Honeywell THM5421C1008 P+I Control, Minimum Off-Timer

VISIONPRO® IAQ TOTAL HOME COMFORT SYSTEM

68-0287—1 32

NOTE: It takes about a week for the thermostat to adjust to

local weather, the schedule, the construction of the

home and the heating and/or cooling system. Each

day it adjusts the next day’s recovery start time

accordingly.

P+I Control

The thermostat microprocessor-based control requires that the

user understands temperature control and thermostat

performance. A conventional mechanical or electronic

thermostat does not control temperature precisely at setpoint.

Typically there is an offset (droop) in the control point as the

system load changes. This is a phenomenon that most people

in the industry know and accept. Many factors contribute to

offset including switch differential, thermal lag, overshoot, cycle

rates and system load.

The thermostat microprocessor simultaneously gathers,

compares and computes data. Using this data, it controls a

wide variety of functions. The special proprietary algorithm

(program) in the thermostat eliminates the factors causing

offset. This makes temperature control more accurate than the

conventional mechanical or electronic thermostats. The

temperature control algorithm is called proportional plus

integral (P+I) control.

The thermostat sensor, located on the thermostat or remote,

senses the current space temperature. The proportional error

is calculated by comparing the sensed temperature to the

programmed setpoint. The deviation from the setpoint is the

proportional error.

The thermostat also determines integral error, which is a

deviation based on the length of error time. The sum of the two

errors is the (P+I) error. The cycle rate used to reach and

maintain the setpoint temperature is computed using the P+I.

The addition of the integral error is what differentiates the

thermostat from many other electronic and mechanical

thermostats. See Fig. 33.

Fig. 33. Proportional temperature control versus P+ I

temperature control.

Minimum Off-Timer

The Thermostat has built-in compressor protection (minimum-

off timer) that prevents the compressor from restarting too

early after a shutdown. The minimum-off timer is activated after

the compressor turns off.

If there is a call during the minimum-off timer, the thermostat

shows “Wait” in the display.

When the minimum-off timer expires, “Cool On” or “Heat On”a

appears in the display and the compressor and fan turn on.

_________

aHeat Pumps only.

Inside Humidity Level

The humidity level in the home is shown in the lower right

corner of the Home Screen.

NOTE: If an outdoor temperature sensor is present, the out-

door temperature will be displayed in this location

rather than the indoor humidity. To see indoor humid-

ity, display the humidity settings menu.

If thermostat is set to Auto Changeover System mode, press

the More key until the humidity level is shown on the screen.

HEATING COOLING

PERCENT LOAD

TEMPERATURE

THERMOSTAT

HEAT SETPOINT

P+I CONTROL

POINT

PROPORTIONAL COOL

CONTROL POINT

THERMOSTAT

COOL SETPOINT

P+I CONTROL POINT

PROPORTIONAL HEAT

CONTROL POINT

M4414

100 50 0 50 100

M23513

Wait

SCHED HOLD CLOCK SCREEN MORE

WED

FAN

AUTO

SYSTEM

HEAT

COOL

Following

Schedule

Inside Set To

Humidity