Radiator placement

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 heating efficiency.
Can anyone add any comments or guidance on this issue?
tia
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Beartums wrote:

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.
--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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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.
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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 winter).
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.
Steve Manes Brooklyn, NY http://www.magpie.com/house/bbs
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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.
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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.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Currently,
windows.
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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.
R
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You like the worst of both worlds? :-)
Nick
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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 doing.
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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.
Nick
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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.
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And restored.

Thanks. Two engineering degrees and 40 years of practice have helped.

Sounds like an engineer :-)

It's hard to argue with reality.

It warms them up.

Absolutely. That's 300 years of settled physics.
Nick
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While entirely correct, this reply does little to explain. An improvement might be:
Yes - but less than we radiate to it, thus the _net_ heat transfer is from us to it, and therefore we feel cold.
-- Regards, 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?
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Beartums,
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 sized right.
Stretch
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And restored.

Thanks. Two engineering degrees and 40 years of practice have helped.

Sounds like an engineer :-)

It's hard to argue with reality.

It warms them up.

Absolutely. That's 300 years of settled physics.
Nick
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And restored.

Thanks. Two engineering degrees and 40 years of practice have helped.

Sounds like an engineer :-)

It's hard to argue with reality.

It warms them up.

Absolutely. That's 300 years of settled physics.
Nick
(I wonder who keeps cancelling this posting...)
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The second time you posted this today Nick , but your ideas still stink.
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And somebody cancelled it for the third time...

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?"
Nick
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Well nick you take the cake, like flooding a basement for added humidity. A true classic
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