I received a call from a client who had been working with a contractor for several months to resolve a recurring problem with a recently installed air-handling unit. The unit had been dropping out on freezestat whenever the outside-air temperature dropped below freezing. After several attempts to find a solution, the contractor declared it a “design problem” and disavowed any further responsibility.

On the next below-freezing morning, I visited the site and noted that the elevation between the outlet of the steam coil and the inlet to the steam trap was less than detailed on our drawings. I suspected there might be a stratification problem caused by the coil's bottom row, which was water-logged and not draining properly. I took several temperature readings. To my surprise, the air temperature was uniform across the face of the coil. However, the air-temperature rise was less than half of what was specified. I checked the steam pressure at the inlet to the steam-control valve. It was 12 psig, which was reasonable. I checked the control valve, and it was wide open. I opened the bypass valve around the control valve, but the supply-air temperature remained about the same.

After a little head scratching, I cut away the pipe insulation with my penknife and discovered the “design problem.” Although the contract documents indicated the steam supply pipe should be 3 in. in diameter, the area in the vicinity of the steam coil was congested, so the contractor reduced the diameter to 1¼ in. to match the control valve. I had the contractor replace the 1¼-in. pipe with the specified 3-in. pipe, reducing only at the inlet and outlet of the control valve. The unit has worked fine ever since.
Donald A. Raynor, PE
Kovacs, Whitney & Associates
Baltimore, Md.

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