Bryant Q674 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

21 60-2485—8

OPERATION

Heat Anticipation/Cool Anticipation

When controllin

g

a heatin

g

unit with a thermostat, the

temperature does not remain exactl

y

at the thermostat

setpoint, but varies within a certain temperature ran

g

e. Heat

anticipation is added to the thermostat to reduce this ran

g

e.

The anticipator is a small resistive heater in the thermostat

that heats when the s

y

stem is on

(

heat mode

)

or off

(

cool

mode

)

. The heat produced b

y

the anticipator raises the

internal bimetal temperature sli

g

htl

y

faster than the

surroundin

g

room temperature. The thermostat

anticipates

the need to shut off the heatin

g

s

y

stem sooner than it would if

affected b

y

room temperature onl

y

.

There are two t

y

pes of heat anticipation, ad

j

ustable and fixed.

Ad

j

ustable anticipation is also called

current anticipation

. See

Fi

g

. 26. The heater is in series with the mercur

y

switch and

heatin

g

primar

y

. The pointer is ad

j

usted to match the current

draw of the primar

y

control, and the correct amount of heat is

added for proper c

y

cle rates.

Fig. 26. Adjustable anticipation heater in series with load.

Fixed anticipation is also called

voltage anticipation

. See

Fi

g

. 27. The heater is in parallel with the primar

y

control and is

not affected b

y

the current draw of the primar

y

. It establishes

the proper c

y

cle rate for an

y

application.

Fig. 27. Fixed anticipation heater in parallel with load.

Heat anticipators add heat durin

g

the call for heat. Coolin

g

anticipators are activated while the air conditioner is off. See

Fi

g

. 28. This heater makes the thermostat think it is warmer than

it reall

y

is, and brin

g

s it on sooner than the bimetal alone would.

Fig. 28. Internal cooling anticipation schematic.

Interstage Differential

The mercur

y

switch of each sta

g

e of heat makes at a sli

g

htl

y

different temperature; that is, the mercur

y

makes the contacts

of the first sta

g

e bulb at one temperature, and the second

sta

g

e bulb at another temperature. The difference between

these two temperatures is the

interstage differential

.

Intersta

g

e differential is the difference between the two make

points when the bulbs are controlled b

y

the heatin

g

of the

bimetal and the action of the heat anticipator. The intersta

g

e

differential is 1.9°F

(

1°C

)

between the sta

g

es of heatin

g

or the

sta

g

es of coolin

g

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 sta

g

es of heatin

g

or

between the sta

g

es of coolin

g

for most models.

Droop

The addition of anticipator heat to the bimetal causes a control

factor called

droop

.

As the weather

g

rows colder, the heatin

g

appliance must

operate lon

g

er and more fre

q

uentl

y

. More heat must be added

to the bimetal. The bimetal now thinks it is warmer than it

reall

y

is, and actual room temperature is controlled at a lower

temperature than the setpoint indicates. At 100 percent heat

load, this droop can be si

g

nificant.

The C815A Outdoor Reset Thermistor is used in heat pump

s

y

stems to minimize droop. Without some method of

counterin

g

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

si

g

nificantl

y

improved thermostat performance when

compared to conventional multista

g

e thermostats. The C815A

is usuall

y

located in the heat pump condensin

g

unit outdoors.

FALL

L1

(HOT)

L2

M5823

STAGE 1 HEAT RELAY

H1 ANTICIPATOR

H1

FALL

L1

(HOT)

L2

M5824

STAGE 1

HEAT RELAY

H1 ANTICIPATOR

H1

RISE

L1

(HOT)

L2

M5825

STAGE 1

COOL RELAY

C1 ANTICIPATOR

C1