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.
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 hat.)
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.
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:
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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 straightforward.
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 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).
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 c paints."
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 than 20%.
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 radiator.
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:
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And, here's what that same radiator looks like from the back:
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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 radiator.
Here's another one... a finned tube baseboard radiator:
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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.
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 convection though.
The "Our house has old-fashioned cast iron radiators warmed by hot water from the boiler. "
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 ridiculous.
After the air is heated by radiation and some conduction, then there is convection.
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.
Nope.
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 ir>
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'
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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 c>
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 this:
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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.
Just what kind of double-line-spacing google-grouping fool are you?
Of course there is "convective heat transfer", but convection itself is not the mechanism how or reason why a radiator transfers heat to the air surrounding it.
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