The Hartman Company Newsletter

Issue: January, 2002
Topic: Better Device Selection for Better Control

Dear Hartman Company Newsletter subscriber,

In every building comfort control system, money is still often wasted by
design engineers and controls subcontractors through the use of sensors or
other end devices that are more sophisticated than necessary, poorly suited
for their applications, or sensors and end devices that fail to measure the
correct variable for optimum control of the system for which they are
applied. Here is a list of references we have developed which you may wish
to consult to help improve the cost effectiveness of your device selection
for heating ventilation and air conditioning (HVAC) controls.

1. Highly accurate, stable and reliable temperature sensors are available
for most HVAC temperature sensing applications that are very reasonably
priced. Still, some designers and contractors require their sensors to have
linear outputs. The need for linear signals from input devices is most
certainly not required, nor is linear response from control loops required
in most instances with the interface capabilities of modern Direct Digital
Control (DDC) systems. These issues are discussed in an article on the
subject by Mr. Hartman that appeared in HPAC Magazine a few years ago. An
on-line copy of this article can be obtained at:

http://www.hartmanco.com/pdf/a27.pdf

2.  Air and water modulating control often wastes energy by under sizing
dampers or valves and then requiring excessive pressure from the fan or
pump that serves the system. This practice is based on the premise of valve
or damper "authority" whereby the concern is that if this authority is too
low, the loop becomes unstable. The above reference provides some
information about why modern DDC systems, when properly designed and
configured, can operate loops with much lower valve or damper authority
with excellent stability, thus reducing the energy use and cost of those
systems. Additional information on this subject as it particularly relates
to valve selection can be found in a recent article by Mr. Hartman that
appeared in HPAC Magazine. An on-line copy of this article can be obtained at:

http://www.hartmanco.com/pdf/a32.pdf

3. A timeless discussion on selection and specification of input and output
devices appears in an article authored by Mr. Hartman over a decade ago
that appeared in HPAC Magazine in 1991. An on-line copy of this article can
be obtained at:

http://www.hartmanco.com/pdf/a22.pdf

4. Finally, the recent Introduction of the new ultra-efficient method of
control called "Demand Based Control" has changed the selection of sensors
that are employed for HVAC control.  To get an idea of the impact Demand
Based Control has on sensor selection, you may wish to review the on-line
interview Ken Sinclair, Editor of AutomatedBuildings.com, conducted
recently with Mr. Hartman. This interview is available at:

http://www.automatedbuildings.com/news/nov01/int/hartman.htm



Linda Zuvich, Editor
THC News 
The Hartman Company



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                          The Hartman Company
              An HVAC Engineering & Technology Development Firm
                        http://www.hartmanco.com