30 November 1998
f. In two-deck multizone and dual-duct multizone systems, given that there is no deadband between
heating and cooling, there may not be sufficient change in the return air temperature between heating
and cooling seasons to provide an indication as to what mode the system is in (heating or cooling). (Note
that the use of two-deck or dual-duct multizones does not comply with federal regulations (10 CFR 435)
which require a deadband between heating and cooling; a bypass multizone is a better choice and allows
for the use of a deadband.) After evaluating several alternative methods for economizer mode initiation,
the method described herein was judged to be the simplest and most reliable, and is therefore the
We will look at two situations: 1) the unit is either served by a dual-temp hydronic system, or HW and
CHW availability is seasonally scheduled (i.e.- only HW is available during the heating season and vice
versa), and 2) HW and CHW are both available year-round.
Case 1: Dual-temp hydronics or scheduled HW & CHW - Make the "cooling mode" decision based
on OA temperature rather than RA temperature. (i.e. - If OA is above X degrees, then assume the space
needs cooling.) In this case, the outside air temperature would be the PV input to EC and the return air
temperature would the CPA input to EC. The outdoor air temperature above which the space will require
cooling could be determined using computer models; however, the value would change as internal loads
changed. In practice, it would seem that an engineering judgement should be made initially and adjusted
later if required.
Case 2: Simultaneous availability of HW and CHW - In this case it is recommended that an
economizer cycle not be used. An example system which does not utilize an economizer cycle is
depicted in Chapter 5.
g. Because of the difficulty of maintaining enthalpy based economizer switchover hardware, the
economizer controller operation is based on dry bulb temperature measurements rather than enthalpy
measurements. The comparison of outside air and return air temperatures for determining the
economizer switchover point is a method of control that uses local weather data for selecting an optimum
dry-bulb temperature difference. An explanation of this method begins with figure 3-7.
Figure 3-7. Design condition for economizer mode operation.
h. The skeleton psychrometric chart shows a return air design condition of 24 degrees C (75 degrees
F) dry-bulb temperature and 50 percent relative humidity (point A). A constant enthalpy line (B-C) drawn
through this condition divides the chart into 4 regions of outside air temperatures and outside air relative
humidities, which are:
(1) Region A, in which temperature and enthalpy conditions are less than return air design
(2) Region B, in which temperature conditions are lower but enthalpy conditions are higher than
return air design conditions.
(3) Region C, in which both temperature and enthalpy conditions are higher than the return air
(4) Region D, in which temperature conditions are higher but enthalpy conditions are lower than
return air design conditions.