Radiant heat in the ceiling vs the floor

[Colbyt]

I wonder how steaks get cooked in a broiler?

To say is doesn't work as well would be a fair statement.

[terry]

Gee whiz. We were wondering how the heat from a radiant heat lamp mounted, say, above a bathtub could warn the person standing below it, guess we were wrong? And those radiant ceiling heaters seen in some retail stores don't work, eh? Even though one can 'feel' the heat from them beaming down.

Also does the radiant heat form the sun; travels up, down, sideways horizontally vertically, at an angle .................. oh forget it Van! You must just be 'having us on' i.e. joking! :-)

And the old chestnut; 'How does radiation (in form of heat/light) travel through a virtual vacuum from sun to earth?

BTW an acquaintance left his 500 watt halogen lamp pointing downwards where it happened to heat up, rather severely, the black rubber surface of a spare tyre! So radiant heat does travel 'down'!

[Doug Miller]

If heat does not radiate down, perhaps you'd care to explain how the broiler element in an electric oven can cook meat.

You're apparently thinking of convection, which does indeed transport heat only upward.

[TimR]

The theory of radiant heat is to not to waste energy heating the air but use radiated heat directly where it's needed.

But of course the hot surface does heat the air near it, and convection does spread that heat.

I haven't been able to find any good description of how much heat gets to your body from the radiation vs from the air. But overhead heat is very commonly used in large spaces like maintenance facilities, garages, etc., where heating all the air would be cost prohibitive.

Cold panels in the ceiling for air conditioning are new to me, but are beginning to be used.

[Bert Hyman]

I've never heard of them; what's the mechanism for cooling them?

Offhand, I'd think they'd only be safe to use in places with very low humidity, otherwise the condensation would be annoying.

[Doug Miller]

Pumped circulation of cold water. The human body radiates heat to its surroundings; when those surroundings include a large mass that's significantly cooler than human skin temperature, you feel cool.

The panel temperature doesn't have to be at or below the dew point to be effective at making the room feel cooler -- it just has to be low enough to absorb a significant amount of the heat radiated from your body. Radiant cooling is markedly less effective in areas of high humidity for two reasons: first, the higher dew point means a higher minimum panel temperature, and second, because they are operated above the dew point, they don't dehumidify as forced-air air conditioners do.

[J. Clarke]

More to the point, heat _radiates_ in any direction, but heat transfer by radiation with small temperature differences is generally a relatively inefficient and slow process compared to convection and conduction. A broiler works because the heat source is only a short distance from the food and the temperature difference is hundreds of degrees. Put the heating element 8 feet away and see what happens.

[J. Clarke]

More to the point, heat _radiates_ in any direction, but heat transfer by radiation with small temperature differences is generally a relatively inefficient and slow process compared to convection and conduction. A broiler works because the heat source is only a short distance from the food and the temperature difference is hundreds of degrees. Put the heating element 8 feet away and see what happens.

[JR Weiss]

Not quite...

NATURAL convection will tend to transport heat upward..FORCED convection, as in a forced air furnace, can transport heat in any direction it is directed by fans, ducts, etc. Wind tends to transport heat sideways in the local case...

[CJT]

The other variable of interest, though, is the heat rise you seek. Just because you can't broil flesh from 8 feet away doesn't mean it's useless as a heater.

[TimR]

As we've tried to become more sophisticated at cooling living spaces like dormitories and military barracks, we've tried to separate the cooling function, the dehumidification function, and the fresh air ventilation function.

Fancoil units used to be cooled with chilled water and have a vent for fresh air. This resulted in an inability to control humidity and mold growth, and mold growth is a pretty hot topic now.

So we went to fancoil units with no vents, and supplied ducted dehumidified air to the space. That let us pressurize the building and control humidity, and let the comfort cooling be adjusted with the fancoil in each room.

The cold plate is an improvement on that idea. Rather than cool all the room air, we want to use radiant cooling to cool just the human in the room. This does require we supply pressurized dehumidified outside air via a separate duct system. You're right, if we don't control humidity it will rain in the room.

I've worked on building renovations with the separate fancoil and dehumidified air systems, but have yet to personally see a cold plate system. I suspect my employer will do one soon, hope so.

[Doug Miller]

Merely "tend to" ??? Seems you misspelled "always". :-)

No such thing. If it's fan-forced, it's not convection.

Wind isn't convection either.

[Jerry - OHIO]

I put it on my ceiling to keep the snow off my roof so Santa Clause wont fall off and sue me. Jr

[Wayne Boatwright]

On Sat 10 Jan 2009 12:00:18p, J. Clarke told us...

One downside is that the heating elements will eventually cause the plaster to yellow in the pattern of the element.

[Jeremy S. Nichols, PE]

So a convection oven does not use convection? Of course it does.

Convection is transport of heat by motion of a fluid. There can be mechanical convection, or there can be convection driven by gravity acting on density differences in the fluid.

[JR Weiss]

OK... Where have you found a general definition of convection that excludes forced convection or explicitly limits it to natural vertical movement?

[Doug Miller]

Consult a dictionary; I think you'll find that convection is by definition the vertical transport of heated fluids under the influence of gravity.

[JR Weiss]

I already consulted several dictionaries. While that is ONE definition of convection, it is NOT the ONLY definition, especially in context:

1.. The act or process of conveying; transmission. 2.. Physics. 1.. Heat transfer in a gas or liquid by the circulation of currents from one region to another. 2.. Fluid motion caused by an external force such as gravity. 3.. Meteorology. The transfer of heat or other atmospheric properties by massive motion within the atmosphere, especially by such motion directed upward.

1. The act or process of conveying or transmitting.
2. (Physics) A process of transfer or transmission, as of heat or electricity, by means of currents in liquids or gases, resulting from changes of temperature and other causes.

1.. a transmitting or conveying 2.. a.. the mass movement of parts of a fluid within the fluid because of differences in the density of different parts b.. the transfer of heat by its absorption by a fluid at one point followed by motion of the fluid and rejection of the heat at another point

transmission of energy or mass by a medium involving movement of the medium itself.

The circulatory movement that occurs in a fluid at a nonuniform temperature owing to the variation of its density and the action of gravity.

1. The act or process of conveying or transmitting.
2. Physics A process of transfer or transmission, as of heat or electricity, by means of currents in liquids or gases, resulting from changes of temperature and other causes.

1 : the circulatory motion that occurs in a fluid at a nonuniform temperature owing to the variation of its density and the action of gravity 2 : the transfer of heat by convection in a fluid

Current in a fluid caused by uneven distribution of heat. For example, air on a part of the Earth's surface warmed by strong sunlight will be heated by contact with the ground and will expand and flow upward, creating a region of low pressure below it; cooler surrounding air will then flow in to this low pressure region. The air thus circulates by convection, creating winds.

[GregS]

Direction has no effect on radiation. It goes all directions.

greg

[Shawn Hirn]

Heat rises, so placing the radiant heating elements in the ceiling makes no sense. What that does is heat the space above the ceiling.