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Community and Q&A

How best to manage minisplit condensate drainage

CatsNPlants | Posted in General Questions on
My first post here.
Profound thanks for this resource.
I have received shipment of the 3-zone ductless minisplit system that I plan to install in my small, block-construction Tampa house, to replace the central heat pump that I decided to stop troubleshooting. I’m in suspense about how my novice-designed system will perform.
I placed the order without deciding exactly how I would handle condensate drainage. The salesperson indicated (when I asked) that yes I could install the system and get it operating “bucket style” then go back later and add condensate pumps, and advised me to just leave plenty of slack in the lines so that I would have room to work. (They don’t sell condensate pumps anyway.)
My plan is to install each of the three wall units on interior walls. One of them could go on an exterior wall, but not opposite the outdoor unit. Each of the linesets has to run through my attic.
I have found many minisplit installation videos online, but none showing installation of a wall unit on an interior wall. That kind of guidance (video + critical comments) would be a comfort.
I originally imagined that I would put a pump on each wall unit and route all the condensate alongside the linesets, out to the outdoor unit. I read on a forum somewhere that the drain line should have a cleanout. (Is that so?) It got me thinking that maybe I should minimize the distance (and complexity) of each drain line–that each wall unit should drain as directly as possible through an opening in an exterior wall. Would that be better?
I considered draining to waste (or toilet tank or laundry room), but ever since hearing that AC condensate is better than “city water” for plants, I’ve been looking forward to using it outside for irrigation. 
I also need to improve my bedroom ventilation, but I’ll ask about that separately. Thank you!

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  1. Expert Member
    NICK KEENAN | | #1

    The best way would be to have it drain through gravity and skip the condensate pump, one less thing to fail. It can drain either to outside or to a sewer pipe. If draining to a sewer pipe it has to be "indirect" -- i.e. with an air gap, a pipe inside a pipe, not plumbed in. Think of a washing machine drain. In any case the drain can never decrease diameter from what it starts at the fixture, and it has to have a continuous downward slope of at least 1%.

    In new construction it's pretty easy to meet those requirements. In existing construction, not always, and that's where the condensate pump comes in. The condensate pump lets you move the water up and over to a spot where you meet the requirements.

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Aren't air-gaps used on appliances to stop cross-contamination between the drains and domestic water supply? Why would a mini-split need one?

      1. Expert Member
        NICK KEENAN | | #6

        I can only speculate why the code is like that, but I can imagine that if your sewer backed up you wouldn't want it going into the coils of your HVAC and being blown around the house.

        1. Expert Member
          Deleted | | #8


  2. StephenW81 | | #3

    I had a similar question. If drained to sewer pipe, it seems like it would need a trap to avoid odors. However, if nothing else drains into the trap, it seems like the trap would dry out in a low humidity heating-dominated climate.

    Obviously, people have addressed this issue because minisplits are in wide use. However, I don't recall any articles about minisplits explaining how people address this issue.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      I don't know how people usually handle the condensate, but for drains that don't see enough water to keep the traps filled, you can use either a Trap-Primer, or a one way Trap-Seal.

  3. Expert Member
    Akos | | #5

    The way I've done this is run the drain line along the lineset to the outside of the house near ground level. Nature takes care of the rest.

    Interior walls are no different, along the wall till you make it out of the house or down into the basement into a floor drain. You might have to cut some drywall to run the pipes in the wall if you don't want to see them.

    Watch that the line never sags and that you maintain a downward slope at all times. Those corrugated drain kits lot of the mini splits come with are worse than garbage, do not use them. I like to use 1/2" poly irrigation pipe but anything smooth and somewhat rigid would also work.

    If you have a drain near by (floor drain or washing machine riser), you can use that instead but make sure you include an air gap.

    Condensate pumps should be a last resort as they can fail. About the only spot I would use them in a basement where you need to pump up hill. If you do use one, make sure there is a float switch in it to disable the mini split in case the pump stops working.

  4. gusfhb | | #7

    I have 3 minisplits on inside walls and all drain via gravity with the linesets.

    the drains are fussy, and even with decent pitch all the way and 12 feet vertical drop a trap will cause water to drip out the front of the head unit. I ended up with a piece of that scotchbrite looking roof vent material in the bottom of the drain to keep bugs out.

    If the washer drain was convenient, that would probably work. The mini split at work runs into the slop sink

  5. FluxCapacitor | | #9

    If you are using Mitsubishi, they make a ceiling unit (MLZ series) that fits between joists and has built in condensation pump.

    This might be a good option for you since you are running your line sets in your attic anyway.

    The built in pump is really quiet because I have never heard it on any of my three MLZ units.

    The MLZ are a little harder to install.

  6. Jack_smith | | #10

    In my experience, (i'm a remodeler) the gravity drains are the way to go if you can. you can get pretty creative on where to terminate it to if you have to. Outside is the easiest but be mindful of where so you don't have green staining on the concrete/siding, under a bathroom vanity, kitchen sink, or if you have access to a stack a trap is pretty simple to install especially if you convert that terrible flex pipe drain to rigid pvc, you can just neatly put it behind an access panel if its in wall. try to avoid condensate pumps at all costs, more moving parts, some are noisy, more drain lines, more fuss, usually by the time you know the pump has failed its time to bring out the shop vac. Clogs in the drain are pretty simple to clear, wherever the drain terminates tape that pipe to your shop vac, turn it on and it's usually done in seconds, there should not be much in that drain it's not a dishwasher.

  7. adrienne_in_nj | | #11

    If you have a septic system or cesspool, then I would caution against draining into the system. I personally wouldn't do it and it may not be allowed in my area anyway. If you have public sewers, then I would check to make sure that you are allowed to drain into the system. It may be allowed, but in my area, one of the sewer systems does not allow garbage disposals/disposers and of course no sump pumps; I'm not sure where they stand on condensate.

    1. Expert Member
      MALCOLM TAYLOR | | #12


      What harm do you see in draining condensate into a septic tank?

      1. adrienne_in_nj | | #13

        Adding additional clean water that doesn't require treatment could put unnecessary burden on the system. Conserving water is a common tip for preventing malfunctions. This includes fixing dripping faucets, etc. I have no idea how many gallons per day would be added for a house in Tampa Florida, and it may not be much, but septic systems are *very* expensive here in NJ and I have seen 10 year old systems fail, so better safe than sorry when dealing with the most expensive system in the house (starting around $20,000 and going up from there.) Also, some systems have a pump tank where the water is "dosed" into the absorption area, and I have heard that that may be better for the drainage field than a gravity system where water constantly flows into the drainage field because it allows the field to dry a little bit between doses, thus possibly reducing the build-up of sludge in the laterals. I'm not sure if this is true, but it seems to me that if there's a chance that it's true, that it may not be a good idea to have condensate constantly dripping into a gravity system, like a faucet dripping 24/7. Most systems will receive no water while the occupants are sleeping or away at work, etc, but not if you drain condensate into the system.

  8. walta100 | | #14

    Adrienne come on now the AC unit is unlikely to put out 5 gallons a day. If 5 people live in the house it is about 1% added load.

    If you know of a city prohibits condensate from the sewer system please post a link to the public notice.


    1. CaptainCurt | | #15

      Walter... I live in Baltimore. The bill for our sanitary sewer is based on the city water supplied to the house. Any excess load on the sanitary sewer needs to be measured and paid for by the home owner. I don't think anyone does that, but that is the code. Conversely, in commercial applications, a company can meter the flow of water to a cooling tower and receive a discount for the water that is evaporated (reduced sanitary load).

      Personally, I would not connect a condensate line to a sanitary line. Too much chance to have migrating sewer gas smell blowing out in occupied space. Draining to a sink or laundry drain is fine... they have traps. And you can use a trap to connect directly, but if it dries out in heat season, you have nothing. Just my humble opinion.

      Capt Curt

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