Honeywell T775 Series 2000, Application Manual

T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS

63-7147—05

6

T775 OPERATIONS

Integral Action

“Droop” and equipment hunting can be minimized by 

summing (integrating) the offset errors over time and adding 

this correction to the output voltage.

A non-zero value for the integral time will allow the controlled 

temperature to try and reach the setpoint value.

The integral time is factory set for 400 seconds and is similar 

to the response time of the T775 Series 1000 models. This is 

a good middle range and should satisfy many applications. 

The integral time can be increased for applications where 

sensed response is slow, and can be decreased for 

applications where sensed response is fast (e.g. discharge air 

control).

As a starting point, an optimal integral time for discharge air 

typically ranges from 12 to 200 seconds. An optimal integral 

time for room control typically ranges from 60 to 2,500 

seconds. The purpose of integral action is to reduce or 

eliminate the offset from setpoint during steady state control 

that is often seen in proportional-only control.

Keep in mind that the controller is most sensitive to throttling 

range. Adjust the throttling range first before making any 

adjustment to integral time. Adjust throttling range to be as 

wide as possible to start, because this will provide the most 

stable control. Remember that the integral will eliminate the 

steady state error so you do not need to have a small throttling 

range to have accurate control. (Integral action allows for 

controlling to setpoint even with a wide throttling range.)

Derivative Action

Proportional-integral-derivative (PID) control adds the 

derivative function to PI control. The derivative function 

opposes any change and is proportional to the rate of change. 

The more quickly the control point (actual sensed 

temperature) changes, the more corrective action the PID 

system provides.

If the control point moves away from the setpoint, the 

derivative function outputs a corrective action to bring the 

control point back more quickly than through integral action 

alone. If the control point moves toward the setpoint, the 

derivative function reduces the corrective action to slow down 

the approach to the setpoint, which reduces the possibility of 

overshoot. The rate time setting determines the effect of the 

derivative action. The rate time is the time interval by which 

the derivative function advances the effect of the proportional 

action. In T775 controllers, the derivative rate time can range 

from 0 to 3,600 seconds. The higher the derivative setting, the 

greater the effect.

For all T775 Series 2000 controllers, the derivative default 

value is factory set to zero (no derivative control). It is strongly 

recommended that the derivative remain at zero (0) unless 

you have a very good reason to adjust it. Derivative control is 

not needed in the vast majority of HVAC applications.

Differential vs. Throttling Range

Differential is used for relay outputs, and throttling range is 

used for modulating outputs.

Setpoint and Differential

The following describes the relationship between setpoint and 

differential for heating and cooling. These settings are 

programmed for each output relay.

HEATING MODE SETPOINT AND DIFFERENTIAL

In heating mode, the differential is below the setpoint. The 

relay de-energizes when the temperature rises to the setpoint. 

As the temperature drops to the setpoint minus the 

differential, the relay energizes.

COOLING MODE SETPOINT AND DIFFERENTIAL

In cooling mode, the differential is above the setpoint. The 

relay de-energizes when the temperature falls to the setpoint. 

As the temperature rises to the setpoint plus the differential, 

the relay energizes.

Throttling Range

The throttling range brackets the setpoint setting, e.g., if the 

setpoint is 72° F (22° C) and the throttling range is 10° F 

(-12° C), then the effective throttling temperature range is 

67° to 77° F (19° to 25° C) . This applies to both modulating 

outputs and floating outputs.

Throttling Range for Modulating High or 
Low Limit 

On models that support this feature, the throttling range for 

the modulating high or low limit positions the setpoint at the 

end

 of the throttling range. For example, with a high (Heat) 

limit at Sensor B of 200° F (93° C) and a throttling range of 

10° F (-12° C), the modulating output controlling Sensor A 

begins to throttle back at 190° F (88° C), and fully closes at 

200° F (93° C). Conversely, the throttling range for the low 

limit begins above the Cooling setpoint in the same manner.

Setpoint High Limit

You can set an irreversible setpoint high limit maximum value 

for any single setpoint temperature value. This prevents the 

user from setting any setpoint above the chosen high setpoint 

limit, which is useful for meeting some local codes.

Adjust the setpoint (at any output) to the desired maximum 

setpoint. Then, simultaneously press the 

HOME,



, and 



buttons, and continue to press all three buttons for five 

seconds to set the setpoint high limit maximum to this value.

NOTE: You must press all three buttons at exactly the same 

time for this action to occur.

IMPORTANT

1. This action sets the maximum setpoint value of 

all

outputs to the setpoint high limit maximum.

2. Setting the high limit setpoint maximum

is 

irreversible

. If you perform the action inadvertently 

and this setpoint adversely affects the control of your 

system, you must replace the controller.