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May 16, 2023

Dan Holohan: Do radiator reflectors work?

Bet_Noire / iStock / Getty Images Plus via Getty Images We always have turkey

Bet_Noire / iStock / Getty Images Plus via Getty Images

We always have turkey for Thanksgiving. I mean who doesn't? My job wasn't to cook it, though; it was to eat it. I loved the way the turkey always made the house smell, and I remember how The Lovely Marianne would carefully lift the aluminum foil off the radiator to baste it, hour after hour.

When we were first married, I asked her why she used aluminum foil to cover the turkey?"

"You have to," she said.

"Why?"

"Because my mother said so," she said.

"Oh."

But then more life took place and I learned about how heat travels and the business with the turkey and the aluminum foil began to make more sense.

Heat travels in three ways: Conduction is what you feel when, usually as a newlywed, you slide the rack halfway out of the oven so you can better baste the turkey. You’re not using a potholder because, well, you’re young.

Hot rack. Cool hand. That's conduction. Ouch.

Convection follows, of course. That's the sensation you feel as the hot air pounds from the still-open oven while you do the Dance of Pain in your kitchen. That's convection. Oh, and you’re waving your blistered hand like a small flag in a futile attempt to cool it. That, too, is convection.

And then comes the third way heat travels — by radiation. As you’re dancing, waving and saying disparaging words, you may notice that you’re feeling hot. This is because the heat radiation leaving that 325-degree oven is traveling in a straight line, just like light, from a hot oven to a cooler place, that being you.

And isn't physics grand?

Mama said to put aluminum foil over the turkey as it cooked. The reason being (even though she never really thought this out) that aluminum foil is an excellent reflector of heat. As the turkey cooks, the foil helps things along by sending the heat that's leaving the turkey right back into the turkey. And by leaving a small air space between the turkey and the foil, you also cut down on the amount of convection that can take place. Convection removes heat from a hot object by allowing cooler air to flow across it. (How's that hand doing?)

And should you ever decide to participate in a Turkey Trot, the race sponsor may drape you in a mylar blanket after you cross the finish line. Its shiny surface will reflect your body heat back into your body and you will feel warm. The blanket will also keep the wind off of your skin. The less convection the better after the race. And who's the turkey now?

And that brings us to radiators. Did you know that there are 6,500 steam radiators in the Empire State Building, which has a Gold LEED rating? Part of what earned that iconic building its Gold rating was the 6,500 reflectors they placed behind those radiators. Rather than heat the exterior wall that backs each of those radiators, the reflectors turn that lightlike heat radiation back into the rooms. Radiator reflectors save energy. They work.

Something else to consider is the Dead Men installed radiators about two inches away from the wall. That was standard procedure back then and the reason was to allow room for convection. If you’re using foil-faced insulation, don't use a product that's so thick it will affect that convection.

And reflectors don't have to be fancy to work. You can make them yourself. A simple way is to tape or glue aluminum foil to sheets of cardboard and then tack it to the wall behind the radiator. You’ll notice an increase in heat coming off the front of the radiator and if you touch the wall behind the radiator you’ll see that it's now cool.

A freestanding, cast-iron radiator puts out about 60% of its heat by convection, and the remaining 40% by radiation. Why heat a cold wall behind a hot radiator when you can redirect that radiant energy to where the people are?

And don't worry about the cardboard bursting into flames. Ray Bradbury wrote a book he called, Fahrenheit 451. That being the temperature at which paper burns. The surface of a steam radiator is typically 215 F., less than half the temperature it takes to set fire to paper. Hot-water radiators are cooler still, so no worries about using cardboard.

Reflective bubble wrap also works well behind radiators, as does foil-faced foam insulation. Polished sheet metal, on the other hand, is not the best way to go because conduction will kick in as the metal gets hot. Some of the heat will go into the wall that the metal is touching. Metal is a great conductor of heat, and it also does not come cheap, so give it a miss for this application.

A contractor friend who posts on The Wall at HeatingHelp.com said, "I think the more polished the surface is the better. I once bought some mirror-finish contact paper to build a solar oven. Any dust or rain deposits really cut down on the power. We found the same issue with reflective foil under radiant staple-up installations. As soon as the foil got dusty the reflectivity was gone from the foil and the heat output through the floor dropped."

Another contractor said, "I used the cardboard and aluminum foil method on a couple radiators. I didn't tack it in place. A couple times a season I reach behind the radiator, pull it out and wipe off any dust with a dry cloth and then put it back in place. It's infinitely customizable, easy to maintain, and cheap. I also keep the reflectors a bit in from the outer edges of the radiator so as to not be very visible."

Appearances also matter, right?

Something else to consider is the Dead Men installed radiators about two inches away from the wall. That was standard procedure back then and the reason was to allow room for convection. If you’re using foil-faced insulation, don't use a product that's so thick it will affect that convection.

And that's another reason why inexpensive marriage of cardboard and aluminum foil shines (pun intended).

A Canadian friend told this story on The Wall:

"The panels definitely work, and pay for themselves quickly. I recommend them without hesitation.

"I spearheaded the installation of reflective panels in my building in 2011. This is a four-storey, 25-unit brick co-op in Toronto, built 1925. We have a Weil-McLain steam boiler with outdoor reset and a Power Flame modulating burner.

"We used Novitherm panels that are designed specifically for radiators. They’re shaped to increase convective airflow in addition to directly reflecting infrared frequencies. The airflow improvement is only an advantage if there is no cabinet over the radiator, of course.

"Thermograms taken a few years before for a different purpose showed heat penetration through the exterior walls where the radiators were. The images were nearly as bright as what we were seeing from our 90-year-old, single-pane, non-weatherstripped steel windows.

"We were able to apply for an energy-saver grant from our natural gas supplier, and the estimate of annual gas savings was about equal to the fuel burned in April of that year (7900 cubic meters). At the time, we were paying CDN$0.33/cubic meter, so the panels paid for themselves in less than two years.

So no matter which way you choose to go, radiator reflectors sure seem like a great idea. They’re simple. They have no moving parts. They save fuel, and they make sense.

Dan is the hydronics contributing editor for Supply House Times. He founded the popular www.HeatingHelp.com web site where you'll find just that - Heating Help. Post your technical challenges on The Wall bulletin board and get input from some of the best in the business. You can reach him at [email protected]. He loves hearing from you!