High Limit Switching - Honeywell Design and Application Technical Reference Manual

Section 6 - W7459 Economizer Module
If the return air were to the left of the enthalpy
curve, you would use the enthalpy curve
instead of the return air conditions. For
example, if the return air conditions were
between D and C and the outdoor was to the
right of the A AND the potentiometer was set
Table 2. W7459D, W7210D and W7212 High Limit Switching.
Percent RH
25
50
60
75
The W7459D, W6210D, W7210D, and W7212
include the high limit function that is not used
on any other economizer modules. It is a high
enthalpy limit that applies to both return and
outside air. It only applies when using
differential changeover, not single enthalpy.
When the return and outside enthalpy both
exceed the high limit curve, (refer to the
psychrometric chart on the previous page) the
outside air dampers are closed to the
minimum position and the mechanical cooling
is energized. In a standard differential
enthalpy control sequence the lower of the
outside and return is selected regardless of
the amount of enthalpy in either.
63-8594-02 Honeywell Economizers

High Limit Switching

Switch Over Point on Enthalpy
Decreasing
83°F ±0.5°F (28°C ±0.3°C)
78°F ±0.5°F (26°C ±0.3°C)
76°F ±0.5°F (24°C ±0.3°C)
73°F ±0.5°F (23°C ±0.3°C)
to B, you would use the B curve instead of the
return air. If the outdoor air conditions were to
the left of the return air you would use the
outdoor air. If the outdoor conditions are to the
right of the return air conditions you use the
return air.
Switch Over Point on Enthalpy
85°F ±0.5°F (29°C ±0.3°C)
80°F ±0.5°F (27°C ±0.3°C)
78°F ±0.5°F (26°C ±0.3°C)
75°F ±0.5°F (24°C ±0.3°C)
A typical application of this limit function is a
building located in a warm, humid climate with
a high internal heat gain such as a laundry or
kitchen. A standard differential enthalpy
control circuit may have extremely warm,
humid outside air being cooled just because
the indoor air is temporarily higher in enthalpy.
The mechanical cooling equipment might be
undersized and the indoor enthalpy then
remains higher than outside longer than the
desired period of time. The high limit function
(only available on some economizer modules)
prevents this from occurring by automatically
switching to return air and turning on the
mechanical cooling during very high outdoor
air enthalpy conditions.
58
Increasing