Service manual

A DJM3.2 MANUAL

temperature for each of the main values for the discharge set-point and Pot Reset Ratio settings.

When using this reset system the discharge temperature will modulate to maintain the minimum

or maximum, and will not operate at the discharge set-point.

DIPSWITCH 3 MUST BE ON TO ACTIVATE THIS FEATURE.

CONNECT “S” TO “SP” IF REQUIRED (to activate set-point on DJM3.2).

DAY OPERATION

• If thermostat made between X and Z, discharge temperature equals dial set-

point plus the maximum reset upwards.

• If thermostat open between X and Z, discharge temperature equals dial set-

point less the maximum reset downwards.

NIGHT OPERATION

• If thermostat made then discharge temperature equals 120F.

• If thermostat open then the heat is off.

NOTE: When the room thermostat calls while operating in night mode, a modulating high limit

feature built into the DJM3.2 will not allow the discharge temperature to exceed 120ºF

if the unit’s heating capacity will allow temperatures to rise that high.

During the night cycle (when the DJM3's terminal “K” is powered) the readouts for the calculated

set-point (across resistor R124) 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” overrides

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 DJM3.2 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.

This operation is difficult to understand because the discharge temperature - as selected on the

DJM3.2 set-point dial - will never be the selected 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. If you refer to the chart located in the “Modulating Room Reset” section on

page 12 of this guide, you will notice that the amount of reset will be split equally at only one

point, that being at about 82ºF. It is therefore important to refer to this chart if you want to know

what the theoretical operating points will be, or else you can use the "Calculated Set-Point

Readout" to read the actual operating points with the room thermostat calling or not calling.

To understand the theoretical set up it is best to use the following example:

The desire is to serve a restaurant dining area by maintaining a space temperature of about 72ºF,

using a room on/off thermostat set at 72ºF. For this explanation we will assume 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. We desire to

discharge at about 85ºF to bring the room back up to 72ºF. The difference between the two

Page 19 Revised: 3/11/99