# Re: How to reduce House heating cost

Posted by nicksanspam on May 13, 2006, 5:42 pm

... 14,000 Btu per hour?

Where I live near Phila, 1000 Btu/ft^2 of sun falls on a south wall on
an average 30 F January day. If 90% of that (900 Btu) enters a \$ square
foot of Dynaglas corrugated polycarbonate glazing and the air behind it
is 100 F, it will lose about 6h(100-30) = 420, for a net gain of 480.

With 16'x24' (\$84) of sunspace glazing, we might collect 184K Btu (54kWh)
of heat on an average day at a rate of 184K/6h = 30.7K Btu/h with a 100-70
= 30 F temp diff and a 30.7K/30 = 1022 cfm airflow between the living space
and the sunspace, with a 16' height diff and a 1022/(16.6sqrt(16x30))
= 2.8 ft^2 vent area. We might open a 2'x2' foamboard damper with a room
temp thermostat in series with a sunspace thermostat and a \$0 2 watt
Honeywell 6161B1000 damper motor that operates for 3 minutes per day,
consuming 2x3/60 = 0.1 Wh/day, for a COP of 54K/0.1 = 540,000.

Nick

Posted by SJC on May 13, 2006, 8:48 pm

I favor both. A south facing sunroom with air heating from the low sun =
in winter
from the side with an active solar thermal liquid/air collector array on =
the roof of
the sunroom with heat pump. That way I can heat the home during the day =
with air
heat and have the warm water with heat pump to warm the house at night.

Posted by nicksanspam on May 14, 2006, 5:06 am

But heat pumps are expensive, with a very low COP, compared to some solar
systems, and a house can store overnight heat from sunspace warm air in
its thermal mass. Big Fins (TM) or fin-tube pipe near the top of an air
heater inside a sunspace can collect and store higher temp heat in a tank
for a few cloudy days and hot water for showers, with help from
a greywater heat exchanger.

We might store overnight heat in thermal mass under a foil-covered ceiling,
eg a flat helix with 10'x4" PVC pipes full of water, with no water movement
on average days and tank water flowing through the pipes on cloudy days,
with a slow ceiling fan and a room temp thermostat to bring down warm air
as needed.

Nick

Posted by SJC on May 14, 2006, 5:13 pm

You might do lots of things. But until you can prove anything actually
works with many installations and hard data, then it is still in the =
might stage.

Posted by nicksanspam on May 14, 2006, 9:28 pm

It's been done many times. This is 300-year old physics :-)

Nick

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