Keeping warm via hot air: works by conduction, but
not conduction like sitting bareassed on a warm
block of steel (much heat transferred to you
per second),

but via super UN-dense air, with minuscule ability
to store heat (per cubic foot) than eg steel.

I recall from a thermodynaics course eons ago
that whatever the materials, the amount of heat
energy flowing from one to the other, per second,
is 100% *proportional* to the temperature *difference*
between the two.

So, while an 80-degree (F) block of steel would
be able to transfer enough heat to you (r butt) to
keep you toasty comfortable even with an open window
with 30 or 40 degrees outside, perhaps, no can do
via hot air. Temp diff must be much greater than
the 99.6 - 80 = 20-degree difference via the block of
steel.

So you gotta blow hot air at you for the same warming,
but so hit that it's stifling (sp?), making you feel
really crappy, even nauseaous.

Then, there's heating via radiation, like by a cast-iron
radiator, an electric (fanless) radiator, the sun, etc.

Those pictures of resort-like ski-areas, all these people
lying around a swimming pool, in their bathing suits,
when it's maybe 30-degrees -- they're kept warm by
radiation from the sun (and by having the wind blocked).

And it's a *much* nicer experience than being kept warm
only by a blast of hot air (sun blocked from warming you).

----

Nice thing about cast-iron radiators is that you can
open the window, allowing nice (cold) fresh air to
circulate through the room somewhat, and yet you're
still warm (via the radiation).



(I grew up in Texas (san antonio), and our house had
forced hot-air, and it would almost make me vomit.
Later, I went way north, and experienced the cast-iron
radiators -- what a difference!!!! YMMV, however )

David