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A G-TRAC2 MANUAL

General Manual Revised: 03/15/99

Page

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 terminal K is powered the readouts for the calculated set point

across "Dtemp" and common, will not be correct. The reading will be that of the set point dial.

This override is difficult to understand but is more functional then the

OR to V override discussed 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 rooms 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 16) 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.

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