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An old radiator Q&A


Dan Holohan
July 16, 2009
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Questions regarding the often elegant, cast-iron behemoths we love so much are as perennial as water hammer and squirting air vents. So, I thought I'd put some straight answers together for you in one convenient place. Here goes.

Q: Can I successfully cut down the size of an antique cast-iron radiator?

A: Maybe. It all depends on how the long-gone manufacturer assembled the radiator. A cast-iron radiator goes together in sections, like a loaf of sliced bread. Each section attaches to the next with round metal fittings called nipples. Nipples looks like very short pieces of pipe, which may or may not have threads on them.

Threaded nipples are unusual in that one side has a left-hand thread, while the other side has a right-hand thread. As the manufacturer turned the nipple one way between the two sections, it pulled both sections tightly together. After a few years of normal use and corrosion, the threaded nipples and radiator sections became one, never to separate again. Because of this, threaded nipples aren't available anymore. If you're looking to reduce the size of one of these old beauties, you're out of luck.

And then there are push nipples, which are still available. A push nipple is a smooth piece of pipe that's beveled. The bevel makes the push nipple wider in the middle than it is at either end. Rather than screw the radiator sections together, the manufacturers who used push nipples pushed one section into the other, taking advantage of the nipple's bevel to create a tight seal.

If your radiator has a threaded rod running between its sections, rest assured it has push nipples. Now all you have to do is get the beast apart. Loosen and withdraw the threaded rod. Next, apply equal parts of patience, pry bar, and elbow grease. If you're careful and persistent you should be successful. Remove the offending section and reassemble the radiator.

If the old push nipples don't look so hot, get new ones. And don't waste your time shopping around for these because there's only one place you can go: Oneida County Boiler Works (Phone: 315- 732-7914). Give them a call. They’ll want you to send a sample of the old nipple (no matter what condition it's in) and they’ll take good care of you. They regularly helps people all across the country, and they assure me that Oneida is the only company around that still supplies these fittings. "If people could get 'em any closer to home, they wouldn't be calling us!" they say. I believe them.

Once you get the new push nipple in place, tighten the push rods, and pull the radiator sections back together. (Gosh, I make that sound so easy.)

Q: What's the best way to disconnect, move, and reconnect radiators for, say, floor repairs?

A: First, take care with those old pipes. Make sure you're using two wrenches when you're loosening the union connections. Assume the position, and then turn one wrench while holding back with the other. Don't take any shortcuts here because if you attack old pipes with a single wrench the torque you create can, and probably will, break the pipe.

A hand truck with a few strategically placed blocks of wood will help you move the old beast out of the way. And if you're planning to take that old radiator down a flight of stairs proceed cautiously, and with plenty of help because an antique radiator can weigh hundreds of pounds.

Q: If I decide to move a cast-iron radiator, where should I reinstall it?

A: Ideally, a cast-iron radiator should be under the window (that's where the greatest heat loss is), and it should be as wide as the window. Its top should never peek above the windowsill because this lessens the convective movement of air around the radiator. Speaking of which, for maximum convective efficiency, the radiator should be 2-1/2 inches away from the wall. It took the old-timers years to figure that out.

Q: What's the difference between a steam and a hot water radiator?

A: It's the way the radiator sections go together. They may be nippled together at both the top and bottom, or just at the bottom.

Older steam radiators have nipples across just the bottom portion of the sections. This is because steam is lighter than air. When the steam enters the bottom of a radiator (as it always will in a one-pipe steam radiator), it flows upward into the sections, displacing the air as it goes.

Hot water radiators, on the other hand, have nipples across both the upper and lower portion of the radiator sections. Even though hot water rises, it doesn't move as quickly as steam. The double set of nipples encourages better circulation of the hot water across the entire radiator and leads to greater efficiency.

Around 1905, when two-pipe steam became popular, contractors began to use hot-water radiators on steam systems. The old steam radiators with their single set of bottom nipples quickly faded and became obsolete.

Q: What's the difference between a one-pipe and a two-pipe steam radiator?

A: As the name implies, a one-pipe steam radiator has just one pipe connected to it, and that pipe is always at the bottom. Both steam and condensate (the water that forms when steam condenses) share this pipe. One-pipe steam systems can use either steam or hot water radiators, however.

Two-pipe steam systems usually have the steam entering through a pipe at the top of the radiator. The condensate leaves the radiator through a pipe at the bottom. Since the steam moves across the top of the radiator, and the condensate drips down along the radiator's inside passages, two-pipe radiators generally provide a more-even sense of warmth.

There will usually be a steam trap (which is an automatic, temperature-sensitive valve) at the point where the radiator and the condensate pipe come together. You should check these with a thermometer once a year. You've looking for at least a 10-degree drop in temperature across the trap. If the trap's not working, you can replace the internal parts. Any good plumbing supply house will be able to get the parts for you.

A two-pipe steam system will almost always use hot water radiators. There is one notable exception, though, and it's called the two-pipe, air vent system. You'll know you have this one if you see two pipes, one on each side of the radiator (at the bottom), and both pipes have hand valves. These radiators also have air vents. From a historical perspective, the two-pipe, air vent system is the missing link between one-pipe steam and two-pipe steam.

Q: Does a two-pipe steam radiator have to have a steam trap?

A: No, but it has to have something to keep the steam from entering the condensate return lines. That "something" may be an internal orifice, a tiny check valve you can't see, a hidden metal ball or a water seal. There were about three dozen companies doing business between 1905 and 1930 that made these steam-stopping gizmos. They're all out of business now. So do not remove any weird-looking device until you've answered three essential questions:

  1. What is it?
  2. What does it do?
  3. What the heck happens if I take it out?

If you can't answer those questions, put your hands in your pockets, and back slowly away from that radiator.

Q: Can I take out a steam radiator and put in a hot water radiator?

A: Yes.

Q: Can I take out a hot water radiator and put in a steam radiator?

A: You can if it's a one-pipe steam system.

Q: Where does the air vent belong on a cast-iron steam radiator? How about on a hot water radiator?

A: If it's a one-pipe steam radiator, the vent belongs on the side of the radiator that's opposite the pipe. Because the lighter-than-air steam will head first for the top of the radiator, you should install the air vent about half-way down the radiator, and not at the top.

Two-pipe steam systems (with the exception of that "missing link" one-pipe, air-vent system) should not have air vents on the radiators. If the two-pipe radiator won't heat without an air vent, check the steam trap. Misapplied radiator air vents can lead to nightmarish system problems.

Each hot water radiator should have an air vent at the top, on the side opposite the inlet pipe. You'll use this vent to "bleed" air from the radiator when you're first starting the system.

Q: Where can I buy antique radiators?

A: Here's a LINK with a lot of options.

Q: Can I repair a leaking cast-iron radiator?

A: It depends on where the leak is and how bad it's leaking. Steam radiators, because they're under much less pressure than hot water radiators are usually the easier of the two varieties to fix.

To begin, first determine where the leak is. This, of course, is easier said than done. Go to your local hardware store and get yourself an inspection mirror, which will allow you to see around corners and up into spaces not viewed within the past 100 years.

If you find a pinhole leak at a push nipple, you can, as you now know, replace the push nipple. If the radiator is cracked, say, after a hard-freeze, you may not be able to repair it at all, however. It all depends on the severity of the crack, and where it is.

Q: Are there stop-leak products for cast-iron radiators (as there are for automotive radiators?)

A: None that you can pour into the radiator, but J-B Weld Company of Sulphur Springs, TX (903-885-7696) may have the answer. The company's literature states that the City of Dallas, Texas used J-B Weld to repair a cracked Caterpillar engine block. That sure got my attention!

A representative of the company told me that old-house owners have reported great success with his company's product, J-B Weld on old cast-iron radiators.

But to fix a leak, you first have to be able to get at it, right? So consider this. Before using J-B Weld, you have to drain the radiator and remove any paint, primer, or rust. Next, you have to thoroughly clean the surface with a non-petroleum-based cleaner such as acetone or lacquer thinner, removing all dirt, grease and oil. Then you have to rough up the surface with a file, mix the two elements of the product in 50/50 proportions, and apply it to a thickness of no less than 1/32-inch. Don't get any on your skin or in your eyes. Finally, you let it dry for at least 15 hours, and see what you've got.

Can you do that?

I asked if the product could take the temperature along with the expansion and contraction common in cast-iron radiators. They told me the product actually "softens" when heated and will move with the metal. It's not the sort of 'softening' you'll notice, though. You’d have to get the temperature up to 400 degrees Fahrenheit, to see that (the product is good up to 600 degrees.) Typically, a radiator in a steam-heating system will get up to about 229 degrees tops. So if you can get at the leak, it sounds like this stuff will work.

The challenge, of course, is that an antique radiator can have more nooks and crannies than a Thomas' English muffin, and a good leak knows where to hide. But if you're in love with that old radiator, it's certainly worth a try.

The company sells only to wholesalers, and only in quantity, but you can buy J-B Weld for about five bucks at most automotive and hardware retail stores.

Q: Any other thoughts on leaks?

Well, I heard from a reader who had success. Here's what Dave Colglazier had to say:

"Hi Dan, I was looking for a way to repair 2 leaking cast iron radiators when I bumped into your article here.  Great information for all who have this problem but don't know how these were assembled.  I also found a response at the Gorilla glue site that mentioned that using that glue for a tank repair was not recommended.  An upholsterer friend had experience with GG so I spoke to him about it and he gave me the remains of a bottle of PL glue by Loctite.  This type of polyurethane glue also expands from water curing but does not have the expanding foam feel to has a much more rubbery consistency in the cured form.  After locating the leaks - both between sections in the push nipples-I decided to inject this PL glue using a hypo needle that was about 1/32".  I sealed the section line with putty, drilled a small hole, and injected the glue into the cavity between the 2 sections...the first time I did it the radiator was upside down and one of them sealed up just fine.  The other one didn't seal and had a very small leak that might have self sealed.  I wasn't content so I drained down the zone leaving it in place and repeated this procedure from near the top after poking the needle through what seems like very resistant rubbery material.  This time the leak sealed and both radiators are in daily use now.  Oh, and although the radiators were upside down when I injected them the first time, I did put them back into the upright position for the glue to cure out since more than likely the leak was at the bottom where the water collects.  Both of these radiators had been installed in hot water systems with the inlet and outlet on the bottom.

"I've also successfully shortened radiators with push nipples and I was fortunate enough to get them apart where I could use the existing nipples but I used pipe thread dope when bringing them back together for added protection.  I also use pipe dope when bringing the unions of valves back together for the same reason not only on the threads but also on the seal surface."

Thanks, Dave!

Q: Do I need to flush my radiators from time to time?

A: No. Hot water radiators operate within a "closed" system where there's little or no corrosion taking place. Flushing these radiators will only cause you to add more water to the system, which will create more corrosion, and so on, and on. Why cause problems?

Steam systems are open to the atmosphere so the radiators do see more corrosion than their hot water brethren. However, cast-iron radiators come with their own dirt-storage compartments, and these can hold many years’ worth of scale and rust.

Take a look at the way your radiators connect to your pipes. Notice how the inlet valve or outlet steam-stopping device is always a bit higher than the bottom of the radiator? This is true even when the valve is installed near the bottom of the radiator.

The scale and rust settle into that low-slung "pocket" and stays there. Keep this in mind if you decide to pitch your one-pipe steam radiators back toward their inlet valves to give you better condensate drainage. Take care not to pitch those old beauties too much. If you do, you just might slosh one-hundred-year's worth of sludge into that inlet valve. You'll know you made this mistake by the water hammer that pounds on your pipes and the condensate that squirts from your air vents onto your ceiling.

That's when it's time to flush the radiator.