Following on (tangentially) from the discussion of warm walls ("Look no rads") what are the mechanisms by which heat is emitted in conventional radiators and in warm floors or walls?
This stems fro an unresolved discussion in
Since there are just 2 of us discussing it there and one of us is struggling to recall ancient school Physics I hope we'll get some more authoritative answers here!
AFAIK. Raidant heat is InfraRed radiation, a waveform that is part of of the electromagnetic spectrum. Convected heat is transported by the movement of air, hot air rises and moves the air around in a kind of shunting mechanism.
A conventional radiators does both. Raidates directly and convects above it circulating warm air around the room. All forms of heat will do both to some extent. But where convection is favourable this form is usually the most useful.
If you have a high power spot lamp the direct heat you feel from it is radiated heat. In a cold room on it's own the lamp will warm the room by some convection and if it is pointing at something it'll heat that by radiation and then that will cause some convection. If you have a lot of low power bulbs the direct heat off them is low but they'll still warm the room by convection if they're warmer than the surrounding air. So, you either have a small amount of hot things which radiate, let's call them, um, radiators, or a larger surface area of not so hot things such as a warmed floor. In the end it's convection that keeps the rooms warm.
In article , visionset writes
I agree conventional radiators do both, you can get a feel for the proportion of each that heats the room by placing a hand near the side of the rad to feel the radiation then placing it above to feel the hot air flow. An old book I read reckoned about 25% of heat was radiated and 75% came from convection but that was probably from the days of 80degC mean rad temperatures. With lower rad temperatures nowadays I reckon it will have dropped to 10% or so, certainly feels about that on my 45degC ones.
I think radiation increases exponentially with temperature and I probably have a thermodynamics book somewhere to prove it but just can't get that much enthusiasm together.
I'm always impressed by the radiant heaters used at market stalls (posh ones !) etc, where a gale can be blowing, but the heating effect is unabaited. The physics is simple, but it still seems neat. You just expect the heat to be blown away ! Simon.
The message from fred contains these words:
Watch where the cat sits. If it sits in front of the radiator it's radiation, if it sits on top, it's convection. Or it can't get on top.
I think its about 80% convection and 20% radiation.
Black painted radiators are not SIGNIFICANTLY better..same goes for black painted heat sinks..
Yes, there is a measurable gain, but its not dominant..not unless airflow is very restricted.
To the 4th power IIRC.
But that is from *absolute* zero..
and I probably
To get significant radiant heat you need VERY high temperatures.
Whats true for a red hot piece of metal is not true for a radiator.
On Thu, 08 Feb 2007 10:59:03 GMT someone who may be fred wrote this:-
The proportions vary depending on the design of radiator. Add a convector strip to the back of a steel panel radiator and the proportion of convected output will be increased. Add a second panel with a second convector strip to the back and that will boost the convected proportion even more.
That was what this specific discussion was about: whether radiant energy drops off linearly with temperature difference or non-linearly. Actually wikipedia gives this
I don't know what rho(?) and epsilon mean - let alone have values for them
- in the formula given for net power radiated. I assume A is area. Working backwards from the example they give of radiation from a human I get a factor for rho*epsilon of 3.7E-8. Applying this to a surface at 29C (302K) with surroundings at 20C (293K) I get 102 Watts per square metre which seems to agree with figures given for UFH of around 100Watts/square metre. Thus it would seem that the output of a warm floor is almost entirely via radiation.
I would have said it was the exact opposite. almost all by convection..but there ya go.
I think your constants are not right.
If something is hot, it'll be radiating heat. Any (liquid) heating system radiates heat. The heated air then moves around the room i.e. The heat convects.
If you huddle around a log fire in an otherwise cold room, you'll be making use, on the most part, of radiated heat. if you leave it on long enough the rest of the room will be making use of convected heat.
Almost all domestic heating ends up being through convection.
Sigma (not rho) is Stefan's constant - value 5.67E-8 W/(m^2.K^4) - this is the fundamental constant linking temperature and power radiated per unit surface area.
Epsilon is the surface emissivity - unity for a perfect black body and probably 0.8-0.9 for a painted radiator. The paint colour makes no difference at these long wavelengths - only shiny metal surfaces give really low emissivity values.
As a rule of thumb I reckon about half the output of a single panel rad at normal working temperatures is radiation (including that from the back which heats the wall and is re-released as convection), and half by direct air convection. Obviously the proportion is lower for DP and 'convector' type emitters.
Radiation plays a greater part in this than many people seem to think. Well insulated rooms feel comfortable at a lower air temperature than in poorly insulated buildings with cold walls. Look up the concept of "mean radiant temperature"...
90% is convection from a rad.
Yep, that is why they have fins on the rear of rads. Have a room which has evenly heated walls, ceiling and floor, and the vast majority of the heat is radiant heat in the room. And the room temperature can be down very low.
We tend to have the room temp up far too high to compensate for draughts, cold spots and radiant heat being extrated from our bodies by cooler surfaces.
Radiant heat is line of sight.
Does the team think: cats can see into the infra-red' Observation; a cat will walk into a room, glance around then settle on/into the warmest place in the room; Telly, lap; window ... wherever: they don't hunt around then return - just go directly to the warmest location.
yes, its a known thing. When viewing in IR, animals hiding in a bush are very visible, when in RGB theyre not. No surprise cats are good hunters.
NT
The message from "Brian Sharrock" contains these words:
Ah, that's memory, that is.
Epsilon and sigma have been explained by another poster.
Your estimate of sigma, the Stephan-Boltzmann linked earlier in that article, is actually a combination of that constant and the emissivity (epsilon). Not a bad result, really.
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