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?
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).
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