Posted by nicksanspam on January 25, 2007, 2:18 pm
Tom writes from MN:
Or night ventilation, or a few $0 window ACs with occupancy sensors?
No good. If a utility turns 3 Btu of coal into 1 of electricity and
you turn it back into 3 of heat, you might as well burn the coal :-)
NREL says 430 Btu/ft^2 falls on the ground and 820 falls on south walls
on an average 17.9 F December day with a 25.5 daily max and a 22 average
daytime temp and a -12 97.5% design temp and a 44.9 deep ground (water)
temp. December is the worst-case month for solar house heating. The
warmest month is July, with a 73.6 average temp and a 63.6 daily min.
Going back to basics, a 48x48x8' tall house with 192 ft^2 of R4 windows
and 30 cfm of air leaks and US Rv walls and ceiling would have a thermal
conductance of 48x48ft^2/Rv Btu/h-F for the ceiling plus 192/4 = 48 for
the windows plus 1344 for the walls plus about 30 for air leaks, a total
G = 78+3648/Rv Btu/h-F.
If it's exactly 70 F for 12 hours on an average December day and exactly
60 at night, it needs 24h(65-17.9)G = 1130G Btu/day of heat. A US average
900 kWh/mo of indoor electrical use could supply 102.4K Btu/day or 4265
Btu/h. Adding 50K Btu/day of solar hot water, we need 1130G-102.4K+50K
= 35.7K+4122K/Rv Btu/day of net solar heat.
If a $ square foot of Thermaglas Plus U0.58 twinwall polycarbonate
"solar siding" with 80% solar transmission with 100 F air inside has
a net gain of 0.8x820-6h(100-22)0.58 = 385 Btu/day, a 48'x8' south wall
can provide 148K Btu/day.
If 148K = 35.7K+4122K/Rv, Rv = 36, something like 9" SIPs or TGI studs
with cellulose fill with G = 179 Btu/h-F. If the electrical gain adds
4265/G = 23.8 F to the outdoor temp, making it 41.7 F, and the house is
60 F for 12 hours per day, we might store 12(60-41.7)G = 39.3K Btu in
C = 39.3K/(95-65) = 1310 Btu/F of thermal mass with lots of surface
cooling from 95 to 65 F overnight, eg 1310/5.12 = 256 hollow 8x8x16"
concrete blocks in a 28x8' wall or core or closet with a 2 watt motorized
damper and a room temp thermostat. This might also act as a downward duct
for thermosyphoning sunspace air. As an alternative, we might fill 81
3-hole blocks with 20 8' thinwall 4" PVC water pipes.
For 5 cloudy days in a row, we might store 5x24(65-41.7)G = 500K Btu in
500K/(140-75) = 7700 pounds or 924 gallons or 124 ft^3 of water cooling
from 140 to 75 F in a 4'x8'x4' tall EPDM-lined plywood box. On a cloudy day
we could pump some water up from the box through a MagicAire SHW2347
800 Btu/h-F duct heat exchanger near the floor in the core. A $0 300'x1"
piece of black PE pipe in the box could act as a DHW heat exchanger.
We might collect 50K Btu/h of DHW with A ft^2 of 140 F bare hydronic
absorber plates or ersatz Big Fins (about $/ft^2) inside the 384 ft^2 of
solar siding if 0.8x820A-6h(140-100)1.5A = 50K, ie A = 169 ft^2. As an
alternative, we might collect hot water in a draindown solar pond in
the attic to the north of a Dynaglas clear corrugated polycarbonate
south roof, under a reflective north roof.
Bad idea. Radiant slabs hinder night setbacks, which save energy.
Posted by Paul M. Eldridge on January 25, 2007, 6:22 pm
In Minnesota, a ground source heat pump (GSHP) would offer low cost
heating and cooling, as well as inexpensive domestic hot water.
Unfortunately, upfront costs are rather high and the system must be
designed and installed by someone well qualified to do the job.
In the case of new construction, the extra cost of a GSHP can be
rolled into the mortgage and what you save on utilities should more
than cover the extra carrying cost; in other words, there should be a
net positive gain right from day one. Moreover, the low operating
costs of a GSHP would offer a measure of protection from volatile
energy prices (thereby providing some peace of mind) and it should
presumably add to the resale value of your home.
Although an air-source heat pump isn't likely to offer quite the same
energy savings given your colder climate, initial costs are much lower
and installation is simpler and in some ways, less risky. If natural
gas is available, I would recommend a dual fuel arrangement where the
air-source heat pump operates down to 25F say, then automatically
switches over to natural gas as temperatures continue to fall (the
exact cutover point may be higher or lower as recommended by your
heating consultant). If you have already determined you want central
air, the incremental cost of going with a heat pump is fairly modest
and you will gain an inexpensive source of heat as well as cooling.
Adding a desuperheater would also allow you to capture a portion of
the waste heat from summer cooling, which can be used in turn to
preheat your domestic hot water.
For an overview on dual fuel systems, see:
Again, as with the GSHP, if the cost of dual-fuel system is included
in your mortgage, your lower energy costs should result in a net
positive cash flow.
Obviously, you'll want to do everything you reasonably can to minimize
your home's heat loss and summer heat gain, i.e., by adding extra
insulation, careful air sealing and ensuring suitable summer shading.
Likewise, you'll want to maximize any potential passive solar gain
(nothing beats the cost of free heat).
Also keep in mind that as you move further away from a conventional
home to one that is increasingly more energy efficient, the economics
of a more expensive heating system will diminish. Some of the more
advanced, energy efficient homes here in Canada require so little
heat, electric baseboard heaters are the preferred heating solution (a
more expensive heating system simply can't be justified). As a rough
guide, larger homes or homes with relatively high heating and cooling
demand would tend to favour a GSHP system; for homes with more
moderate demand, a dual fuel system (natural gas furnace/heat pump)
might be a better solution. Lastly, homes with very little heating
and cooling demand might be best served by a conventional gas or
electric furnace and, if required, a standard central air system.
Posted by Robert Scott on January 25, 2007, 8:32 pm
On Thu, 25 Jan 2007 18:22:03 GMT, Paul M. Eldridge
I don't think air-source heat pumps are an energy saver anywhere. They are used
mostly in southern climates that rarely need heat.
At 25F you are already down to a COP of nearly 1.0. You might as well just use
electric resistance heat. The times when this can be used in Minnesota are very
rare. You would be much better off burning natural gas all the time. Forget
heat pumps unless you can work against 35F and above most of the year.
Posted by Paul M. Eldridge on January 26, 2007, 12:07 am
On Thu, 25 Jan 2007 20:32:27 GMT, ---@--- (Robert Scott) wrote:
I'm afraid your impression of air source heat pumps is in need of
updating. The current minimum federal standard for air source heat
pumps is a HSPF of 7.7 (climate regions IV and V). That translates to
be a seasonal COP of about 2.3.
I live in Halifax, NS which falls under region IV. Halifax is rated
at 7,861 heating degree days and Minneapolis-St.Paul comes comes in at
7,882 HDD, so we're basically at par. Last year, my heat pump, which
has a HSPF rating of 7.2, achieved a seasonal COP of 2.45. In
fairness, last winter was warmer than normal, so its COP in an average
winter would be closer to 2.1.
There are air source heat pumps available now with a HSPF rating as
high as 10, which puts their seasonal COP at over 2.9.
That's not really the case. Consider, for example, the Friedrich
M18YF ductless heat pump. At 47F, it produces 21,600 BTU/hr
(6.33 kW) and its COP at this temperature is 3.66. At 17F, heat
output declines to 16,600 BTU/hr (4.86 kW), for a still quite
respectable COP of 2.81. At 32F (the half-way mark between these two
temperatures), we can reasonably assume its COP falls somewhere in the
range of 3.2.
So how do their relative operating costs compare? After all, from the
consumer's point of view, that's likely to be the only metric that
One therm of natural gas contains 100,000 BTUs or 29.3 kWh(e). At 90
per cent conversion efficiency (e.g., a high efficiency condensing
furnace), our net heat gain is 26.4 kWh(e) per therm. If a therm of
gas costs $.90, our equivalent cost per kWh is $.0341.
If electricity costs $.10 per kWh, at 47F, our reference heat pump
produces heat at the equivalent of $.0273 per kWh. At 32F, assuming
our estimated COP of 3.2 holds true, the cost climbs to $.0312 per
kWh(e), and when the outdoor temperature falls to 17F, it rises to
$.0356 per kWh(e). That suggests the economic balance point, at
these assumed rates, would be somewhere in the range of 22F.
I should add these are conservative numbers. According to the DOE's
Energy Information Administration, the average 2005 residential retail
price of electricity in MN was $.0828 per kWh.
CenterPoint Energy, one of the largest distributors of natural gas in
Minnesota is currently selling natural gas at $.06 per therm.
If natural gas costs $.06 per therm and electricity retails for
$.0828 per kWh, our economic balance point falls well below 17F and
if our customer happens to opt for a mid efficiency furnance, it drops
further down again.
Posted by Louis Geoffroy on January 26, 2007, 5:41 am
On Fri, 26 Jan 2007 00:07:52 GMT, Paul M. Eldridge
Your impression of air source heat pumps is in need of updating.
Here in Montreal, I'm using the Fujitsu 24RL which is the same
model as the M24RF of Friedrich. This model use the new gas R410A, it
has a HSPF of 10 and a SEER of 18. The heating rating is 27600Btu/hr
at 47F and the cutoff temperature is officially at -4F but in practice
at -8F. I did cutout only one this year at that temperature.
It's -1F outside now, and the heat pumps is still working at a
COP of around 2.0 and heating the complete house. Before at 0F, my
house was using around 90kw/day with electric resistance heat , now
it's using 47Wk/day with the heat pump.