With a lot of windows, you want supply register bellow windows, ducting in the crawlspace makes this much easier.

    Make sure to air seal and insulate your rim joist while doing the stem walls.

    The multi position air handler that James linked to is a good option. All manufacturers make an equivalent unit, Mr Cool has a similar DIY (Universal DIY) version with the quick connect lines sets. These air handlers look much closer to a standard furnace so lot of the off shelf HVAC bits can be used. I would still home run with such a small place. The slim ducted units are cheaper but you have to get more bits fabbed.

  10. waltyl | | #13

    Alright, well I ran the coolcalc- attached is the output. Looks like ~200 cfm per bedroom and ~600 cfm for main room is recommended. Also seems a 15k unit is just about right for heating, but coolcalc spec'd a 20k+ cooling system. Heating is my primary concern here, not really planning on cooling the house in summer too much (again, olympic peninsula in washington state not far from the water with a steady summer breeze), so again, thinking a 15k is plenty. A question arises though, should the CFM rating from Coolcalc dictate my decision on equipment (i.e. looking for a unit that has an equivalent output? Most slim duct units seem to have a maximum output well below the CFM requirement that Coolcalc revealed... I am seeing 12k slimduct units in the 300-400 cfm range for high fan speeds. Is this an issue? Finally, given the CFM requirements of each room, would 4" ducts to bedrooms and a 6" duct to main living space get me in the zone. Alternatively I could run two 4" supplies to main room.

    As for the sip roof, yes I am aware of the issue with ridge rot and have taken necessary precautions. There is full dimension car decking on top of rafters, covered in 6mil plastic with foil backed panels (raycor panels- do not have osb sheathing, just structural members and foil facing, 7 1/4 in thick, claimed to be r-52) glued together and down w/ seams thoroughly taped. Then a continuous 1/2 ply decking and metal roof. It was built according to architect and engineer specs and following raycore manufacturing recommendations.

    As for crawl space insulation we went with Joto-Vent so the crawl is perimeter vented, so I believe I am already too far down the path of insulating between joists and the crawl will remain unconditioned.

    I will definitely take a close look at the multiposition units- I had kind of ruled them out in favor of the slim duct units.

  11. twoodson | | #14

    I’ve installed two. If you found GBA and are building your own house, will have no problem. The install is easy, it will take you longer to get tools together. I just purchased equipment on Craigslist / Facebook / eBay and resold it.

    Most HVAC techs will do an inferior job vs what you can muster on both the labor and design side.

    1. aunsafe2015 | | #19

      "Most HVAC techs will do an inferior job vs what you can muster on both the labor and design side."

      Sadly, this is too true. When I had a mini-split installed the tech did a nitrogen pressure test but didn't have a micron gauge. So for his vacuum, he was just going to let the pump run for 30 minutes and call it good.

      That was when I decided I needed to learn how to DIY a mini split.

    2. bfw577 | | #21

      A diy job with the proper tools will most likely be superior to most pro hvac installs. You have all the time in the world to double check everything and take your time. I triple evacuated to a deep micron level and pressure tested with nitrogen multiple times. I pressurized to 500 psi and let it sit for 24 hours and then deep vacuum and another 24 hours. With diy you have the time to absolutely make sure your system is leak free and properly charged.

      I bought all pro hvac tools from various online sources for around $700. The most important one is a good flare wrench and digital torque wrench. You can rent a nitrogen tank from your local airgas right online.

      I kept the tools and have since installed dozens of units for friends/family at affordable prices.

      1. waltyl | | #22

        Thanks for the input bfw577! where are you located? Need a weekend sidehustle?

  12. vpc2 | | #17

    Some of the DIY Mini Splits are very efficient and are not very hard to install (youtubes galore). Helped on one install that took 3-4 hours.

    Having the option to put in two heads and zoning is very cost-saving and super efficient for both heating, cooling and dehumidifying! (3 options). Zoning with Wifi and cell phone is very common and easy. This technology seems to be a SeaChange event.

    Also very low noise. Generally drilling the 3" or so hole for the 2 small pipes and wires may be the most challenging task.

    You could/should get government rebates (state and federal) for installation and electric power upgrades.

    One issue currently most (all) do not use the new gas refrigerant as of the new year so in years to come may cost more for recharge if needed. Not sure if any are out yet or obtainable on a one-off buy, with the new gases. Likely they built millions with the old gas yet to be sold.

    Most 12k btu units run on 120v and seem to be the more efficient, 20 to 25 SEER in better models.

    They even make universal units (central) with this new method and also works well but not as efficient generally or easy to control.

  13. Patrick_OSullivan | | #20

    Point to note regarding handling refrigerant, etc. If you are comfortable doing all the work and acquiring the tools for a non-DIY unit, you still may not need to handle any refrigerant.

    The systems are precharged for a maximum line set size. For most units, I've seen this at 25 ft. However, Fujitsu units often come with a higher charge. Both of the units I installed myself were charged for up to 45 ft. That extra 20 ft. can often make a big difference and greatly simplifies the process, tools, material, and license required.

    1. waltyl | | #23

      Thanks Patrick, you are totally right. I looked into a Fujitsu 12k hander and condenser setup (ADUH12LUAS1 & AOUH12LUAS1) and it seems the condenser is charged for up to 49ft of lineset which is more than adequate for my situation. I'm having some trouble finding a controller for this unit, but other than that it looks like I could get this system with an install kit (lineset, whip, disconnect, etc.) for about 3000 dollars. Also this unit can be installed vertically or horizontally as someone mentioned above. Conservatively, with duct work and registers, and tools for pressure test, vaccum down, I could estimate 5,000 dollars all said and done. This is <50% of the cheapest bid I received from local HVAC contractors.

    2. Expert Member
      DCcontrarian | | #24

      My understanding is that the pre-charge isn't for "up to" a certain length, it's for exactly that length. You have to keep the linesets that length and coil the excess somewhere. See also post #2 by Walta.

      You can estimate the length of the runs by planning the layout and then running something similar -- garden hose, PEX, romex -- and marking and measuring the length.

      1. waltyl | | #25

        Here is the portion of the manual related to additional charge for aforementioned condenser. I am not talking about a pre-charged lineset, just the r410a already in the condenser from the factory. Are you saying that coolant would need to be let out of the system if line set ends up shorter than 49ft? There doesn't appear to be any mention of that in the manual.

        1. Expert Member
          Akos | | #26

          The refrigerant pre-charge is for a range not a specific length. Somewhere in the specs there is a min length as well, on some units this is very short and you are unlikely to ever hit it.

          With longer lengths you also loose a bit of capacity, something to check if you are near rated max.

        2. Expert Member
          DCcontrarian | | #27

          I'm talking about the DIY units that come with pre-charged linesets. They're not meant to be shortened. They can be lengthened by coupling linesets together.

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