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Community and Q&A

Retrofit a stuffy room in a ductless condo

tyaruss | Posted in General Questions on

I have a new condo with ERVs and a ductless minisplit.  One of the bedrooms is isolated like an interior cul de sac.  It’s 100 sq. ft. and has its own FCU for heating and cooling, but it gets no fresh air or cross ventilation.  And the air is not mixed well at all without an additional continuous fan.  It gets stuffy and humid at night, and too warm in the winter, even with the heat off.  I think I have two options:

1)  Install an ERV.  While this will help keep the air fresh and dry, I don’t think it will keep the room from getting too warm in the winter.
2)  Install a through-wall transfer fan and a door vent on opposite sides of the room.  It’s next to the living room which is fresh and cool all night because of its ERV and leaky kitchen and bathroom fans.    Products such as the Tjernlund AireShare and the Panasonic WhisperValue DC fan and TRB look good, but the CFM may be too high and noisy to run continuously all night.
I don’t need to equalize temperatures.  I just need to get fresh dry air moving and mixing through the room, slowly and quietly all night.  I appreciate any opinions and suggestions.  Thank you.

P.S.  Cracking a window is not an option because the room is on a busy city street with traffic most of the night.  And leaving the door open doesn’t change anything, nor is it desirable for undisturbed sleeping.

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  1. Expert Member
    Akos | | #1

    You can probably use a Panasonic whispervalue DC fan. These fit inside a standard 2x4 wall and have adjustable flow rate with a little switch. These don't hum like a lot of bathroom fan, very quiet especially on the lower flow setting.

    There are a number of in line through the wall transfer fans but these tend to be louder as they use an axial fan.

    The best would be to run a supply duct to the existing ERV.

  2. tyaruss | | #2

    Running a duct from the existing ERV would be great, but there is no route without zig zagging a new fur-down along the living room ceiling. That would also interfere with the living room FCU intake air.

    The WhisperValue looks promising because it can mount in a 2x4 wall and run continuously, and it's very quiet. But 50 CFM is a lot for 100 sq.ft. That's about four exchanges per hour, which seems like too much. Am I wrong?

    It's still cold here, but the AC still ran several times during the night. That's how well insulated that room is and how warm a single body makes it during the night.

  3. Expert Member
    Akos | | #3

    You'll find that 50CFM is not too much air, there is no such thing as too many air exchanges per hour.

    There are some smaller fans that are designed for cooling electrical cabinets. You can get these in 24V DC and run it off a small wall wart power supply. Lot of them also come with a replacable filter. (see . I've mostly been around these in industrial settings, so I can't say how quiet it would be in a house.

  4. tyaruss | | #4

    I've found a number of fans that would work great. But I need a transfer grille because the crack under the door is insufficient. My search for one that dampens sound and light has produced only one, the Return Air Pathway from Tamarack. With all the clever "sound mazes" and acoustic baffles in large scale commercial products, I'm skeptical of the effectiveness of the corrugated cardboard insert, and the visibility of the cardboard is unappealing. The reviews on sound dampening are mixed, from vary bad to just OK. Are there other products out there that I missed?

  5. Expert Member
    Akos | | #5

    Those honeycomb inserts work supprisingly well. They do mostly filter out higher frequencies so some sound will still come through.

    You can DIY a decent quiet transfer duct yourself by using a stud cavity (find one without wires).

    Cut a slot for one register at the bottom of the wall and another one on the other side near the top of the wall. You want to air seal both top and bottom plates first. A 14x3 toe kick diffuser works well for this.

    Through these slots install duct liner or duct board (stuff you make plenums out of) against the back wall. You don't need full height of the cavity, a couple of feet is enough. Better performance if you can get the duct liner on all four sides of the cavity, back wall is the most important though.

    Complete by installing the registers.

  6. tyaruss | | #6

    This is really good information. I would bet that each stud bay has fire blocking somewhere in the middle, so I may only be able to separate the two registers by a couple of feet. But I really like the duct liner idea, so that's worth investigating.

    Tamarack's testing data indicated the sound reduction was very good, so perhaps the bad reviews are from folks unrealistically expecting too much. I just want good privacy when people on either side are reasonably quiet.

    1. Expert Member
      Akos | | #7

      If your walls are standard construction, I would stick to the off the shelf tranfer grill. If you are only flowing 10 or 20 CFM of air, you block off a good section of it to get better isolaiton.

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