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