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Re: PTAC or mini split

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  • LarenCorie
    Posted by: Martha Olson mbo1955@peoplepc.com ... Hi Martha; What is that argument? -Laren Corie- Natural Solar Building Design and Solar Heating/Natural
    Message 1 of 4 , Mar 19, 2013
      Posted by: "Martha Olson" mbo1955@...

      > There's an argument for installing the larger unit because
      > I am designing the attic for the possibility of a loft space.

      Hi Martha;

      What is that argument?

      -Laren Corie-
      Natural Solar Building Design and
      Solar Heating/Natural Cooling/Energy
      Efficiency Consultation Since 1975
      www.ThermalAttic.com (many new
      photos and pages, coming soon)

      Read my Solar house design articles in:
      -Energy Self-Sufficiency Newsletter-
      www.essnmag.com

      Home base-LittleHouses YahooGroup
      http://groups.yahoo.com/group/LittleHouses/

      Founder-WoodGas - Power from wood
      http://groups.yahoo.com/group/WoodGas

      Founder-RefrigeratorAlternatives YahooGroup
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    • Martha Olson
      1a. Re: PTAC or mini split Posted by: LarenCorie larencorie@axilar.net larencorie Date: Tue Mar 19, 2013 7:12 am ((PDT)) Posted by: Martha Olson
      Message 2 of 4 , Mar 20, 2013
        1a. Re: PTAC or mini split
        Posted by: "LarenCorie" larencorie@... larencorie
        Date: Tue Mar 19, 2013 7:12 am ((PDT))

        Posted by: "Martha Olson" mbo1955@...

        > There's an argument for installing the larger unit because
        > I am designing the attic for the possibility of a loft space.

        Hi Martha;

        What is that argument?


        The argument is that there will be a larger conditioned space and I plan to install a timed thermostat (I know - heat pump style) so it will be good to ramp up the heat more rapidly. The counter argument - of course - is a smaller unit will extract more moisture from the air.

        Some of the discussion I've read is that it is not good to overwork the mini splits.

        Remember I am not building a superinsulated house. Energy savings will be because it is small, has minimal windows on the NW side and 3 large windows on the SE side. Also a lot of the NW wall will be closet space and kitchen cabinets. The homeowner can retrofit the porch (SW side) with sliding glass doors to make it a sun space. Although my handyman says used sliding glass doors are harder to come by these days.

        Martha
      • LarenCorie
        Posted by: Martha Olson mbo1955@peoplepc.com ... Hi Martha; Heating/cooling system capacity, is not determined by space . It is about heat losses and
        Message 3 of 4 , Mar 24, 2013
          Posted by: "Martha Olson" mbo1955@...

          >> There's an argument for installing the larger unit because
          >> I am designing the attic for the possibility of a loft space.

          > The argument is that there will be a larger conditioned space

          Hi Martha;

          Heating/cooling system capacity, is not determined
          by "space". It is about heat losses and gains, which
          are functions of characteristics of the building envelope,
          rather than volume. For instance, a 600ft² house with
          6/12 pitched gable roof, would only gain about 150ft²
          of building envelope surface area, much of that in (pre-
          -sumably) well insulated roof area. With an average
          insulation value of say R30, and a design temperature
          of 18°F (Chattanooga, 97½%), it would only require
          a heating system capacity increase of about 250BTU
          per hour, not thousands. That amount is insignificant,
          and you already have it well covered, with a smaller
          mini-split. What is your calculated heating system
          capacity at design temperature? One of the biggest
          mistakes, that is often (usually) made is over-sizing
          heating/cooling systems for energy efficient houses.
          This results in too little circulation, and too much
          cycling during the heating season, as well as way
          too little dehumidification during the cooling season,
          which lead to a cold and clammy environment.

          > I plan to install a timed thermostat

          That is a good idea.

          > so it will be good to ramp up the heat more rapidly.

          That is a sales pitch you will hear from heating
          guys who don't want to bother with calculations
          for proper sizing, and have not yet developed an
          understanding of the difference demands of energy
          efficient houses (especially small ones). Oversizing
          is not some little greedy enhancement, of just selling
          the customer more than they need, so that they get
          great performance, and will always be happy. It is
          a technical error, that can cause big problems, in
          how the system performs, and what it is like to
          live in the house..

          > The counter argument - of course - is a smaller
          > unit will extract more moisture from the air.

          That is only one of the very important reasons
          why an energy efficient house (which will require
          the system to run WAY less, even if it is properly
          sized) needs to have a properly sized system, not
          an oversized system. So, since you already know
          why, then do not oversize the heat pump. It looks
          like (guessing at values), at design temperature,
          even if the house is quite leaky (¼ACH) that it
          would only need about 8000BTU/hr. And, about
          2000 of that would actually come from about a
          third as much electricity as an average America
          household, and body heat (leaving 6000 for the
          heat pump). Use as much electricity as an average
          American home, and a small house like that (not
          super insulated) will heat itself when it is about
          36°F outdoors, with no heater, and no Solar
          gain contribution.

          > Some of the discussion I've read is that it is not
          > good to overwork the mini splits.

          Energy efficient houses do not over work them.

          Proper sizing does not over work them.

          > Remember I am not building a super insulated house.

          I think I understand (basically) what you are
          building, and also the climate in which you are
          building it. In your climate, good insulation,
          but not "super insulation" is the optimal choice,
          along with Solar tempering, and ideally a small
          sunporch, to provide isolated, highly controllable
          Winter Solar gain, and to reduce Summer Solar
          gain, on part of the South wall. Using that strategy
          can provide all but a very small amount of heating,
          even in January, the toughest Solar heating month
          in SE Tennessee. In that climate, even better
          performance is possible, but requires consideration
          for heat storage (and maybe other things)

          > Energy savings will be because it is small, has minimal
          > windows on the NW side and 3 large windows on the
          > SE side.

          How far (degrees) is that from true South?

          > Also a lot of the NW wall will be closet space and
          > kitchen cabinets.

          That should have no significant effect,.

          What is your roof and wall construction?

          > The homeowner can retrofit the porch (SW side) with
          > sliding glass doors to make it a sun space.

          A sunspace is excellent (significant Solar gain, with good
          control, and no heat loss) Clear double glazed aluminum
          sliding glass doors (white or bronze finish) at about $250
          each, can be set in a very simple timber frame, so they
          only need a simple quarter-round (or similar) as trim. It
          looks great, and costs less than the cheapest 2x4 walls.
          But, SW is a tough orientation in that climate, unless the
          glass can face SE, at the South corner. Even that may
          be tough (Summer) depending on the actual direction,
          shading, etc.

          > Although my handyman says used sliding glass
          > doors are harder to come by these days.

          Yes. With remodeling down, there are fewer
          being removed. However, you do not need
          used. New ones are cheap. I am currently
          doing a design for a client in NE Oklahoma, a
          climate similar to yours, in which we will use
          13 of them, cheaper than wood frame walls.

          -Laren Corie-
          Natural Solar Building Design and
          Solar Heating/Natural Cooling/Energy
          Efficiency Consultation Since 1975
          www.ThermalAttic.com (many new
          photos and pages, coming soon)

          Read my Solar house design articles in:
          -Energy Self-Sufficiency Newsletter-
          www.essnmag.com

          Home base-LittleHouses YahooGroup
          http://groups.yahoo.com/group/LittleHouses/

          Founder-WoodGas - Power from wood
          http://groups.yahoo.com/group/WoodGas

          Founder-RefrigeratorAlternatives YahooGroup
          http://groups.yahoo.com/group/RefrigeratorAlternatives
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