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Re: Desperate for advice on replacing dead 255K BTU furnace in 3200 sq foot house

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Posted by nicksanspam on November 11, 2005, 12:48 pm
 


My Canyon (2 miles from Oakland) CA friends don't believe in insulation,
nor anything else in the building codes :-)


... 255K Btu PER HOUR seems like a lot. NREL says 1050 Btu/ft^2 of sun
falls on a south wall on an average 48.7 F January day in San Francisco
with a 55.6 daily max. ASHRAE's 99% winter design temp is 35 F, ie it's
warmer 99% of the time. Solar house heating is _extremely easy_ in SF.


Trust and believe me. Stretch and Turtle are OK, but orthodox. SQlit
is dogmatic, mransley sometimes acts like an arrogant idiot, Meehan
and Radwinski could learn more about heatflow, and so on.


... 255K Btu/h for 3200 ft^2? Does this house have walls? :-)
You probably need a blower door test and more insulation.


Lovely :-)


That's OK in CA. A square foot of R1 south window with 90% solar transmission
would gain 0.9x1050 = 945 Btu on an average January day. On a 24-hour living
space, it might lose 24h(65-48.7)1ft^2/R1 = 391 with a net gain of 554. On
a low-thermal mass sunspace, it might lose 6h(70-52) = 108, netting 837.


Congratulations! You have a solar-heated house! It needs more thermal mass,
maybe upstairs or below the ceiling, with an insulated wall between all
those windows and the 24-hour living space and airflow between the new
"sunspace" and the living space that stops at night.


You might "open windows" with a thermostat and an exhaust fan...

NREL says 830 Btu/ft^2 of sun falls on a south wall on an average 52.2 F
February day in San Francisco with a 60.8 daily max. A square foot of R1
south window would gain 0.9x830 = 747 Btu and lose 24h(65-52.2)1ft^2/R1
= 307, for a net gain of 440. On a low-thermal mass sunspace, it might
lose 6h(70-56.5) = 81, for a net evil gain of 666 on an average Feb day.


My advice: forswear thy fossil fuels and improve the solar performance.
It's easy, with high-school physics and simple arithmetic.

Nick


Posted by Iain McClatchie on November 11, 2005, 7:36 pm
 

For folks who don't know the Bay Area, Canyon, CA is a famous
place.  Check it out:

http://maps.google.com/maps?oi=map&q=Canyon,+CA

It's the green spot between Pinehurst Rd and Shephard Canyon
Rd, right in the middle where it looks like there are almost no
roads.  If this guy's got a view of the bay, then he's probably on
the south-west side of Manzanita Way.

Decades ago, a bunch of people decided to build there against
the official wishes of Oakland.  They build houses and I think
even short roads, completely without city support.  I think they're
on-grid now, after decades of wrangling, but maybe not.

Anyway, I just wanted Nick to know that just from riding around
in the area I can say it gets a *lot* colder there at night than it
does in San Francisco.  This guy probably has a monster furnace
because that was cheaper than insulation back when the house
was slapped together.

Nick> Solar house heating is _extremely easy_ in SF.

Solar house heating *during the day* is easy near SF.  It's the
nights that get you.  This guy's probably got 800+ ft^2 of glass
and probably sees days of 20+ degree-days heating.  That's
400k BTUs a night just through the glass, he might have to
double that for the total (tons of fresh air).  The furnace is just
a bit oversize for such a leaky house, but the house is a bad
design in an environment of pricey fuel.

And thermal storage is hard.  Note that this guy is sitting
more-or-less on top of the Hayward fault, probably on a sloping
lot with poor footings, and probably has fairly weak construction.
Placing a few dozen tons of thermal mass on the second floor,
nevermind the ceiling, would be irresponsible (although very
much in character for the neighborhood).

If he's seriously lucky, he's got room on the north side of the
house for some thermal storage, maybe even a basement
cut into the hill with a floor that can take some load.  If that's
the case, maybe he could get some sort of loop going
between the downstairs storage area on the north side and
the upstairs southern living space.

Nick> ... 255K Btu/h for 3200 ft^2? Does this house have walls?
Nick> :-)  You probably need a blower door test and more
Nick> insulation.

Yep.  That could make a serious dent in about half the problem.
But that glass is going to be expensive to replace.  If he's near
broke, I think it might be reasonable to consider insulation
against the glass at night.  Ugly, inconvenient, and cheap.

Nick> Congratulations! You have a solar-heated house! It needs
Nick> more thermal mass, maybe upstairs or below the ceiling,
Nick> with an insulated wall between all those windows and the
Nick> 24-hour living space and airflow between the new
Nick> "sunspace" and the living space that stops at night.

Your low-mass sunspace is probably a nonstarter, as it would
mean he has to give up a sizeable fraction of the house for all
but ten hours of the day (and most of those hours he's not home
during the weekdays).  My guess is he's got the master
bedroom on the south side as well.

Nick> You might "open windows" with a thermostat and an
Nick> exhaust fan...

Yep.

OP> I am looking to cut costs b/c I'm pretty broke right now,
OP> but not at the expense of safety, comfort level, or
OP> decreasing the value of the house by not getting the
OP> right furnace installed.  Meanwhile we have no heat,
OP> and I've no idea which (if any) of these contractors I
OP> can trust.

Nick> My advice: forswear thy fossil fuels and improve the
Nick> solar performance.   It's easy, with high-school physics
Nick> and simple arithmetic.

Thermal storage will cost a lot more than a bigger furnace
and burning more fuel, especially in the short term.

My advice:
1) More insulation, especially attic insulation.
2) Blower test, and then patch those leaks.  This could lead
    to getting a new door or something, but hopefully it's just
    weathersealing.  PG&E might pay for the blower test.
3) Thermostat-controlled exhaust fan, for those hot days.
4) Insulation on the windows at night.  This is ugly, but maybe
    you could do something creative like have interior shutters
    that hinge down from the ceiling with a drawstring.  The
    insulation could be 2 inch thick extruded polystyrene slabs
    covered in cloth to make them look nice, and also to help
    get a reasonably airtight fit between the insulation and the
    window frame when deployed.  This could save 2-3 therms
    ($-5) per night, if you're disciplined about getting those
    shutters down every night before you go to bed.
5) Sweaters.

A big furnace is going to be cheaper than thermal storage.
My wild guess is that you're now burning 1500 therms/year
for heating.  You can probably get that down to 600-700
therms/year without thermal storage, assuming the window
insulation.  So that says you can probably get away with a
half-size furnace, maybe 100k BTU/hr.  The money you
save versus a replacement 225k BTU/hr furnace might
offset most of the cost of the insulation updates.


Posted by Iain McClatchie on November 12, 2005, 8:25 am
 Nick> You might "open windows" with a thermostat and an exhaust fan...

If I'm right about that 800 ft^2 of glass, and if 400 ft^2 of it is
passing
sunlight, then he's getting 332k BTU on those February days you
quoted.  While the sun is up, he'll lose 50k BTU through the windows.
Assuming he loses a good chunk elsewhere and has no significant
thermal storage he'll need to vent over 1000 cfm at 80F (from outside
55F).  Not exactly an 8" NuTone bathroom fan.

Nick> My advice: forswear thy fossil fuels and improve the solar
Nick> performance.  It's easy, with high-school physics and simple
Nick> arithmetic.

...and many, many tons of water.


Posted by nicksanspam on November 12, 2005, 11:28 am
 

... 400 ft^2 of single glass might pass 0.9x250x400 = 90K Btu/h in full sun.


... and lose (70-56.5)400/R1 = 5.4K Btu/h.


... Significant improvements are possible.


... cfm = (90K-5.4K)/(80-55) = 3384, approximately. With your command
of numbers, you might consider a different preoccupation, eg surgery :-)


Maybe 3 $2 20" window box fans, if he wanted to waste all that solar heat.
Or 2 $5 2470 cfm Lasko fans, or a single whole house fan.


With enough insulation, it wouldn't need any water, nor windows.

A 32' R32 cube with no air infiltration would need 24h(65-48.7)5x32^2/32
= 62.6K Btu on an average Jan day in SF. It might come from 18.3 kWh/day
or 550 kWh/mo of internal electrical usage, 2/3 of the 833 US average.

With no internal heat gain, it might stay warm for 5 cloudy days with
5x62.6K/(130-80)/2000 = 3.13 tons of water cooling from 130 to 80 F in
a 3.13x2000/1024/62.33x12 = 1.2" layer under the 4th floor ceiling. If
A ft^2 of R1 sunspace glazing with 90% solar transmission over the R32
south wall gains 945A Btu/day and loses 6h(130-56.5)A = 441A and makes
62.6K Btu net, A = 124 ft^2, eg an 8'x16' sunspace or $00 patch of
polycarbonate solar siding.

Solar house heating is extremely easy in San Francisco.

Nick


Posted by nicksanspam on November 12, 2005, 10:39 pm
 
The 1st floor ceiling would be better, with less insulation above it and
less heat loss to the outdoors.


ie 945A - 441A = 62.6K makes A = 124 ft^2.



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