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PTAC or mini split

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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 1 of 4 , Mar 20, 2013
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      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 2 of 4 , Mar 24, 2013
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        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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