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Re: Spa (Hot tub) Renovation - Gas? Solar? - Page 5

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Posted by daestrom on October 30, 2004, 1:14 pm

This assumes that the inside surface absorbs and re-radiates all IR though.
Since glass is not completely opaque to IR, I think this may off.  If the
surface is not completely opaque to IR, some of the IR from your body goes
right through to the outer pane, and even outside.  While some of the IR is
absorbed in the glass and re-radiates at a surface temperature of 57.5F.

I found an interesting article, http://irc.nrc-cnrc.gc.ca/cbd/cbd052e.html  
In table I, for 0F outside they show a similar number for the temperature
drop to the inside surface.  About half-way through it, they discuss the
radiant heat transfer mode between the inner and outer pane.  Their
calculations show that about 63% of the heat transferred from the inner pane
to the outer is by radiant heat transfer.  This is due to the two pane's

But it doesn't discuss the transmission of IR from one side directly through
to the other (which is what I'm wondering about here)

And then the 'ugly' question of just what to use for the 'outside'
temperature comes into it.  At night, sky-lights might be between you and
the upper atmosphere.  On particularly clear nights, the IR losses might be
extreme.  Windows in walls might allow radiant heat-transfer (to a limited
degree) between your body and the snow-covered ground, trees/structures at
20F, or even the sky.

Much easier to just 'close the curtain' than to calculate all those
particulars ;-)  If I could just convince my wife....

Interesting.  I wonder if a 'home-made' version would work?  I wonder what
factors are important to such an instrument (diameter? surface finish?
emissivity of coating I suppose?)


Posted by nicksanspam on October 30, 2004, 3:11 pm

Seems reasonable, esp for ordinary glass, which only transmits about 25%
at 2 microns (2150 F), about 5% at 3 microns, 3% at 10 microns (80 F)
and so on. Low-iron glass transmits about 80% at 2, less than 10% at 3,
3% at 10, and so on down.

You may be wearing clothes, and standing far away from the window...

I think theirs was homemade. The kit also contains a plastic garbage bag
bolometer, vs the usual $K device for measuring airflow :-)

Just emissivity, I suppose. Most surfaces are close to 1...


Posted by TimR on November 1, 2004, 8:18 am

you must be correct and nick wrong, because it is extremely common to
have people next to windows feeling cold.

The outside temperature may easily be 70 F lower than inside, if you
are considering heat transfer to the surrounding objects.  If you
consider transfer to the sky, it is even lower (low enough to account
for freezing water at air temperatures well above 32 F).

We have to consider radiant transfer through the glass.  Treating the
glass as a surface is obviously misleading.  (just think how much heat
comes IN when the sun shines <grin>)

I've handled complaints from a large office building.  It has a decent
VAV system with plenty of capacity for heating.  (Don't do much
heating even in the winter, but it has reheat coils and they work)
The office workers who sit next to the windows, and refuse to draw the
blinds, complain about being cold all winter, in Virginia.  The air
temperature is fine, the electronic controls monitor the space and
prove this.  We've tried raising the air temperature several degrees,
but it isn't enough to offset the radiation heat loss out the window.
The body feels cold even when it isn't, because it senses the rate of
heat transfer.

Posted by daestrom on November 1, 2004, 10:40 pm

While I'm glad you agree with my earlier posts, I have to say that I'm not
so sure anymore.  My research shows that glass is indeed opaque to IR
(wavelengths of about 2.7 microns and longer).  Water outside with no glass
will, as you point out, freeze on particularly clear, low-humidity nights
because of radiant cooling.  The old weather forecast saying, 'clear and
cold' is the layman's explanation for it.

And things like humans radiate almost 100% of their radiant heat in the
infrared, longer than the point of glass being opaque.

The heat gain from sunshine is different because more than 96% of the sun's
radiant energy is given off in wavelengths *shorter* than 2.7 microns.  So
almost all the energy (about 86%) can come directly through a perpendicular
piece of glass.  The objects inside the window, re-radiate at much longer
wavelengths, that are blocked by the glass.

I've been looking into this a bit further, and as Nick had already pointed
out, the inside surface temperature of a double-glazed window can run in the
mid 50's (F).  So if you stand in the middle of a large, empty room with 70F
air and 70F walls, and one window, you have a small portion of your body
radiating towards the window's inside surface, and most of your body
radiating towards other surfaces.  You feel 'comfy'.  But if you step over
towards the window, the total portion of your body radiating towards the
window's inside surface rises drastically.  Although the surface area of the
window is constant, the solid-angle of how much of your body radiates to it
rises.  If your body were a single point and the room a sphere, the solid
angle rises with 1/R^2 where R is the distance from you to the window.

I think in your large office building situation, you have some workers
sitting right next to the window (say, within 3 feet).  And a window that is
6-8 ft high means the solid-angle exposed to the window is a significant
fraction of the total spherical surface.  So that person may have 40% of
their body exposed to a ~50F radiant surface and the other 60% to 70F
surfaces.  While a person halfway across the floor (who can barely see the
window behind their cubicle wall) is radiating perhaps 10% to the ~50F
surface and 90% to the 70F surfaces.  Hence, the window person 'feels' a lot
colder because they are losing more heat through radiation.

Closing commercial, plastic blinds can help this situation, *IF* the blinds
themselves are warmer than the window surface (probable).


Posted by nicksanspam on October 28, 2004, 12:04 pm

How cold? To investigate this, you might make TA = 70 F and
TR = 70-R0.66(70-30)/(R20+R0.66) = 68.7 in that BASIC program,
for an R20 wall on a 30 F day.

Then again, warm air rises. Harry Thomason used a central supply with the
heat source in the basement and a long slotted grill near the ceiling to
make a thermal chimeny and floor returns near outside walls to make good
natural airflow.

This keeps the outside walls slightly cooler. We might make up for that
by slightly raising the room temp, and end up using less energy.


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