A
G-TRAC2
IOM-25
26 of 42
Mar 99 R7
NOTE:
When the room thermostat calls while operating in night mode, a modulating high limit feature built into
the G-TRAC will not allow the discharge temperature to exceed the setting of the night SP pot (100° -
120°F), if the unit’s heating capacity will allow temperatures to rise that high.
During the night cycle when the G-TRAC's ter
and common, will not be correct. The reading will be that of the set point dial.
ussed above. The control is in
effect a two level control with both levels modulating to maintain each of their desired room discharge temperatures. If the
room thermostat is not calling then the G-TRAC control will maintain a constant discharge temperature by modulating the
burner at a lower discharge "calculated” set point. If the room thermostat is calling then the discharge temperature will be
set to maintain a higher "calculated" set point, again by modulating the burner to maintain the higher discharge
temperature.
To understand this operation, we need to realize temperature as selected on the G-TRAC set point dial will never be the
discharge temperature. Instead of using the modulating room reset thermostat to obtain a linear room reset operation, this
system takes advantage of the "modulating" reset function at its two extremes - maximum reset up and maximum reset
down.
The following is an example of a theoretical set up:
Assume the goal is to serve a restaurant dining area with intent of maintaining a space temperature of about 72ºF
using a room on/off thermostat set at 72ºF. For this example we will suppose there is a quantity of fresh air for
ventilation purposes that operates at 10% minimum fresh air or at a 58ºF mix box temperature. In order to satisfy
the cooling conditions of the restaurant in its busy hours, the desire is to discharge air at about 56ºF into the space.
When the restaurant is not crowded, the space may cool down depending on solar load, etc.
When the room thermostat calls for heat, the space temperature has fallen below the desired 72ºF set point of the
room thermostat. It is now necessary to discharge at a higher temperature then 72ºF to satisfy the room’s needs.
For this example have unit discharge at about 85ºF to bring the room back up to 72ºF.
Bandwidth is the difference between the two selected discharge temperatures - 85 - 56 = 29º
band width
. Referring
to the table found in Section XVII (page 21) in the section for a 70º base discharge set point, you find a minimum
discharge close to 56ºF and a maximum discharge close to 85ºF. This would be found under the 70°F discharge set
point area with pot 5 set at 3. The minimum discharge shown is 59°F and the maximum is 88°F.
If the mix box temperature set at 58ºF, the minimum temperature should be just below that setting. Otherwise as
the dampers modulate open to give cooling to the space, the heat will be on, thus warming up the discharge air.
To get the 56º-discharge set point we desired, note our theoretical selection is 57ºF with the set point dial at 70ºF.
If we turn the set point dial down 1º to 69ºF we will then have the desired 56º low discharge and when the room
thermostat calls the discharge temperature will rise to 87ºF.
NOTE:
With the above theoretical settings, the set point will read 69º
but the control will never try to operate
with a 69º discharge
. It will operate with a 56º discharge when the room thermostat is not calling and
with an 87º discharge when the room thermostat calls.
During the "Night" mode when the room thermostat calls, the discharge temperature will go to 120º if the unit heating
capacity will allow it. There is a high limit function built into the G-TRAC to stop the night heat from going above the 120º
discharge.
See Diagram D2 below
.
Содержание EngA G-TRAC2
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