The case of the cold steam radiator

By Ray Wohlfarth


While talking with the maintenance director about purchasing boilers, he asked if I could meet one of his technicians at a nearby elementary school. The building had a low-pressure steam heating system and no heat in one classroom.

When I arrived at the building, the technician thanked me for coming and explained the problem. The classroom had a single steam radiator and the radiator never warmed. The first place I wanted to see was the boiler room. The boiler room had two large cast iron sectional boilers and I noted the steam pressure was set to two psi. We then visited the classroom, and it looked like it was converted from a storage space. On the outside wall was a large steam radiator with a steam trap on the outlet and a self-contained temperature valve on the inlet. I felt the radiator, and it was cool. “Did it ever heat?” I asked, trying to see if this had been an ongoing problem.

“It stopped working a few years ago,” he said. “Anything happen then that could have caused this?” I asked. He shrugged. The technician replaced the thermostatic control valve and the steam trap element twice without success. He had plugged in a portable electric heater to heat the room but the single heater did little to warm the room. When a second was added, it tripped the electrical breaker. The teacher and her classes were relocated to the gymnasium, and she and the gym teachers were not happy.


The investigation begins

I closed the radiator valve and removed the cover and thermostatic element for the angle steam trap on the radiator outlet. Be careful when doing this, as steam can burn you quickly. Third-degree burns can happen within a second when exposed to steam. The trap cover and element are removed to see if other traps connected to the condensate pipe are leaking through and filling the condensate pipe.

Steam traps work on pressure differential. If the pressure is the same on the inlet to the trap and the outlet, there will be no flow through the trap. This is called steam stall. Conversely, the higher the pressure difference between the trap inlet and outlet, the more capacity the trap has.

The steam trap outlet had a small whisp of steam coming from the vertical condensate pipe. This is normal and called flash steam. If traps were leaking through in other areas of the building, there would be a steady plume of steam back feeding through the condensate pipe. There are a couple of rules I follow when troubleshooting steam systems. The first is a rule I borrowed from Dan Holohan, the smartest steam guy I know: the problem and solution are rarely in the same room. The second rule is that using your hydronic system logic for troubleshooting steam systems will not end well.

Although both steam and hydronic systems use a boiler to generate heat, they operate differently. Due to the changing states in a steam system (water to vapour and back to water), it is closer in operation to an air conditioning unit than a hydronic boiler.

The next step was to see if steam was fed to the radiator. After opening the radiator valve, I saw steam chugging through the radiator outlet after a minute. This eliminated the steam side of the system from the problem, and I had to concentrate on the return or condensate piping. We reassembled the steam trap and opened the radiator valve.


Getting to the bottom of it

The technician and I traced the condensate pipe from the cold radiator through the building. Directly below the classroom was a stairway. The pipe dropped through the stairway into a crawl space below the school. We crawled through spiderwebs and over pipes and rocks to where the condensate pipe from the cold radiator was connected to a horizontal pipe. Following the horizontal pipe, we found the culprit. A clevis pipe hanger had come apart, and the condensate pipe was sagging. The asbestos abatement company that removed the asbestos pipe insulation a few years ago must have removed the pipe hanger. The timeline corresponded to the time the radiator stopped heating. “I forgot about that,” said the maintenance tech. This sagging pipe allowed condensate to collect and caused water seal in the piping, similar to a P trap on a plumbing system. Steam systems must breathe and the water seal prevents that from occurring.

While the steam system is off, the pipes and space above the boiler water are filled with air. When the boiler starts, water expands at 1,600 times its volume and becomes steam.

The steam pushes the air through the piping at around 20 to 30 miles per hour or 32 to 48 kilometers per hour and out the air or condensate tank vent. After the boiler shuts off, the steam condenses, and air rushes back into the system from the air or condensate tank vent and fills the pipes. I like to think of it as the system breathing, and the sagging pipe did not allow this section of the steam system to breathe. Air was trapped in the condensate pipe, and steam could not enter.

The maintenance technician and I lifted the condensate pipe and re-attached the pipe hanger. We also raised the elevation of the piping to have more of a pitch toward the boiler room. The radiator was nice and warm when we walked upstairs to the classroom. The maintenance technician could not believe that was the solution. When working on steam systems, you have to think holistically and consider the entire system.

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