A simple way of killing convection and radiation.
One of my yellowed heating textbooks has a wonderful drawing of a Victorian radiator that has what looks like a teapot cozy over it. The cozy is made of heavy drapery material and it has a drawstring on its side, giving the folks living in the house a way to control the amount of heat by raising and lowering the curtain. The long-dead writer explains that this is a simple and elegant way to control radiators that are oversized for the space they’re serving, which I thought was delicious. Even then, heating contractors could be, well, let’s just say conservative.
The radiator cozy was a simple way of killing convection and radiation. Considering that many of those old radiators ran on one-pipe steam, and since a one-pipe steam supply valve offers but two choices (open or closed), the cozy delivered relief from having to bend over and twist again and again.
I thought about that Victorian invention of convenience as I stood in a Manhattan apartment and looked at this gorgeous marble window seat, inside of which was a squat, cast-iron radiator. The person who owned this apartment had lots of money, which I thought was wonderful, and he had decided to spend some of that money (a lot of it) on encasing his steam radiators within stone that would have made Michelangelo smile. The permanent residents of Green Wood Cemetery should have it so good.
I was there because the wealthy fellow was cold. He had asked me to take a look, and since I grab every opportunity I can to poke around the homes of the fabulously wealthy, I accepted.
“It’s very cold in here,” he said.
“Yes,” I agreed.
“Why do you suppose that is?”
“Well, you’ve encased your radiators in marble,” I said.
“I know, doesn’t it look grand?”
“Absolutely,” I said.
“I hated the way the radiators looked without the marble. They’re quite ugly.”
“But why is it so cold in here?” he asked.
“Well, you’ve encased your radiators in marble,” I said.
“Yes! And doesn’t it look fabulous?”
We went around like that for a while until we were finally able to stumble beyond interior design and into heating. I explained how a freestanding, cast-iron radiator puts out about 40% of its heat by radiation and the rest by convection. I waggled my fingers straight at him, voodoo-style, to demonstrate the principle of radiation. Then I wheeled my right arm through space, just like Pete Townsend does, in what I thought was a decent visualization of convection. Finally, we both gazed again at the marble sarcophagi that held his radiators.
“It’s not going to work like this,” I said. “You’ll have to have holes drilled in the top and the bottom of the stone to let in the air. And that’s not going to do much for the radiation part.”
“Don’t be absurd!” he sputtered. “Do you have any idea how much that marble cost?”
“Well, it cost a lot,” he said.
And because of this, he honestly believed he was entitled to a bye week from the laws of physics, he being a rich guy and all.
Perhaps you’ve met this guy.
When I was growing up in the apartments of Manhattan we had steam heat and there were no covers, marble or otherwise, on any of our radiators. I know this because of black-and-white family photos and one very vivid memory of the day I touched that blazing beast.
These days, parents who live with big, cast-iron radiators worry about their children getting burned. My parents handled things differently. My brother and I were each, at a time of our own choosing, allowed to touch any of our hot radiators. Brother Ed and I needed to do this just once because our parents hadn’t raised dummies. We each let out shrieks of horror that summoned Mom, who then proceeded to spank us for touching the hot radiator. And in this way, we learned.
It was different then.
Some companies spotted an opportunity in all this mother-and-child sadomasochism and came out with grand lines of radiator covers. They made them of metal and they made them of wood and they advertised them as a way of hiding those hideous cast-iron radiators from sight, while improving the efficiency of same. These companies advertised directly to the homeowners in many magazines and newspapers and they did very well for themselves. Radiator cover-ups are ubiquitous.
I was roaming the Internet the other day when I spotted a question on one of those sites where you can ask anything and lots of folks will answer. Then the group gets to vote on which answer wins. It’s democracy at its best.
So in answer to the question, “Do radiator covers improve efficiency?” the congregation came up with these Top Two Answers:
Top Answer.“Radiator covers do not reduce the efficiency of radiators unless for faulty and ill-designed installations. These covers usually provide an invaluable service as they reduce the risk of damage to the radiators as they provide safety from burns as radiators can heat up to 180° F. They also are energy efficient as they prevent heat loss as they normally direct the heat toward the interior of the room, where it can flow more efficiently, rather than be absorbed instantly by the ceiling or the adjacent wall.”
Alternative Answer.“Radiator covers are known to reduce the efficiency of heat transfer. This is because the radiator cover slows the movement of warmth out of the radiator and into the room. They are, however, available in a wide range of styles as well as designs.”
There you go. Take your choice. And note how the top answer (the public has spoken!) explains how a radiator cover prevents heat loss by directing the heat toward the center of the room. That concept catches my fancy. If you can round up your Btu, like a wagon train being attacked or a herd of cows, you can send them into the core of the room and save a bunch on fuel. Just keep those Btu away from the walls and ceiling. A good radiator cover will do that for you. That’s why it’s the Top Answer.
The Alternative Answer, however, is closer to the truth. Once the Victorians died and took with them their radiator cozies, heating engineers took a closer look at those radiator covers and did some testing. They learned that if you placed something as simple as a shelf over the top of a free-standing, cast-iron radiator, you’d have to add 20% to the size of the radiator to get the same output that you would get if the shelf wasn’t there.
That’s because the shelf impedes the convective flow of air over the hot metal. It’s like closing the damper on a baseboard radiator. Less air movement means less heat transfer. But then I suppose the Top Answer people might say that the shelf has the ability to whirlpool the Btu into the center of the room and away from those Btu-robbing walls and ceilings.
The heating engineers also tested that classic radiator cover you and I have seen on so many jobs. It’s the metal one with the solid, convection-stopping, hinged top and the front that’s perforated with a gazillion holes. Use one of those and you’ll have to add 30% to the size of the radiator to get the same output you would get without the cover.
The cover that works best has holes at the bottom front, a solid facing and holes at the top, or at the top of the facing, providing there’s enough space for the air to create that all-important chimney effect. You probably know that instinctively. Think of all those convectors you’ve seen in your life. They work best when the air can move through them. Or think about how many times you’ve urged your customers to pull that couch a few inches away from the baseboard radiators so the air can get at all those hot fins.
None of this is complicated but people will always go out of their way to hide radiators and that will always be a wonderful source of commerce, for both the radiator-cover folks and you.
That said, perhaps the Top Answer to the radiator cover question should be (as it so often is in life): It depends.