I originally became interested in fan heat in about 1975.  As a young engineer of 27, I found myself assigned to do testing work on an existing high-rise office building in downtown St. Louis.  I was attempting to determine why the design discharge air temperature of 55°F could not be achieved on the leaving side of a certain air handling unit.  The unit was a high pressure draw-thru unit with roll filters, heating coil, cooling coil, and supply fan serving a two-pipe induction system with induction terminals in the building perimeter offices.  While I was making temperature readings to quantify the problem, the chief engineer at the facility saw me working and stated bluntly that he already knew the source of the problem.  However, since he had no degree in engineering (or as he put it – no sheepskin), no one would believe him.

Though I was not in the mood for his advice, I had no choice but to listen since at this point, I was a captive audience.  The chief engineer proceeded to tell me that the source of the problem was the high pressure drops across the filters and coils.  These pressure drops, because of the friction taking place, was causing the air to heat up in transit through these devices, raising the supply air temperature.  Though this argument was not correct, I had neither the patience nor the inclination at the time to try to explain why.

As an instructor in the ASHRAE Professional Development Seminar “Air System Design and Retrofit” starting in 1982, many questions arose with regard to the source of fan heat and where in the duct system this heat appears.  These questions were asked by competent mechanical engineers, thus indicating the confusion on this issue in our industry.  After I grew weary from presenting the same explanation at seminar after seminar, I published the first paper on the topic in the January 1989 issue of Heating Piping and Air Conditioning magazine entitled “Fan Heat: Its Source and Significance”.  Now I could simply use it as a handout at each seminar.

The articles to follow were printed again in HPAC Engineering magazine (the new name of the same publication) in August and September of 2005 with the title “Fan Heat and Pump Heat: Sources and Significance”.  Pump heat was added here because although the processes seem quite similar, the end results are not.  One thing good about articles like this is they are relatively timeless; the physics and thermodynamics of a process simply do not change over time.  Enjoy!

Article Author: Jerry Williams
Blog Post: Michael Mosbacher