Our house has old-fashioned cast iron radiators warmed by hot water from th
e boiler. We're painting one of the rooms in which the radiator has been pa
inted over with flat latex wall paint to match the walls. I'd rather not pu
t another layer of paint on as that will probably reduce the efficiency of
the radiator but I can't leave it the old color.
My first thought was to pull it out and have it sand blasted but that would
be a big, messy job. I'm sure that just a wire brush wouldn't get into all
the spaces in the radiator.
Second thought was to get some chemical to remove the paint; sort of like n
aval jelly but for paint instead of rust. Does anyone know of a good chemic
al to use? (We still have a bottle of sulfuric acid left over from my wife'
s fireworks experiments a few years back but I'm not going to mess with tha
Final idea for the morning is to build a decorative but well-vented frame a
round it, like we have in the dining room. However, that would probably red
uce the efficiency of the radiator more than another thin coat of paint.
Any ideas on the best way to proceed?
On Thursday, May 22, 2014 12:41:11 PM UTC-4, Pavel314 wrote:
the boiler. We're painting one of the rooms in which the radiator has been
painted over with flat latex wall paint to match the walls. I'd rather not
put another layer of paint on as that will probably reduce the efficiency o
f the radiator but I can't leave it the old color.
ld be a big, messy job. I'm sure that just a wire brush wouldn't get into a
ll the spaces in the radiator.
naval jelly but for paint instead of rust. Does anyone know of a good chem
ical to use? (We still have a bottle of sulfuric acid left over from my wif
e's fireworks experiments a few years back but I'm not going to mess with t
around it, like we have in the dining room. However, that would probably r
educe the efficiency of the radiator more than another thin coat of paint.
I did some googling and found the following:
A common myth is that too many layers of paint will reduce the heating effi
ciency of a radiator. According to my research this just isn't true with on
e exception. Metallic paints apparently reduce the ability of the radiator
to transfer heat, so you may want to avoid those kinds of paint.
From http://www.dundee-memorialpark.org/ohm/index.php?cat=Painting&artPainting Radiators I.txt
I agree with Vic Smith on this one.
Radiators work by CONVECTION. The hot new paint will convect warm air
into the room just as efficiently as hot iron or hot old paint.
But, check to see if the old paint is oil based or latex. If it's oil
based, you might need to apply a coat of oil based primer first, and
then paint with the new colour of latex paint. Latex paints don't
generally stick well to oil based paints.
I literally have hundreds of feet of cast iron baseboard radiators in my
apartment block, and all of those baseboard radiators are painted.
I wouldn't worry about another layer of paint but:
Strypeeze works but it is messy as hell and you'd wind up wire brushing and
scraping. Also, it's basically methylene chloride in gel form. Meth isn't
bad as acetone or MEK but you wouldn't want to take a bath in it.
You'd want to pull the radiator anyway and while I had it out, I'd just take
it to get sandblasted. If you have trouble finding someone to sandblast,
check the tombstone guys. They're often willing to break the monotony of
engraving little angels or whatever and clean up old iron. There isn't
anything on a radiator to mask or fiddle around with so it's
On Thursday, May 22, 2014 11:36:00 PM UTC-4, rbowman wrote:
I can see going to the trouble of removing paint if it's
necessary because the old paint is in such condition that
you can't just paint over it. But the stated reason for
removing the paint was concern over efficiency. That's
incorrect. Another layer of paint will have insignificant
effect on heat transfer. And even if it resulted in the water
leaving the radiator 1 deg warmer, so what? Even that heat
is not totally lost, like it went out the window.
It just flows on through the system.
If the paint is sound, just paint over it and save a lot
of time and/or money.
US Department of Commerces National Bureau of Standards
July 19, 1935.
It will appear that as far as their effect on the performance of
radiators is concerned, paints fall into two classes. First, those in
which the pigment consists of small flakes of metal, such as the
aluminum and bronze paints, most commonly used for painting radiators,
which produce a metallic appearance and will be called metallic paints.
Second, the white and colored paints, in which the pigment consists not
of the metals but of oxides or other compounds of the metals. Thus white
lead paints, or those containing compounds of zinc or other metals, will
be called non-metallic paints. These non-metallic paints are obtainable
in practically all colors, including white and black, while the metallic
paints have the color of the metal or alloy of which the flakes are
We will state at the outset the principal conclusion, which will be
explained in more detail later, that the last coat of paint on a
radiator is the only one that has an appreciable effect. And that a
radiator coated with metallic paint will emit less heat, under otherwise
identical conditions, than a similar radiator coated with non-metallic
paint. In order to obtain the same amount of heat from the two radiators
just considered the temperature of the one painted with metallic paint
must be somewhat higher.
Science proves that the finish of a radiator affects its heat output in
There is a principal known as "emissivity" that enables experts to
measure the ability for heat to leave (or radiate from) the surface of
Levels of emissivity vary between finishes of radiators. Painted
radiators have a higher level of emissivity than bare metal radiators,
meaning that painted finishes absorb and release heat more than bare
metal finishes. Matt finishes have a higher level of emissivity than
gloss radiators. Even the colour of the finish can affect the level of
emissivity. For instance, black paint has a higher level of emissivity
than white paint. However, the difference in the emissivity of radiators
is negligible and would only be realised in laboratory conditions.
Only a chrome finish has a noticeable affect on the heat output of a
radiator as chrome has a very low level of emissivity. The chrome
coating works on the same principal as the space blankets (the silver
insulation blankets) used to keep athletes warm. The chrome coating,
whilst looking beautiful, does reduce the ability of the radiator to
radiate heat. Chrome (chromium plated) radiators are proven to emit
approximately 20% less heat than the equivalent sized radiators in a
I don't think that considering a radiator's emissivity is a useful way
to know if painting a radiator is a "good" thing from a heat-transfer
POV. If you held a thermometer or pointed one of those cheap
battery-powered spot infrared thermometers at a radiator surrounded by a
vacuum (ie - no air or gas or atmosphere between the sensor and
radiator) then you'd be experiencing and measuring emissivity of the
radiator, and the color and surface characteristics of the radiator
would play a huge role in your reading.
But radiators mostly don't heat rooms by emissivity. Direct thermal
transfer of heat to the surround air, and then air movement in the room
and transfer of heat from the air to objects in the room, and primarily
to surfaces where the room is losing heat such as floor, ceiling and
walls, is how radiators work.
A barely perceptible breeze produced by, say, an ultra-low-power/low
speed fan directing air past a radiator can have orders of magnitude
difference in how much heat is extracted from a radiator per unit time
in a room with stationary air. (you can measure heat extraction by
knowing the difference in water temperature between the inlet and outlet
of the radiator, all other things being equal such as water flow rate).
On Friday, May 23, 2014 10:09:15 AM UTC-4, HomeGuy wrote:
You left out the most imporant and essential part:
"And since neither the boiler efficiency nor the heat wasted in the pipe li
nes is appreciably affected by small
changes in radiator temperatures, practically the same amount of fuel is re
quired to supply the heat in each case. In
other words, while it may be desirable for various reasons to avoid the use
of metallic paints on radiators, no appreciable
saving in fuel will result from the use of non-metallic rather than metalli
Also note that the rest of their discussion just throws
out examples based on theory, with them freely admitting
the numbers used are not actual, just apparently numbers
pulled from nowhere for the purposes of showing an example.
In other words, they don't show any real world data from
testing that they measured.
It's not clear if the above 20% is based on the *total*
heat emitted or just 20% of the radiated part. If it's on the
radiated part, then the overall total impact is way less
A fan would have huge impact compared to paint color, that's
for sure. But even then, as the other article points out, it's not
like if you extract more heat from the radiator, it's getting heat
that would otherwise go out the window. The extra heat not extracted
just leaves the radiator and flows on into the rest of the system.
Before there is any hot air that convects, the air has to be heated, and
the radiatior does that by radiation. That's why it's called a
While unradiated heat isn't lost and goes back as warmer water to the
boiler, it will take longer to heat a room, and this matters when you've
set the heat down overnight, when it turns cold maybe several times in
a season, if you've turned the heat down when you go away, or whenever.
The idea that it's only 1 degree warmer is just speculation afaik.
What if it is 20 degrees? (If the water temp at the boiler is 180,
that's a 110 degree difference to start)
OP, instead of asking these people I'd call a reliable heating company
or two that does hot water and steam heat. Just ask for advice. Call
between 4 and 5 when they've usually done all their work for the day and
are not under pressure to do something. Ask them whether it matters
and how you should clean the rads. if they ssay it matter.
As to the guy quoted who said he'd done research, I'd like to know what
sort of research he did. The link didn't work fo me.
No, Micky, that's ridiculous. Radiators work by convection.
And, radiators are called radiators for much the same reason that latex
paint is called "latex" paint. Somebody call it that and the name
stuck. Happens all of the time.
Here, this is what a typical cast iron baseboard radiator looks like
from the front:
And, here's what that same radiator looks like from the back:
What do you have on the BACK of a baseboard radiator? Why it's fins!!!
The ONLY reason to put fins on the back of the radiator is to increase
the amount of air CONVECTION out the slots near the top of the
radiator. Cold air comes in at the bottom of the radiator, gets warmed
by contact with the finned back side of the radiator and convects out
the slots near the top of the radiator. Those fins on the back of the
radiator greatly increase the amount of surface area over which
convection occurs, so MOST of the heat actually comes from the hot rear
surface of the radiator, not from the hot front surface of the radiator.
Heck, you could even INSULATE the front of the radiator and still have
MOST of the heat coming out the slots at the top of that insulated
radiator because of the larger surface area at the back of the
Here's another one... a finned tube baseboard radiator:
Notice that the tube has fins on it. Those aluminum fins are there to
increase air CONVECTION.
If that radiator worked by radiation, it would be dumb as a bag of
hammers to put a great big steel plate in front of those fins to block
the radiant heat from getting into the room, wouldn't it? And yet,
that's exactly what this company has done. So, we're left to conclude
that either the engineers working for this company are all morons, or
our supposition that radiators work by radiation is flawed. In fact,
it's the latter. Radiators work by convection, not radiation. The heat
flows by conduction from the water to the copper tubing to the aluminum
fins, and thereafter by convection of air into the room.
And that's why you can paint that "Heavy Duty Cold Rolled Steel Cover"
on that radiator without concern about affecting the efficiency of heat
transfer into the room.
On Saturday, May 24, 2014 5:35:54 PM UTC-4, nestork wrote:
They work to some degree by both convection and radiation, it's
not limited to one or the other. Any time you have a hot object
it's going to radiate heat. More heat is likely transfered by
The OP didn't say he had a baseboard radiator. He said:
"Our house has old-fashioned cast iron radiators warmed by hot water from the boiler. "
Which I think that probably means this:
You are forgetting conduction. If the heat was not conducted from the
water to the tube and then to the fin, there would be no convection. The
fins increase the conduction to they can radiate the heat to the air
where it is carried into the room by convection.
I think Ed and Trader should have convinced you that it's not
After the air is heated by radiation and some conduction, then there is
How do you think the air gets heated. Do you think the iron convects?
But not here.
No it's to increase the amount of air that touches the radiator and gets
hot by conduction and radiation. After the air gets hot, it rises and
is replaced by new air, but that's after it gets hot.
But you're not thinking of the OP's radiator, anyhow.
No, by that time, the air has been warmed and it rises. The cover is
there to make it look okay. I'm sure it does lower the heat output.
Why do you think there are only two choices?
How did it get from the fins to the air? Did you not notice that you
left that step out?.
Oh, it's very possible that he was. But regardless of the style of cast
iron radiator, the method of heat transfer from the hot water to the air
in the room is by convection, not radiation.
No, even though SOME amount of heat transfer by radiation is going to
occur by virtue of a temperature difference, it's almost wrong to
mention radiation and convection in the same sentance here. You see,
heat transfer by conduction and convection are proportional to the
temperature difference. Not so for radiation. The rate of heat loss by
radiation is proportional to the temperature (in degrees absolute)
raised to the 4th power, or temperature squared squared. So, heat
transfer by radiation is tiny for normal temperatures that we see in our
every day lives. It only becomes important when the stuff we're looking
at is hot enough that it GLOWS, like red hot element on a stove. At
those higher temperatures, heat transfer by radiation quickly increases
rapidly because it's magnitude is proportional to that temperature ^4
term that makes the radiation component very big very fast.
Look at this:
'Heat transfer - Wikipedia, the free encyclopedia'
where it says under the section on radiation:
The Stefan-Boltzmann equation, which describes the rate of transfer of
radiant energy, is as follows for an object in a vacuum :
For radiative transfer between two objects, the equation is as follows:
where Q is the rate of heat transfer, ε is the emissivity (unity
for a black body), σ is the Stefan-Boltzmann constant, and T is the
absolute temperature (in Kelvin or Rankine). Radiation is typically
only important for very hot objects, or for objects with a large
No I didn't. I wholeheartedly agree that heat moves from the copper
pipe to the aluminum fins by conduction, and from there is lost to the
air by convection. I just wasn't comfortable agreeing that heat
transfer from the water to the copper is by conduction.
No matter how fast the water is moving in that copper pipe, there is
always going to be an infinitesmally thin boundary layer on the inside
diameter of the copper pipe. That boundary layer is SUPPOSED TO consist
of stagnant molecules of water or at least water molecules that aren't
moving very much.
So, if the copper pipe is to get hotter, that would require the copper
atoms in the pipe to vibrate more vigorously. And, why would they do
that if they're only in contact with those boundary layer H2O molecules
that are supposed to be relatively stagnant? That is, how does the
kinetic energy of the water molecules make it to the copper atoms if
there's a layer of stagnant water molecules between them. I just didn't
want to go there because I don't know enough about it to conclude that
it's conduction and conduction alone that gets the copper hot.
Actually, a cast iron "radiator" heats by 2 different methods. It
heats nearby objects by radiation and air by conduction, which in an
enclosed model causes movement of heated air by convection. An
enclosed (shrouded)radiator has significantly less radiated heat
output than an open radiator but is more effective as a "convector". A
radiator "radiates" just under half of it's heat output at best.
The larger the surface area exposed to the air, the more heat is
transferred to the air in contact with the fins, and the hotter the
air gets, the better the convection flow. If the paint insulated the
fins too much, there is less conducted heat transfer to the air, so
less convection. In the real world, it takes a LOT of paint to reduce
the heat transfer by a sensible amount
The common "treatment" of cast iron radiators in years past was a thin
coat of aluminum paint, which had very little insulating value and at
the temperature of the radiator was "effectively dark" - radiating
heat quite well. Although non-melallic paints in theory heat better,
the fact the radiator will run hotter due to less transfer of heat
causes it to radiate almost exactly the same amount of heat - so the
type of paint used is in practical terms irrelevant. The
effectiveness of the "radiant" heat was often enhanced on open cast
iron radiators by installing a "reflector" behind the radiator.
Aluminum tube and fin "baseboard" heaters are more optimized for
convection. Convection causes air flow - but the transfer of the heat
to the air to cause that convection is by conduction and radiation of
heat from the surface of the fins to the air. The shape of
the"reflector" behind some tube and fin radiators concentrates the
"radiated" heat out the front of the unit. There is always some
measure of "radiation" from any hot object.
Sorry, I realize that I didn't answer your question, and this web page
won't let me correct my response. It asked me to type in a "Captcha",
and I must have mistyped the answer because it told me "Sorry, try
again.", but it didn't give me another Captcha. It gave me a blank line
and was waiting for a response. Perhaps I didn't give the page long
enough to load. I'm on dial-up.
Heat transfer from the aluminum fins to the air is by convection. The
science of heat transfer doesn't concern itself with the interactions of
the atoms of aluminum and the molecules of oxygen and nitrogen in our
air from which that heat transfer ultimately occurs. It only concerns
itself with the rate at which heat moves from one place to the other.
But, to say how heat is transferred from the aluminum to the air, I
would draw an analogy to the dark lines and dark spots that gradually
show up on the walls of a house with a smoker in it. The extremely tiny
particles of soot the smoker exhales act very much like molecules of
oxygen and nitrogen in the air. When those tiny particles of soot hit a
wall, in a Newtonian world, they would bounce off. In Newton's world,
if you bounce a ball against a wall, the temperature of the wall is
irrelevant. The ball bounces off the wall with the same energy
regardless of the temperature of the wall. In fact, those particles of
soot don't bounce off the wall. They hit the wall and stop, and that's
perfectly consistant with quantum mechanics where every form of energy
is interchangable. In this case, the kinetic energy of the soot
particles is being sucked up by the cold wall, which theoretically
should be ever so slightly warmer as a result of the impact.
And, of course, exactly the opposite would happen at the surface of the
aluminum fins. The oxygen and nitrogen molecules would hit the fins,
but bounce off with much more energy than they hit the aluminum with.
And, so far, on every single web site I've posted on, there is always
someone that claims that the dark lines and spots that form on the
exterior walls of houses smokers live in is because of tiny amounts of
condensation that form over the colder studs and still colder drywall
nails or screws. But, no one has been able to explain why the soot
particle would stick to a microscopically small droplet of water. Water
has surface tension. Water molecules want to stick together so strongly
that you can "float" a razor blade or even a dime on the surface of
really cold water. Many bugs exploit this quality of water in that the
bug can walk on top of the water cuz of that surface tension.
So, if someone tells me that the soot particles in a smokers house are
sticking to tiny droplets of condensation, I look at a picture like
and ask them why it wouldn't bounce off the water's surface. Surface
tension is what's supporting the weight of that bug. How would a soot
particle that's billions of times smaller and lighter break that water's
surface tension? And, the soot particle would have to break the water's
surface tension and be "wetted" by the water in order for the water to
hold on to it.
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