Why do my vertical steam risers bang so much?

By Daniel Karpen, P.E.

The problems that I find in steam heating systems never fail to amuse me. I was called in by the owner of a four-story brownstone of about 4,000 square feet in the Park Slope area of Brooklyn. He told me that he had very severe steam pipe banging problems.  

It is not unusual to see steam pipe banging problems in horizontal steam lines. However, in this case, the steam pipe banging problems were in the vertical risers. This is a very serious problem that is not terribly unusual.  

So where did I start? I needed to look at three things: 1.) the BTU output of the boiler 2.) the BTU rating of the steam radiators and 3.) the actual use of fuel by the building.  

I went into the basement, accompanied by the owner of the brownstone. The boiler was a Weil McLain model EGH-105, fired by natural gas. The input to the boiler was 450,000 BTU per hour, and its rated output was 360,000 BTU per hour. The boiler was relatively recently installed, about 2010.

The owner of the house had it entirely gutted and renovated several years ago. New double glazed Low-E windows were installed on the northern front and southern rear sides of the house. The ceiling of the upper floor was insulated heavily with thick closed cell polyurethane spray foam insulation.

The house was almost a passive house. The peak heating load was the lowest I have ever seen in any building of its size. In February 2015, the fuel usage for building heat was only 230 CCF of gas. This amount is astonishingly low. That winter was one of the coldest in the past 70 years.

Now, let’s look at the steam heating system. There are two main steam lines leading to the radiators at the front and rear of the house. Each main steam line is 2 inches in diameter. According to Dan Holohan’s book, “The Lost Art of Steam Heating,” each steam line has a capacity of approximately 92,640 BTU per hour. The total capacity of two steam lines is about 183,280 BTU per hour.  

This boiler was oversized by a factor of two to feed the main steam lines. 

Every radiator in the house was measured to determine its BTU output. BTU ratings of radiators were taken from standard charts in accordance with Holohan’s book, “EDR, Every Darn Radiator.” The total output of the radiators was determined to be about 81,360 BTU per hour. 

This boiler was oversized by a factor of four to feed the radiators.

There were two risers; one at 1-½ inches in diameter and the other at 1-¼ inches in diameter.  Holohan gives the BTU rating of vertical risers for one pipe steam systems. 

In this case, the maximum capacity of the risers was determined to be approximately 96,480 BTU per hour. It appeared that the original design of the steam system was correct.

When you have excessive steam flowing up a riser that exceeds its capacity, you will have condensate held up in suspension in the vertical risers. You will have water hammer, spitting vents, uneven heat and water level problems in the boiler.  

However, the actual average peak heating load at the beginning of 2015 was a small fraction of the capacity of the output of the boiler. From February 5, 2015 until March 6, 2015, the usage of gas for heating was approximately 110 CCF. At 103,000 BTU per CCF, the hourly average usage of gas was only 16,278 BTU per hour. Thus, the boiler was oversized by a factor of 27.6!

What was I supposed to do? The owner did not want to spend the money to install a smaller boiler, as there was no return on this investment. So, I recommended the installation of a Honeywell Vaporstat Model L408J controller.  

One might be able to overcome the steam pipe banging problem by setting the steam pressure extremely low at no more than 2 ounces. The idea is to keep the boiler pressure very low so that condensate can fall down against the extremely low steam pressure and not be held in suspension. It should be noted that the highest steam velocity occurs at start up at zero pressure.

Installers of steam boilers need to perform the necessary inspection work prior to the selection of a steam boiler. If in doubt as to what to do, recommend to the owner of the building that they hire an engineer to do the heating load analysis. In every case, it is money well spent. 

Daniel Karpen is a professional engineer with over 30 years of experience in energy conservation engineering. He is based in Huntington, New York, and he is licensed to practice engineering in New York state. He can be reached at 631-427-0723 or www.danielkarpen.com.

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