XPost: rec.arts.comics.strips
From: tkoenig@netcologne.de
Paul S Person schrieb:
> On Sun, 11 Jan 2026 17:23:03 -0500 (EST), kludge@panix.com (Scott
> Dorsey) wrote:
>
>>Thomas Koenig wrote:
>>>
>>>What is R-20 and R-30? Another kind of liberty unit? :-)
>>
>>Yes, it's a measure of thermal resistance. Ironically, it appeared in the
>>US at a time when we were actively moving toward the metric system, back
>>in the seventies.
>>
>>Nobody outside of construction folks actually use it. It was never mentioned
>>in my thermo class. I saw how it was calculated and it was kind of a mess.
>
> Can I take it that actually measuring the effect with various
> materials is not how its done?
If I understand it correctly, the R - value is the inverse of
the U-value (which I am more familiar with).
If you have a temperatre difference across a wall (or something
similar) there will be a heat flux across it. The heat flux is
given as power per area, so it has the dimension of W/m^2 K.
q = Q/A, where q is the heat flux, Q the heat and A the area.
For a solid wall, the temperature difference ΔT will be proportional
to the heat flux. This can be expressed in two ways:
q = U * ΔT or q = ΔT/R (if that it what it is), where U has
the unit of power per area and temperature, dimension W/(m^2 K)
(in liberty units probably BTU/(ft^2 deg F) or something like that.
High U values mean high heat conduction.
How do you calculate U? In a simple case, a homogenous wall made
of a simple isotropic a wall with thickness L will have
U = λ / L, where λ is the thermal conductivity of your material,
which is a property you can look up. Metals have high thermal
conductivity, isolation is specially designed for low thermal
conductivity. Air has very low thermal conductivity, but if left
to circulate, can transport a large amount of heat; therefore many
insulating materials (including clothing) basically keep air from
moving. (During an internship in the US, I actually once
looked up a thermal conductivity which was given as BTU per square
foot for a temperature gradient of one degree Fahrenheit per inch -
not even internally consistent. I returned the book and politely
asked for SI units).
If you look at walls made up of layers different materials, the
U value is the sum of the reciprocals of the individual walls.
It's not called U any more, but the formula then is
U = 1/(L_1/λ_1 + L_2/λ_2 + ... + L_n/λ_n)
So, if you know the design of your wall and the individual values
of λ for your materials, you can calculate U. If you're using
R (which seems to be the inverse), the formula is easier, you
just add them up.
>
> (eg: closed room, various panels, heat/cold source on the outside,
> thermometers on the walls/windows inside and out at the same height)
>
> The /real/ question is: how is the thermal efficiency claimed for a
> given oil furnace computed? Does someone just pull it out of his *ss
> or is it something Marketing comes up with? Particularly measure
> claiming to take into account the existing ducts.
You can determine that pretty well from measurements of flue
gas composition and temperature.
>
> Note that the thermal efficiency of a radiant electric heater is,
> IIRC, 100%. I once explored this a little bit to try and figure out
> the relative economics, but got lost in the details. Also note that
> the current push against oil furnaces, at least up here, is based on
> air pollution, not inefficiency or inadequacy.
If the heating oil has low sulfur content and the nitrous oxide
values are controlled, that should not be a problem. Here, there
are mandatory annual measurements by a chimney sweep to control
CO and NOx.
--
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--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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