Operation; Heat Anticipation/Cool Anticipation; Interstage Differential; Droop/Outdoor Reset - Honeywell T874 Product Data

OPERATION

Heat Anticipation/Cool Anticipation

When controlling a heating unit with a thermostat, the
temperature does not remain exactly at the thermostat
setpoint, but varies within a certain temperature range. Heat
anticipation is added to the thermostat to reduce this range.
The anticipator is a small resistive heater in the thermostat
that heats when the system is on (heat mode) or off (cool
mode). The heat produced by the anticipator raises the
internal bimetal temperature slightly faster than the
surrounding room temperature. The thermostat anticipates the
need to shut off the heating system sooner than it would if
affected by room temperature only.
There are two types of heat anticipation, adjustable and fixed.
Adjustable anticipation is also called current anticipation. See
Fig. 26. The heater is in series with the mercury switch and
heating primary. The pointer is adjusted to match the current
draw of the primary control, and the correct amount of heat is
added for proper cycle rates.
H1 ANTICIPATOR
H1
FALL
Fig. 26. Adjustable anticipation heater in series with load.
Fixed anticipation is also called voltage anticipation. See
Fig. 27. The heater is in parallel with the primary control and is
not affected by the current draw of the primary. It establishes
the proper cycle rate for any application.
FALL
H1 ANTICIPATOR
H1
Fig. 27. Fixed anticipation heater in parallel with load.
L1 L2
(HOT)
STAGE 1 HEAT RELAY
M5823
L1 L2
(HOT)
STAGE 1
HEAT RELAY
M5824
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Heat anticipators add heat during the call for heat. Cooling
anticipators are activated while the air conditioner is off. See
Fig. 28. This heater makes the thermostat think it is warmer than
it really is, and brings it on sooner than the bimetal alone would.
RISE
C1 ANTICIPATOR
Fig. 28. Internal cooling anticipation schematic.

Interstage Differential

The mercury switch of each stage of heat makes at a slightly
different temperature; that is, the mercury makes the contacts
of the first stage bulb at one temperature, and the second
stage bulb at another temperature. The difference between
these two temperatures is the interstage differential.
Interstage differential is the difference between the two make
points when the bulbs are controlled by the heating of the
bimetal and the action of the heat anticipator. The interstage
differential is 1.9°F (1°C) between the stages of heating or the
stages of cooling for most models.
Mechanical differential is the difference between the make
and break points of each switch. The mechanical differential
for the T874 is 1°F (0.6°C) between the stages of heating or
between the stages of cooling for most models.
Droop
The addition of anticipator heat to the bimetal causes a control
factor called droop.
As the weather grows colder, the heating appliance must
operate longer and more frequently. More heat must be added
to the bimetal. The bimetal now thinks it is warmer than it
really is, and actual room temperature is controlled at a lower
temperature than the setpoint indicates. At 100 percent heat
load, this droop can be significant.
The C815A Outdoor Reset Thermistor is used in heat pump
systems to minimize droop. Without some method of
countering this situation, the occupant could notice the
temperature offset.
Outdoor Reset
Some T874/Q674 models are intended for use with the
C815A Outdoor Thermistor. The outdoor thermistor provides
significantly improved thermostat performance when
compared to conventional multistage thermostats. The C815A
is usually located in the heat pump condensing unit outdoors.
21
L1
(HOT)
STAGE 1
COOL RELAY
C1
L2
M5825
60-2485—8