[Greenbuilding] Ductless Heat Pump Performance

nick pine nick at early.com
Tue Dec 6 07:15:24 PST 2011

Paul Eldridge <paul.eldridge at ns.sympatico.ca> writes:

>I'm not inclined to alter the appearance of our home in any material way...

You might enjoy a lawn sculpture.

Paul Ladendorf <paulladendorf at ...> wrote:

> I think I'd be better off... building an super-insulated shed attached to 
> the house and put a tank in it and blow the heat through a radiator.

A superinsulated tank inside a solar-collecting shed that gets cold at 
night? You could circulate warm air through a shed attached to the house 
during the
day, with a heat distribution radiator to warm the house with tank water on 
cloudy days. You could preheat water for showers with a $60 pressurized 
13-gallon plastic pipe coil in the tank.

>Then I can use the attic for my music studio.

There you go :-) How much heat does your house need at 65 F on an average 
28.9 F December day in South Bend with the sunspace doors closed? Divide 
each exterior surface in square feet by its R-value and add those numbers 
up? How many square feet of R40 attic ceiling? How many square feet of R24 
walls and R? windows? Your 640 cfm 50 Pa blower door measurement corresponds 
to about 32 cfm of natural air leakage (enough for 6 full-time occupants in 
a clean house), which adds about 32 Btu/h-F to the house conductance.

Or buy 2 or 3 $15 120 V 1500 watt space heaters with thermostats and keep 
the house 65 F for 24 hours with no help from the sun or the woodstove and 
record the outdoor temperatures and the electric meter readings before and 
after? If the house is 65 F indoors and the average outdoor temp is (say) 40 
and the difference in meter readings is (say) 35 kWh, the house conductance 
is 35x3412/(24h(65-40)) = 200 Btu/h-F...

So it would need 24h(65-28.9)200 = 173K Btu on a 28.9 F day, or 866K Btu for 
5 cloudy 28.9 F days in a row, eg 866K/(140-80)/62.33 = 232 ft^3 of water 
cooling from 140 to 80 F in a 6'-wide x 4'-tall x 10'-long plywood tank with 
a $100 folded 18'x20' EPDM liner inside a lawn sculpture with a 12'x24' 
footprint and a $750 24'x24' twinwall polycarbonate south wall with a 60 
degree slope and a 24'x21'-tall insulated north wall.

NREL says 430 Btu/ft^2 of sun falls on the ground and 580 falls on a south 
wall on an average December day with a 32 F daytime temp in South Bend, so 1 
ft^2 of R2 twinwall with 80% solar transmission and a 60 degree slope would 
receive 430cos60+580sin60 = 717 Btu/day. With 100 F air inside, it would 
collect about 0.8x717-6h(100-32)1ft^2/R2 = 370 Btu, so a 24'x24' south wall 
would collect 213K Btu/day.


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