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Community and Q&A

Ductless Minisplit and Positive Pressure

Garsagil1 | Posted in General Questions on

Hi everyone!

First of all, thanks for your help, this community replies fast and is extremely helpful, thanks a lot.

I am having issues with second hand smoke and one theory as a solution is to have a window fan to generate positive pressure (and seal as much as you can the walls).

I live in a condo with Window AC Units in the walls (instead of windows). I know that Window AC Units generate negative pressure to push out the hot air they generate.

Having a window fan during summer in Chicago with super high humidity is going to be a problem. I am pushing humid and hot air inside a house I want with low humidity and low temperature, so I have the kitchen humid and hot, and then AC units and dehumidifiers running.

Now, a simple question. If I need positive pressure and a window fan does the job, would a ductless mini split generate positive pressure?

If the “machine” is outside, it shouldn’t generate negative pressure like an window unit, right? It would work as a window fan but that actually pushes cold air.

Hopefully someone can confirm this, I am willing to change all AC Wall units if that will create positive pressure.

Thanks again!

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  1. charlie_sullivan | | #1

    A ductless minisplit will have no effect on pressure. If you created positive pressure with a fan, the ductless minisplit will give you cooling without changing that positive pressure created by the fan.

    But a window A/C should be possible to configure for no effect on pressure either, so I don't recommend spending a lot of money on a ductless minisplit before looking at the window A/C more carefully.

  2. Garsagil1 | | #2

    So we don't have any technology that will generate positive air pressure with cool air?

    There is no AC unit that will generate cool air outside and push it inside the house?

    The window fan alone just pushes hot and humid air...

    1. charlie_sullivan | | #4

      If you had a location outside to mount a water chiller such as a Chiltrix unit, you could put that chilled water through a fan coil on your intake air, as wall as running it to wall-mount fan coil units for cooling.

  3. charlie_sullivan | | #3

    Here's a picture of a window A/C that has a damper, a little door that opens between inside and outside air:

    The way that's configured, it would exhaust cooled indoor air outside, and make the problem worse. If you have a door like that, there should be a lever to control it. You want it full closed, maybe even sealed shut with foil tape if you can assess it.

    It's possible that some have the configured differently, and they suck in outside air. If you wanted to rig something crude and ugly, you could have a duct from outside supply some of the air getting sucked into the indoor intake, into the filter and evaporator. If you found a way to do that, that would cool and dehumidify your supply air from outside and solve the humidity problem you mentioned on the other thread.

  4. Garsagil1 | | #5


    I am looking at this machine:

    What if I connect that machine with ductwork to my house? The machine is generating cool air and pushing it through the hose... shouldn't that be positive pressure?

    1. charlie_sullivan | | #10

      If you locate that machine outdoors and run a duct from it into your house, it will provide some cooling and create positive pressure. Maybe way too much.

  5. Jon_R | | #6

    Agreed, you are adding some heat to the kitchen. But about like cooking in the kitchen.

    Ideally, your outdoor air supply would dump the hot/humid air right next to your AC indoor intake. This would probably require a hole in the wall.

    > generate negative pressure like an window unit

    Window AC units typically don't create negative indoor pressure. The hot air blowing out the back came from outside. Mini-splits are similar - no effect on indoor pressure*.

    * - OK, technically, AC does cause some stack effect increases and decreases in pressure.

  6. Garsagil1 | | #7

    The biggest issue I have is the humidity. The heat I can take it, but the humidity is going to generate mold (I had a mold issue when I bought this house).

    SO the fan is doing the trick to keep everything in positive pressure. To reduce humidity I can get an extra dehumidifier in the kitchen (increasing the heat as well). I can do that, but I just wonder how there is no AC Unit that gets air from outside, cools it, and pushes it inside a house, how come we don't have that technology? Makes no sense to me.

    Thanks again!

    1. Expert Member
      BILL WICHERS | | #8

      There are units that do what you're looking for, but they're going to be much larger than you need, and they're expensive. We use them commerically to pre-condition air we use to keep telecom facilities under a slight positive pressure for dust control purposes. They have very high latent heat removal capabilities, since they are intended to do heavy dehumidification.

      In your case, just use a small fan, and let your regular A/C system handle the dehumidification. If you need more than that, add a supplemental dehumidifier. The dehumidifier will add some heat to the space, but will ensure that you don't have humdiity issues regardless of conditions.


    2. Jon_R | | #9

      You could buy a small ducted mini-split and duct it such that some of the return air comes from outside. This is a lot more work than adding another portable dehumidifier.

  7. Expert Member
    NICK KEENAN | | #11

    The reason you're having trouble finding equipment to do what you want is that what you want to do is not a usual way of handling issues like this. As I said in the other thread, exhaust every avenue for air sealing before installing any equipment. Your home will be quieter, cleaner and more energy-efficient.

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