William Bahnfleth wbahnfleth at psu.edu
Wed Aug 23 17:50:45 PDT 2006

This is more or less correct, but the full 
explanation requires the introduction of some 
additional jargon.  The throttling range is a 
parameter of proportional control, the set point 
being the other.  For a system controlled 
proportionally, there is only one value of the 
cooling load for which the set point temperature 
is exactly satisfied.  If the load is less, space 
temperature will drop below set point and if it 
is greater, it will rise above.  The difference 
between this operating point and the set point is 
called error or offset.  Offset can be reduced or 
eliminated by adding a second, integral, 
component to the controller.  In integral 
control, the time integral of error is used to 
drive the space temperature back toward set point 
regardless of load.  Proportional + integral (PI) 
control is a type of control action that does not 
inherently require the use of digital controls, 
but has become commonplace because of them.

A good explanation with illustrative diagrams can 
be found at the Iowa Energy Center's DDC Online web site:


Bill Bahnfleth

At 08:20 PM 8/23/2006, John Aulbach wrote:
>To quote Shakespeare (and the DOE-2.1E mnual)
>"Throttling Range specifies the number of degrees that
>room temperature must change to go from full heating
>to zero heating and/or full cooling to zero cooling."
>The zone thermostat setpoint is assumed to be at the
>middle of the throttling range. (DOE-2.2 manual add)
>In other words, if you set your cooling t'stat at 72°F
>and set a throttling range of 3 degrees, you would be
>at full cooling at 73.5°F space temp and full cooling
>off at 70.5°F (as it is explained).
>I believe this has to do with the "slop" in the T'stat
>(which DDC controls virtually eliminates).
>But it also comes into play when you swing from
>cooling to heating and vice versa (such as a VAV
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William P. Bahnfleth, PhD, PE, Fellow ASHRAE

Professor of Architectural Engineering
Director, Indoor Environment Center

The Pennsylvania State University
104 Engineering Unit A
University Park, PA 16802 USA

voice: 814.863.2076 / fax: 814.863.4789
e-mail:  wbahnfleth at psu.edu

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