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 t.)
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.
US Department of Commerce?s 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 composed.
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.
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Science proves that the finish of a radiator affects its heat output in varying degrees.
There is a principal known as "emissivity" that enables experts to measure the ability for heat to leave (or radiate from) the surface of an object.
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 painted finish.
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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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