Warm air rises, and cool air falls near exterior walls. Why fight nature?
IMO, a radiator near an exterior wall is less efficient, since it's in
slower air, since it has to fight downgoing air. It also loses more heat
to outdoors, keeping the wall warmer than a central radiator would and
making turbulent vs laminar flow near the wall, which increases the wall's
film conductance. Harry Thomason knew this, but we've mostly forgotten it.
Warm air rises over hot spots, slides along the ceiling to cooler spots,
drops to the floor, slides back to hot spots, rewarms and rises. Warm
ceilings radiate heat to the rest of the room with a surprisingly high
linearized conductance: 4x0.1714x10^-8(460+70)^3 = U1.
We might keep a room with an 8'x8'/R16 = 4 Btu/h-F exterior wall 70 F on
a 30 F day with a central radiator that heats C cfm from 70-dT to 70 F,
where CdT = (70-dT/2-30)4 = 10-dT/8. If 70 F air flows down a 8' wide
virtual duct that extends 1' from the wall into the room and 70-dT air
flows out the bottom and C = 16.6x1x8sqrt(8dT), dT = 0.163 F, the wall
loses (70-0.163/2-30)4 = 159.67 Btu/h, ignoring the turbulent flow. A
radiator below might make it lose (70+0.163/2-30)4 = 160.33 Btu/h. No big
deal, altho the difference is larger for windows with less insulation.
OTOH, warmer walls allow lower room air temps, for the same comfort level.
A cube with 5 70 F walls and a 70-0.163/2 = 69.919 F wall and a radiant
temp of 70-0.163/2/6 = 69.986 F would need 70.0104 F room air for equal
comfort compared to 70 F air and 70 F walls, according to ASHRAE 55-2004,
so the loss from the warmer wall outweighs the gain from cooler air.
With 180 F water in 5 Btu/h-F-ft fin tube, the central radiator might have
159.67/(5(180-69.84)) = 0.29' of tube in slow-moving air near the floor.
But the bouyancy force of a column of warm air in some sort of chimney
above a tube can move air by fins at a higher velocity and raise their
water-air conductance. If a foot of fin tube has a conductance of 5 Btu/h-F
= A(2+V/2) in V = 0 mph air, its effective area A = 2.5 ft^2. (I counted 43
2"x2" fins per foot, about 2.4 ft^2, including both sides.) Fin tube near
the floor in a closet or stairwell or inside wall with an A ft^2 vent at
the bottom and top and an 8' height diff between them and a dT temp diff
from room to chimney air should make C = 16.6Asqrt(8dT) = 47Asqrt(dT) cfm
flow with velocity V = 0.01136C/A = 0.533sqrt(dT) mph, so 1' of 180 F tube
would lose (180-69.84)2.5(2+0.533sqrt(dT)/2) = 551+73.4sqrt(dT) = cfmdT
= 47AdT^1.5, ie dT = ((11.73+1.56sqrt(dT)/A)^(2/3). With a 2"x12" slot,
A = 0.167, so dT = (70.4+9.36sqrt(dT))^(2/3). Plugging in dT = 10 on the
right makes dT = 21.5 on the left. Repeating makes dT = 23.4, 23.7, and
23.7, so 2.6 mph air might come out of the top vent at 69.8+23.7 = 93.5 F,
moving 47(0.167)23.7^1.5 = 906 Btu/h of heat, so we only need 159.67/906
= 0.176' of fin tube, ie 40% less than fin tube in free air.
The closet might be a nice place to dry clothes.
An excellent example of why you should keep calculators away from
people with internet access and too much time on their hands.
All of your calculations mean nothing with respect to heating a room
evenly. Efficiency and comfort are not necessarily the same thing.
Amen, my brother !
His calculations are meaningless in EITHER domain. Nick has
one skill set, and one skill set only - piling bullshit upon bullshit
upon bullshit, and falsely claiming that he can 'prove it by some book
Oh - and writing inane little pretend-programs that suppose
and propose to support his babble.
Just because you can't understand a concept, doesn't mean it's babble.
Just means you should be comprehending instead of typing.......
Nick has been a foundation of this group longer than your daddy has been
letting you use his computer .....
Dir., Green Trust
On Wed, 23 Mar 2005 18:36:10 GMT, Steve Spence
Depends which group of Nick's obnoxious
cross-posting-for-fame-and-dollars groups you refer to.
In MY group, SEHVAC, which **I** helped found **LONG** before
Nick bumbled into it, Nick is regarded as a complete asshole
space-cadet, whose foundations lie only in mock-science, and who has
*ZERO** credibility for his bullshit, which has been disproven
Oh, I almost forgot - kiss my ass. Learn who you're talking
to before you speak, junior.