My wife and I are having a discussion about radiator placement. We are in
the process of having our basement renovated. The rooms in the basement
have an 8-inch "shelf" about a foot and a half from the floor along the
outer walls of the building (part of the footing, since we had to dig down
to make the ceilings high enough). Currently, our contractor has placed
our hot-water radiators on the shelf under windows. My wife is convinced
that the radiators will never properly heat the rooms since heat goes up.
I believe that the room is actually warmed by being filled with heat
(otherwise, you would need radiators on all walls and even in the middle of
the room to heat the floor).
While I believe the optimal placement of the radiator would be on the
floor, the sizing of the radiator is MUCH more important, and the saving of
floor space in this case is more important that the minimal increase in
Can anyone add any comments or guidance on this issue?
Time will tell, but assuming you will heat full time, then I would guess
it will work. The deeper you go the more even temperature and better
insulated the wall. Need the floor you have almost 8 foot of soil as
insulation. The wall above and just below ground will be the coldest.
At worse you may want to add some sort of fan to mix the air. It can be
a ceiling fan or a floor fan. It would not need to be large.
It will work just fine. Way back in the mid 60's I worked for a major
manufacturer of heating products. Two of them were Window-line and
Sill-line systems. They are installed in buildings all over the country.
In your case, it may work better than the floor.
As you said, heat goes up, not out, so it probably doesn't make a big
difference. Most of the heat in that room will be cooler air dropping
from the ceiling (which is why ceiling fans work well even in the
I once had a basement apartment with steam heat. Since the boiler was
on the same floor but about 40 feet away, the radiators were mounted
up near the ceiling. It was the hottest apartment I ever had.
Radiators literally radiate radiant heat and with air convection will
heat fine. I hope a load calculation was done, the real issue is enough
btu. If you mix different types, cast iron and baseboard then you will
have an issue. But if it was planned by a pro who knows, [ many don`t],
you will be fine.
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.
Nick your numbers mean nothing , Do you live in a house, have you ever
worked on one, have you ever done any heating or remodeling. Im suprised
you dont tell them to flood the floor for humidity like you have been
My calcs showed very little difference in energy or comfort for radiators
near or far from walls. We might redo them for windows. But it looks like
we can heat a house with 40% less fin-tube if it's below a hot air column.
That could be a nice capital savings. My kitchen has a 2'-high commercial
"convector," an empty box with fin tube near the bottom which moves more
heat than the same length of baseboard fin tube. Lots of HVAC people can
look at the specs and see that convectors have more output and pick one
when there isn't enough wall space for baseboard, but fewer seem to know
why convectors have more output, or how to build-in vs buy one.
Humans radiate at the same rate, but warmer surfaces radiate more back.
Air is transparent to radiation. Warm air makes no difference to radiation,
but warmer windows help. So do drapes and clothing. If you sit near a large
cold window, the surface cools you a lot more than if you sit farther away.
The mean radiant temp is the average of the surroundings weighted by solid
angles to a body. An 8' cube with 5 70 F walls and an 8'x8' R2 window with
an R2/3 inner air film and an inner glazing at 30+2(70-30)/(2+2/3) = 60 F
makes the mean radiant temp in the room center about (5x70+60)/6 = 68.33,
and we need to turn the thermostat up to 71.28 to make it feel like a 70 F
room with no window, according to the ASHRAE 55-2004 comfort standard, and
this increases the room heat loss by about 1.28x8'x8'/2.67 = 31 Btu/h.
If a radiator under the window makes the glazing 70 F and destroys the inner
air film, the 70 F room heat loss increases by 10x8'x8'/2 = 320 Btu/h, 10X
more than letting the window stay cooler and raising the room air temp.
You misquoted. The OP was right, and wrong, for "placement" in closets.
Or he could be thinking of conduction where a smooth surface feels cooler
than a textured surface of the same temperature due to the contact area
actually against the skin. People often associate certain materials as
being cooler than others even though they are at the same temperature.
While entirely correct, this reply does little to explain. An improvement
Yes - but less than we radiate to it, thus the _net_ heat transfer is from us
to it, and therefore we feel cold.
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
Heat doesn't rise, HOT AIR rises. That is why a hot air balloon works.
Half of the heat from the radiator is convection, the other half is
radiation. The convection part will heat the air next to the radiator,
which will rise up to heat the cold window above it. The radiation
part will heat the entire room, including the sunken floor. Your
current configuration should work fine, as long as the radiator is
I'm afraid you are an ignorant judge in this domain.
Arrogant too... You often have helpful ideas, but we
could easily come up with a long list of Ransleyisms
like "Is 1 kWh equivalent to 450 Btu?"
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