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Ventilation for Ductless HVAC Systems in Mild Climates

cjwagner | Posted in Mechanicals on

I know that California in particular a lot of new “ADU’s” and small homes being built with ductless HVAC systems. I hear so much about heating these units with ‘Mini-Splits’ but no-one (energy consultants included) seems to mention ventilation. Lets assume a mild, coastal climate and a relatively open floor plan. 

-Are HRV’s worth the cost in such a climate and such small spaces (especially considering most are designed for ducted systems) ?
-Anyone using “exhaust only” or “supply only” systems successfully? 
-Has anyone come across a good return air filtration system for wildfire smoke that can be set up without a ducted system? 

I have read https://www.greenbuildingadvisor.com/article/designing-a-good-ventilation-system
 But I think that article is little biased towards relatively large homes in colder climates. 
I have also researched the Lunos units but they seems expensive and more advantageous for larger homes with many individual rooms instead of open floorpan. 

Thanks for the input,
Christian Wagner 
Petaluma CA, climate zone 3C

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    In milder climate without a lot of humidity, ventilation with heat recovery is probably not worth it. The issue is that it is much easier to set up proper ventilation with an ERV/HRV. If you look at the cost of a quality bathroom fan, something like a Panasonic intellibalance ERV or their whisper comfort ERV is not all the much more extra cost.

    Some (probably most) ceiling mount cassette mini splits have a fresh air input right on the housing.

    You can go with something like this:

    https://ashp.neep.org/#!/product/29607

    Connect the ERV to the fresh air input on the unit, split the stale air pickup from the ERV and run one pipe to the bathroom as exhaust fan and one the kitchen area at least 8' from the stove. Install boost switches in both bathroom and kitchen.

    The nice part about the above unit is that you can connect branch ducts to it and divert up to 50% of the flow. You can use these branch ducts to feed the bedroom and bathroom.

    Since the blower on mini splits runs continuously, the unit would then distribute the fresh air supply from the ERV through the ADU.

    For wild fire some, you will want a filter box on the fresh air intake. Preferably something that can take at least 3 filters. You want a merv8 pre filter, a activated carbon filter for smells and a HEPA filter for small particulates. You would only need the carbon and HEPA during fire season, the rest of the time it can be left out as they are pricey. The nice part about having a pre-filter is that replacement filters for these tend to be cheaper than the one in the unit.

  2. exeric | | #2

    i agree with Akos on this. I live about 80 miles away from you in Lakeport in Lake County. It's a very slightly colder climate than yours because its more protected from the marine influences than your location. But I still think you can use it as a comp because my house is also small. It used to be a thing at GBA to try to use the Panasonic bath fans set to their ventilation modes to ventilate homes. I bought one and that ventilation mode has VERY low power. Even an average tight home just doesn't have airflow to supply it air, as measured by CO2 in the house.

    So I put in a makeup vent (also used for for the range hood) and the low power mode wasn't enough to open its spring mechanism. It didn't work to ventilate the house at all. I could have used the high power mode but a bathroom fan isn't made to be run all the time. And even in sunny California it gets cold in winter (I know you know this) and you don't want a bathroom fan running full blast all the time bringing in cold air.

    I think you'll find out that the ventilation mode of a Panasonic fan just isn't sufficient and full blast is unworkable. I tried it and went to a Panasonic ERV. I'm very satisfied.

  3. cjwagner | | #3

    Thanks for the responses! Eric yes I could see your point about needing the ERV and also to provide more options for ventilation different areas as you noted in the other thread.
    I think I'm realizing im dealing with a specific scenario that is making the ducted options difficult as my register would be in a vaulted great room mounted on the wall. I didn't think these registers had options for fresh air inputs as Akos noted with the ceiling cassettes.
    If I was to figure out how to house the ERV and run the ductwork are there wall mounted registers that can allow for fresh air input as well?

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