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  1. #1
    Join Date
    Feb 2004
    Posts
    33

    Central supply, ducted returns in passive houses

    Passive Houses have very low heating and cooling loads because of R60 walls and ceiling, carefully placed triple glazed windows and minimal infiltration. Passive Houses do not recirculate air - fresh air comes in through the HRV, gets distributed, and then exhausted. It's very hard to appropriately size HVAC equipment for such a house. Mini-splits are attractive because their inverters let them work efficiently at very low levels. For energy savings you need to get by with just the fans on the HRV, and not run a standard air handler.

    One strategy for cooling and dehumidifying such a house is to dump fresh air in a central hall way cooled by a mini-split, then use jump ducts from the hall way to the rooms and finally exhaust stale air from the rooms through the ducts back to the HRV. This is the opposite of what Lstiburek generally recommends where you push air into the rooms and exhaust using jump ducts to centralized collection points.

    Does anyone see problems with using a central supply and ducted returns? Would this run afoul of any building codes?

    Thanks!

  2. #2
    Join Date
    Jul 2008
    Location
    Alabama
    Posts
    658
    That doesnt make sense....why would you want to have it installed backwards???

    It would probably never work right.....plus it would be an installers nightmare.

    Some installers have a hard enough time putting it in the right way to start with, let alone backwards.
    You're only as good as your customer will allow you to be.........If they want junk, sell them junk, but make your junk look neat!!!

  3. #3
    Join Date
    Feb 2004
    Posts
    33
    I think the logic is to work around the limitations of the mini-splits which are the only thing on the market that come close to matching the load. An inverter based mini-split can work efficiently as low as 3000 btu/hr, where the smallest central unit is 6x that, not counting the ridiculously expensive IQ drive units. The duct based mini-splits are an option, but most of these very limited in terms of static pressure, plus they are not as efficient (and get less efficient as you go up in static pressure ability).

    If you used a mini-split in the hall using the standard bedroom to hall flow, they you would always be putting warm, moist air into the bedroom, and then cooling and drying the air just before you exhausted it.

    Sounds like the best option is going to be a ducted mini-split for each floor with the usual flow. Thanks.

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