Iain 332k BTU on those February days...
Nick 90K Btu/h in full sun.

Iain lose 50k BTU through the windows.
Nick 5.4K Btu/h.

We're in agreement here, Nick.

Iainhe'll need to vent over 1000 cfm at 80F (from outside 55F).

I calculated 1500 cfm for venting the incoming radiation as heat at 80F
from 55F, averaged across the few hours of insolation. But I figured
he'd lose some through the rest of the house. Your number, below,
is at the peak heating rate and assumes no other loss. Both of us,
incorrectly, assume that the full area of the window is insolated.

Nick cfm = (90K-5.4K)/(80-55) = 3384, approximately.

"Cancelling" air's 0.02 BTU/ft^3-F and 60 minutes/hour, so nobody can
follow your units.

Iain...and many, many tons of water.

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

And *right here* is where you miss your opportunity to help a lot of
people.

The windows make this house. The interior was deliberately opened
up so that most rooms could see the incredible views. If the house
had no windows, your penpal would have bought a different house.

As you keep pointing out with your cube analysis, we could live
comfortably on far less energy. In fact, people all over the earth
do just that every day.

But you advocate going farther down the path of form following function
than most people care to. Ultimately, that gets you ignored by most
folks, and defeats any aspirations you might have to lowering overall
consumption. Windows are not put in houses primarily to gather heat.
They are there to make us happy.

Engineering analysis that says not to put in the windows is not useful.
To be useful, analysis must minimize energy consumption while
constrained to other, more important goals, like having a nice view or
cheery lighting most mornings, or having the house look a certain way.

A house in a 4000 degree-day environment can get through cold sunny
days just fine with solar heating alone so long as it can store the
heat.
If the heat will be stored in the habitable envelope, the water will
only
swing about 10 F before things get uncomfortable. We can fairly easily
guess the volume and surface area of this storage, and with R1
windows it is essentially a foot-thick sheet of water the size of the
windows. Modern glassy houses thus need to contain very large
amounts of water, or even larger amounts of something else to store
their heat. This isn't usually practical in a retrofit.