Steam Unit Heaters Can Present a Challenge

HVAC SteamUnitHtr

They built the place in the 1930s and heated it with steam. It's always been a commercial building - part office, part factory.

They heat the offices with one-pipe steam radiators and the factory with steam unit heaters nowadays. A fellow who's been there for years remembers when they used to heat the factory with pipe coils.

"They ran back and forth over there," he said, pointing to a pockmarked brick wall. "There must have been a couple of hundred feet of pipe in that coil. It was wonderful standing next to those hot pipes - much better than these things." He pointed at the unit heaters, suspended from the ceiling.

They removed the pipe coils in the early-1980s because they were leaking at the joints. "It just didn't pay to fix them," he said. "The owners figured the heaters would be better, but we've had problems ever since."

Unlike the one-pipe radiators in the offices, the old pipe coils in the factory worked as two-pipe units. It's not unusual to find this mixture of one- and two-pipe steam in older buildings. The two systems can work well together, if you follow a few rules.

The Dead Men didn't use steam traps with the old pipe coils. They didn't need steam traps because they drained the coils into a gravity wet return. The Dead Men knew enough to use traps for just two situations: when heaters drained into a dry return on a two-pipe steam system, or when heaters drained into an atmospheric return on a system with a condensate- or boiler-feed pump. That's it.

Steam traps are there to keep the steam from winding up in a place in which it doesn't belong. If you're draining into a gravity return, the steam has nowhere to go so you don't need traps. If you use steam traps where you don't need them you can wind up with big problems. If you trap a gravity-return line, for instance, you'll probably back water up into the heating equipment, create water hammer, cause the boiler to go off on low-water or, if there's an automatic feeder, flood. How does that sound to you?

Don't use traps if you don't need them.

The Dead Men knew they'd have to use air vents, though. Here, they put the vents at the outside of the pipe coils. The Dead Men realized that if steam was to get in, the air had to first get out. They were really good at venting air in the old days.

In this factory, the Dead Men supplied the steam from an overhead main and let it flow back and forth through the horizontal coils. The steam condensed as it went, carrying the condensate along to the gravity wet return and back to the boiler.

The trick with this system was to make sure the coil was higher than the system's "A" Dimension. The "A" Dimension in a gravity return system is the vertical space between the center of the gauge glass and the bottom of the lowest horizontal, steam-carrying pipe. Here, that would be the air vent at the bottom of the pipe coil. The "A" Dimension has to be at least 28 inches for gravity return.

When they replaced the pipe coils with unit heaters, they ran new dry returns. They put an air vent and an F&T trap at the outlet of each heater and ran the returns back to the boiler. You see, they needed traps now because they had switched from wet to dry returns. Without the traps, steam would have zipped from the return of one unit heater to the next, and that would have led to air-binding problems.

The problem they were having now was that whenever the unit heaters were working, the people in the office were miserably cold and no one knew why.

I looked up at the new unit heaters and noticed the installer had used the Hole Method of pipe sizing. In case you're not familiar with the Hole Method, it works like this. The installer looks at the size of the hole in the unit heater, and then pipes a line the exact size of the hole back to the steam main. The Hole Method makes piping very easy. Any idiot can do it.

Unfortunately, it never works with steam heating systems.

You have to look in old catalogs to appreciate why. During the Steam Era, unit heater manufacturers went to great pains to show the fitter what size the piping had to be. For instance, if you look in a 1954 unit heater catalog you'll see these specifications for a 260,000 BTU\Hr. heater:

The heater's inlet and outlet are both 2-1/2".

The main supplying steam to the heater has to be at least 3".

The horizontal runout between the main and the riser to the heater has to be at least 3-1/2" (4" nowadays).

The riser from the horizontal runout to the heater has to be at least 3".

The steam trap has to be 1-1/4".

Piped this way, the pressure drop through the piping to this unit will be two ounces per 100 feet of equivalent length. This was a piping standard for steam heating systems in the old days. It's what allowed the Dead Men to run their systems at such low pressures. In 1954, the unit heater manufacturer went to great pains explaining pipe size. They wanted their unit heaters to work well with the other types of radiation they knew the Dead Men would use on a typical one- or two-pipe steam heating system.

Not so nowadays. Check out the same unit heater in a current catalog. Here are the specifications:

The heater's inlet and outlet are both 1-1/2" (perfect for hot water, not so good for steam).

There are not other piping specifications (you're on your own, pal).

Assuming you'll use the Hole Method, your pressure drop through the piping leading to the unit heater will now be two pounds per 100 feet instead of two ounces per hundred feet. Big change, eh?

You see what's happened? Unit heater manufacturers are building their equipment for hot water, but they're still rating them for steam. Will they work on steam? Sure, they will, but you need higher pressure to make them work at their rated capacity.

Steam pipe sizing is very subtle. You can get just about any steam load you'd like through any size pipe if you have enough pressure. Look at what they manage to do with 3/4" pipe and 125 psig in the typical dry-cleaning shop. But in a steam heating system, the boiler pressure should be less than 2 psig if you expect things to work well and economically.

Here's what happened in that commercial building I was telling you about. Whenever they cranked up the boiler pressure to overcome the pressure drop of that undersized unit heater piping, all the quick vents on the office radiators would snap shut and not reopen. If they lowered the pressure at the boiler, they would get heat in the office, but not in the factory. You see, steam air vents and system pressure are married to each other. And pipe size has everything to do with system pressure.

What did they wind up doing? They repiped the heaters, of course. This time, though, instead of using the Hole Method, they checked out a good steam heating book and got the right sizes. Now it works fine.


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