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Ventilation for an ERV and Multi Mini-split Passivhaus Design

RyanOlehausen | Posted in General Questions on

Hello,

I’m designing a passive house for my master’s capstone project which I hope to build in the near future as well.

I’m looking for general recommendations and confirmation of the adequacy of the design at this point. It’s too early for specific model recommendations.

I’m designing the house for Charleston, SC (IECC Zone 3). The floor plan is attached. The northern end of the house is the living area while the southern end contains the bedrooms, bathrooms, and office. I plan to put a wall-mounted mini-split in the kitchen and master bedroom, both on the eastern wall (top of page), connected to a single 2-ton heat pump.

The 2 remaining bedrooms and upstairs office would then require an 8000btu/hr heat pump supplying a ducted system which would take suction from the hallway (the lowest capacity air handler I can find is 12000btu/hr so it’ll be slightly oversized).

For ventilation, I planned to use an ERV to take return from both bathrooms, the laundry room and the living room and supply all 4 bedrooms. (The kitchen wall-mount should push kitchen air toward the living room return for the ERV.)

Please let me know if there are any recommendations to improve the ventilation flow. I’d like to reduce ducting if possible. I expect at least half of ducted unit will go to the office since it’ll be a high point in the house (attic), high significant window surface area, and high ceilings. Is there enough cooling in the living room with just once wall-mounted unit located in the kitchen (open concept) supplying 1000ft2 (15000ft3)? Ceilings fans should help to circulate air.

I used loadcalc.net to estimate heating/cooling loads, meeting PHIUS IECC Zone 3 recommendations for R-values. 

(I’ve already looked at integrated ERV/mini-split designs like the Minotair Pentacare V12, but it seems to be drastically undersized for my needs.)

The master bedroom and living room have high vaulted ceilings, hence separate wall units.

Thank you!

-Ryan

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Replies

  1. this_page_left_blank | | #1

    I think you should have a return in the kitchen, and make the one in the living room a supply. I know you said the minisplit head will drive the air toward the return, but there's a few problems with this idea. You want to extract cooking smells and particulates as much as possible. By blowing this across two rooms, you're going to mix this air pretty effectively with the rest of the air. The minisplit doesn't really blow a lot of air in the first place, plus a lot of the time it won't be running at all.

    I'm not familiar with climate zone 3, but your load numbers seem high if you're sealing and insulating to passive house levels. I'm guessing the cooling numbers dictate the load, and that cooling load is driven more by glazing than outdoor temperature. You have a very modest amount of glazing, one might even say scant.

    1. RyanOlehausen | | #2

      Thanks for the advice! Great point regarding the kitchen/living room supply/return locations. I assumed double pane/low-e windows, but the master bedroom and living area have high vaulted ceilings (27 and 21 ft, respectfully). I understand the bedroom ceiling height might be a bit absurd. I was trying to keep a 30 degree roof slope to optimize solar panels on a south facing roof. I may just truncate and make a small attic above.

  2. Jon_R | | #3

    I'd use 15+ CFM/person to the bedrooms.

    Don't neglect dehumidification and distribution of it.

    1. RyanOlehausen | | #4

      Thanks, Jon. Shouldn’t the use of ERV vice HRV (and returns in the bathrooms) account for the dehumidification factor?

      Using ASHRAE standard 62.2, I’d need 108 cfm for 4 ppl in a 2600 sqft house. Granted this doesn’t discuss distribution between rooms. Assuming 2 in master, 1 in each bedroom and office; I’m at 75 cfm. Send remaining ~25 cfm to living area?

      1. Jon_R | | #5

        An ERV doesn't add as much humidity as an HRV, but it doesn't dehumidify.

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