Steam velocity kills a heating system

BY PATRICK LINHARDT
STEAM HEATING AUTHORITY

THE CALL: One of my favorite customers called me about a job we looked at years ago. I like this customer because he loves to talk about history and there is so much about steam heating that involves history.

At the time we originally looked at his job, he had installed a boiler that was only half the size required, because the building owner wanted to get a few more years out of the boiler that was already there. The strategy was to replace the older boiler when it died with another boiler of half the size. Then the building owner would have a nice multiple-boiler system that would provide better energy efficiency and partial heat in case one boiler needed repair.

As with most plans, however, this didn't happen exactly the way it was conceived. When the older boiler finally died, another contractor convinced the building owner to just make one big boiler out of the small boiler. The beauty of commercial cast-iron sectional boilers is that it can be done.

In this case, it went from six sections to 12. But it only lasted a few years before many of the sections had cracked. The building owner was not too happy with that other contractor at this point, and my customer wanted me to come out and take a look.

The troubleshoot
This building is not far from my office; it is built right alongside the canal that used to run between the Ohio River and Lake Erie. When steam engines took over carrying freight in the early 1900s, the city of Cincinnati filled in the canal with a subway system that never carried a passenger (according to my customer, construction was stopped before it was completed due to some scandal or financial scare).

Along about that time, the steam system was installed in this building — it is only about 75 ft. deep but runs at least a city block. The old timers used a downfeed two-pipe distribution system. An express steam supply main runs straight up out of the basement boiler room to the ceiling of the second floor.

It then branches off into two steam mains that run the length of the front and back of the building. Downfeed risers carry the steam down to the radiators, while the air and condensate flow back to the basement boiler room through the return piping.

To get to the boiler room, you have to walk through a bit of history in the remaking, a shop that restores and rebuilds wooden car bodies. An auto body shop that works with wood instead of plastic and Bondo — you don't see that everyday.

My customer showed me the cracked sections in the boiler room. The bottoms were loaded up with chunks of sediment that looked like peanut brittle. The boiler sections had failed because of excessive fresh water make-up. Sediment buildup from the fresh water acts as insulation between the flame and the water, causing the cast iron to overheat and eventually crack. All 12 sections were either cracked or so loaded up they had to be replaced.

I looked around for a cause and it didn't take long for me to find it. A glance at the near boiler supply-side piping was all it took.

Three 4- in. supply risers were connected to a 4- in. header that had a 4-in. takeoff to the express main. That didn't make any sense. I don't know of any modern boiler manufacturer that recommends three boiler supply risers of a certain pipe size connected to a header of the same pipe size.

After looking in the installation manual for the boiler, we found that the recommended minimum pipe size for the risers is 5 in., and the header is 8 in. What happens when too much steam tries to get through too small a pipe? The velocity of the steam increases and carries water with it as it rushes out of the boiler.

So much water was being carried up into the system that the boiler feed system was working overtime. Fresh water was being made up during the call for heat to keep the boiler running. All that excess water was being wasted to the drain as it overflowed out of the boiler feed tank during the off cycle.

The follow-up
The building owner confirmed the large amount of fresh water being used. His water bill showed thousands of gallons per month in excess of normal usage after the installation of the extra sections. This bit of history was our "smoking gun."

I calculated the velocity of that boiler output through the 4-in. header and the result was even worse than I anticipated. Steam was zipping along at more than 200 ft. per second, or about 136 miles per hour. Modern boiler manufacturers design the header size for about 50 ft. per second. The old timers went as low as 10 ft. to 20 ft. per second.

Excess velocity — and this job had plenty of excess — causes water to be carried out of the boiler with the steam. Water was going everywhere it wasn't supposed to be.

After the new boiler was installed and the near boiler piping corrected, the system still had problems. All that water in the supply piping had created massive amounts of rust and scale.

That gunk had been down-fed to the valves, piping, strainers and traps. My customer was there for two more weeks working on T&M replacing and cleaning out all the rust and scale.

But afterward, the building heated like it hadn't in years. Steam, not water, was now gently moving through the supply pipes. Historically, steam systems work better that way.

Patrick Linhardt is the sales manager at Aramac Supply in Cincinnati. To order his book, "Linhardt's Field Guide to Steam Heating," visit steamupairoutwaterback.com or call 513/703-5347.