[Greenbuilding] Distibution and radiant heat and Morse stove solution

[nick pine]

RT <ArchiLogic at yahoo.ca> writes:

> Once a house is well insulated/air-sealed and its massing, orientation and  
> landscaping have been reasonably well considered, the building heat load  
> will be so minimal that occupancy gains and solar gains should provide  
> most of it (speaking "typical"-sized houses, say 1600 - 2800 sf) in Cold  
> Climate Regions, say, > 7500 HDD/yr)

Bullshit.

Then again, Sustainable Building Industry Council masonry salesmen say
reasonably well considered lukewarm sunspaces with LOTS of thermal mass
(bricks, according to SBIC Brick Institute of America members :-) can give
a max 60% solar heating fraction, even here in PA, with a mere 4954 FHDD. 

Then again, "most" can leave a lot. If a department store sells a $1000 shirt
with a 60% discount, it's still a $400 shirt. Direct gain houses with minimal
fuel bills in cold climates cost a lot, because windows on living spaces lose
heat on cloudy days. It's better to focus on absolute fuel consumption than
fractional savings.

If cloudy days are like coin flips (a reasonable assumption, according to my
TMY2 simulations) a house that can keep itself warm for N cloudy days in a row
has a 1-2^-N solar heating fraction, eg 50% for 1 day, 75% for 2, and so on... 
90% requires -ln(1-0.9)/ln(2) = 3.3 days, 95% requires 4.3...

A direct gain house in Minneapolis (7981 FHDD) in December will cool from
70 to 60 F in -RCln((60-17.9)/(70-17.9)) = 0.213RC cloudy hours, where RC
is the house time constant in hours, so a 95% solar fraction requires RC
= 4.3x24h/0.213 = 484 hours = C/G, where C is the house thermal mass in
Btu/F and G is the house conductance in Btu/h-F (with nicely cancelling
units: C/G = (Btu/F)/(Btu/h-F) = h.) 
     
A 40'x60' house fully-loaded with room temperature thermal mass might have
a 6" floorslab with 6"/12"x40x60x25 = 30K Btu/F plus 2(40+60)x8x5 = 8K Btu/F
hollow concrete block walls, with C = 38K Btu/F and G = C/484 = 78 Btu/h-F.

With A ft^2 of U0.25 south windows with 50% solar transmission (a low U-value
to maximize cloudy day heat storage) and 820 Btu/ft^2 of sun on the south wall,
24h(70-17.9)(0.25A+Gmax) = 410A makes A = 12.8Gmax, where Gmax only includes
the ceiling and wall and ventilation conductances and 0.25A+Gmax < 78, which
makes Gmax < 5.7 Btu/h-F, with A = 72 ft^2 of south windows. In a perfectly 
airtight house, a 30 cfm 90% ERV adds about 0.1x30 = 3 to Gmax, leaving 2.7
for walls and ceiling, eg 5928 ft^2 of 5928/2.7 = R2196 walls and ceiling,
eg R5/inch Styrofoam walls and ceiling, 36.6 feet thick :-)

> the remaining auxiliary heating requirement will be so small that
> a wee woodstove is quite adequate as the auxiliary heat source.

After a few cloudy days, the wee woodstove needs to supply all the house heat.
How much wood do you have to feed it over a 7981 FHDD Minniapolis winter, if
the house walls are less than 36 feet thick? :-)

> True, one could probably get close to 100% of the building's heat load  
> from occupancy/solar gains by making the house look like one of Nick  
> Pine's wet dreams, but curiously enough, most people seem to like having  
> windows on their homes and have their houses look like houses rather than  
> Grade 6 science experiments gone awry.

It's fun to live in science experiments gone awry :-)

Nick
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