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
http://wiki.diyfaq.org.uk/index.php?...Heating_Design
(Section "Convection or radiation?")

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!

"John Stumbles" wrote in message
news
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
http://wiki.diyfaq.org.uk/index.php?...Heating_Design
(Section "Convection or radiation?")

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.

--
Mike W

On Feb 8, 10:35 am, John Stumbles wrote:
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?


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

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.

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.
--
fred
Plusnet - I hope you like vanilla

On 8 Feb, 11:50, "adder1969" wrote:
On Feb 8, 10:35 am, John Stumbles wrote:

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?

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.

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:


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.

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.

--
Skipweasel
Pay no attention to that man behind the curtain.

John Stumbles wrote:
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
http://wiki.diyfaq.org.uk/index.php?...Heating_Design
(Section "Convection or radiation?")

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!

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.

fred wrote:
In article , visionset
writes
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.

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

To the 4th power IIRC.

But that is from *absolute* zero..

and I probably
have a thermodynamics book somewhere to prove it but just can't get that
much enthusiasm together.

sm_jamieson wrote:
On 8 Feb, 11:50, "adder1969" wrote:
On Feb 8, 10:35 am, John Stumbles wrote:

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?
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.

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.

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:-

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.

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.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Thu, 08 Feb 2007 10:59:03 +0000, fred wrote:

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.

That was what this specific discussion was about: whether radiant energy
drops off linearly with temperature difference or non-linearly. Actually
wikipedia gives this
http://en.wikipedia.org/wiki/Black_body_radiation
as related to the 4th power of the body's temperature (Stefan-Boltzmann
law), and the net heat output related to the difference between the heat
output of the warm body and that of its surroundings. In all cases of
course the temperatures are absolute so the difference between a radiator
at say 75C and a warm floor (or wall) at say 25C is actually the
difference between (273+75)^4 (approx 14.7E9) and (273+25)^4 (~7.9E9)
which is a bit under 2:1. In other words a radiator radiates about twice
as much heat per unit surface area as a warm floor or wall does.

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.

John Stumbles wrote:
On Thu, 08 Feb 2007 10:59:03 +0000, fred wrote:

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.

That was what this specific discussion was about: whether radiant energy
drops off linearly with temperature difference or non-linearly. Actually
wikipedia gives this
http://en.wikipedia.org/wiki/Black_body_radiation
as related to the 4th power of the body's temperature (Stefan-Boltzmann
law), and the net heat output related to the difference between the heat
output of the warm body and that of its surroundings. In all cases of
course the temperatures are absolute so the difference between a radiator
at say 75C and a warm floor (or wall) at say 25C is actually the
difference between (273+75)^4 (approx 14.7E9) and (273+25)^4 (~7.9E9)
which is a bit under 2:1. In other words a radiator radiates about twice
as much heat per unit surface area as a warm floor or wall does.

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.

Thus it would seem that the output of a warm floor is almost entirely via
radiation.

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.

John Stumbles wrote:

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.

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"...

--
Andy

"visionset" wrote in message
...

"John Stumbles" wrote in message
news
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
http://wiki.diyfaq.org.uk/index.php?...Heating_Design
(Section "Convection or radiation?")

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.

90% is convection from a rad.

"adder1969" wrote in message
ups.com...

Thus it would seem that the output of a warm floor is almost entirely via
radiation.

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.

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.

"Guy King" wrote in message
...
The message
from fred contains these words:


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.

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.

--
Skipweasel
Pay no attention to that man behind the curtain.

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.

--

Brian

On 8 Feb, 14:47, "Brian Sharrock" wrote:
"Guy King" wrote in message
...

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.

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:

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.

Ah, that's memory, that is.

--
Skipweasel
Pay no attention to that man behind the curtain.

John Stumbles wrote:

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.

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.

Link he

http://en.wikipedia.org/wiki/Stefan-Boltzmann_law


--

Frank

On Thu, 08 Feb 2007 13:24:08 +0000, Andy Wade wrote:

John Stumbles wrote:

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.

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"...

I agree I read somewhere that the balance is about 1/3 radiative to
convective on a hot radiator.

Clearly with the more fins and cooler temperatures the proportion of
radiated heat will be even less.

On something like a warm floor or wall i suspect that the proportion of
radiative heat might be quite a bit more. Although the temperatures are
lower the areas are also much greater. The convection will also be less
effective as much of the upper half of the wall will be trying to heat air
that is already warm and flowing up past it!

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards