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Community and Q&A

How do YOU mix the Air in your Home?

homedesign | Posted in Mechanicals on

In North Texas, the standard is forced air…that’s all we know how to do.
Air conditioning is almost mandatory and forced air has been the method.

I have read posts from cold climate folks that use radiant heat or a fireplace as the heat source and they do not even have an air conditioner or duct system.

The forced air systems that we have around here are MONSTERS.
The units are usually large and the duct work looks like an overgrown octopus.

The typical attic is the worst place for these units and the maze of ductwork…

As enclosures improve… the size of the required air conditioners is coming down.
A Not-So-Big house could be serviced by a single mini-split ductless AC.

The problem with Mini-split Ductless is that it will not mix the air from room to room.
It seems to me that if the ductless unit was located in the “right” place … the air can be “mixed” or transported from room to room with a properly designed ventilation “path” (similar to Passivhaus)

Martin Holladay once described the mixing (or lack of) in his home.
I believe he said if the bedroom doors were closed … heat distribution did not happen.
That seems a little too primitive.

I think the easy answer for larger more expensive homes would be Ducted mini-split….
What about the more affordable homes?

My question is for you Cold climate folks….
If you do not have forced air……
How do YOU “mix” the Air in your Home?

Jump ducts along the “exhaust path” ?…. from hallway to bedrooms? …then duct from bedrooms to bathroom?

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  1. GBA Editor
    Martin Holladay | | #1

    People have lived for thousands of years without any equipment to mix air in their homes. I'm not quite sure what your concern is.

    1. Is your concern uneven temperatures? This concern is usually addressed by proper design of the heating and cooling system. It is certainly possible to heat with hydronic heat, and cool with ductless minisplits. As long as the system is properly sized, installed, and commissioned, there is no reason to think you will have wild temperature variations from room to room.

    2. Is your concern the accumulation of odors or noxious substances? If so, that's why we have mechanical ventilation systems. These systems don't have to mix the air from room to room, although they usually do, to some extent. If you have a smelly, humid room, make sure that the room is ventilated.

    3. Did I miss something? Is there some magic reason why you want to mix your living room air with your bathroom air? What's the virtue of mixing?

  2. homedesign | | #2

    1. Yes, uneven temperature .... balancing temperature without the typical( for Texas)forced air system
    Mixing WAS a poor choice of words.
    I should say distribution of Heat or Coolth when using only 1 ductless unit in a home with multiple rooms.
    Provisions must be made for when doors are shut as in a closed door problem you mentioned once about your home.
    2. odor is not the concern...I assume that air is exhausted through the bathroom or kitchen and then to the outside
    3. I should not have said Mix... perhaps heat flow strategy is better?

  3. user-723121 | | #3


    A highly insulated, airtight building envelope with proper window placement and shading will take you a long way towards a comfortable living environment. If that doesn't work you could always put a fan with an ice block in each room.

    Is there any use of evaporative cooling in your area or is the humidity level too high?

  4. user-757117 | | #4

    Tansom windows over interior doorways?

  5. user-757117 | | #5

    Dang, should read "transom windows..."

  6. GBA Editor
    Martin Holladay | | #6

    The better your thermal envelope, the less likely your home will have uneven temperatures. Comfort complaints and room-to-room temperature variations often indicate high levels of air leakage.

    Is the problem in Texas that no one bothers to perform Manual J and Manual D calculations? If you follow the necessary steps, namely,
    1. Calculating room-by-room heat loss and heat gain, accounting for window glazing, window shading, and window orientation, and
    2. Careful sizing of the heat distribution system so that each room receives the required heat (or cooling airflow),

    ...then I can't think of why you would be uncomfortable.

  7. homedesign | | #7

    Thank you Lucas
    that's the idea

    Of course we do manual J for or forced air systems

    I am not talking about a typical forced air system...I want an alternate strategy
    I am thinking more low-tech .. like Lucas mentioned transoms over doorways etc.

  8. homedesign | | #8

    When I visited my inlaws farmhouse in Upstate New York,
    there were louvered transfer grilles in the floor of the upstairs bedrooms.
    Combined with a single wood stove in the downstairs living area...
    It worked
    low tech..... few or no ducts

    This is probably obvious to you cold climate guys...
    Foriegn to someone who grew up with ducted forced air.

  9. GBA Editor
    Martin Holladay | | #9

    The basic answer to your question — "My question is for you Cold climate folks.... If you do not have forced air......How do YOU "mix" the Air in your Home?" — is, we don't. We have a measured number of feet of hydronic baseboard in each room.

    Many Passivhaus builders are going with a single heat source and air-conditioning source -- for example, located in the central stairway. The key to making this work is to have an impeccable, very airtight envelope, with thick insulation. Operation of the HRV provides a little bit of air mixing.

  10. homedesign | | #10

    Are you saying that with a good enclosure and a Single Minisplit ... the HRV ventilation is enough to provide comfort in all rooms on different sides of the house, with an upstairs and downstairs etc. even if doors are closed?

    Surely you still need transfer grilles or transoms etc.
    Some method to balance temperature?

  11. GBA Editor
    Martin Holladay | | #11

    I'm saying that Katrin Klingenberg is doing it in Urbana-Champaign, Illinois.

    Keep your windows shaded, and get your air leakage down to 0.6 ach50.

    But -- it's an experiment. As I said, the HRV moves air around the house to some extent.

  12. homedesign | | #12

    Do you remember seeing transfer grilles or room to room transfer ducts in the Urbana home?

    Maybe I will ask my question over at PHIUS bulletin board.

    I would love to get rid of the "ducted monster" (typical forced air)and be able to heat and cool with a single mini-split
    The Minisplit Unit would not look too bad in a hallway or a stairway.
    Installed price should not be too bad either.

  13. GBA Editor
    Martin Holladay | | #13

    As far as I know, just ventilation ducts. The fresh outdoor air is supplied to the bedrooms and the living room. The exhaust air is pulled from the bathroom(s) and kitchen. Therefore air is always circulating between rooms.

  14. Riversong | | #14


    In cold climates, not only do we not need AC but exhaust-only negative-pressure ventilation works since that prevents the condensation caused by exfiltration. But a negative pressure system won't work in a hot climate with cooled spaces.

    With exhaust-only ventilation and make-up air inlets remote from the bath fans in bedrooms and living rooms and 1" spacing under all interior doors, there's good air movement throughout the house.

    I would think that, with careful design and layout, a mini-split and an ERV could work in tandem to distribute conditioned air. Transfer grills and hallway ceiling plenums are alternatives to 1" gapped doors. But the door gaps allow the mice to travel freely without having to chew holes.

  15. Riversong | | #15

    By the way, as a pacifist, I would never use a forced air system.

    I think it's wrong to force the air to circulate, so I always try to convince it to do so.

  16. user-626934 | | #16


    Here's a link to the research presentation on the Fairview II house in Urbana from the 2009 Passive House conference.

    The presentation doesn't show all of the data, but a quick look tells me this:
    1) Indoor average temperature did not exceed 74F on May 19-20, 2009, even when outdoor highs reached 87F and 93F...and this is with no cooling needed from the mini-split!

    2) On a couple of really hot days in July (high temps of 99F and 102F), average temperatures ran a little bit high, and temperatures in the upstairs bedrooms ran really high for several hours on the second day....but this is with the mini-split running only occasionally. Longer run-time on the mini-split might have avoided the overheating.

  17. user-757117 | | #17

    For animal lovers: Air-permeable cat-flaps between rooms.

  18. user-757117 | | #18

    Maybe a custom vented laundry chute or dumb-waiter.

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