Bryant T874 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

15 60-2485—8

thermostats. The schematics can be field-modified as

re

q

uired

(

EX: if T874C is bein

g

used, eliminate second

sta

g

e of heat

)

.

For heat pump circuits:

Cool chan

g

eover valve—operates on coolin

g

. The revers-

in

g

valve or rela

y

is activated either b

y

movin

g

the s

y

s-

tem switch to COOL

(

manual chan

g

eover

)

or b

y

a

mercur

y

switch that makes on a temperature rise

(

auto

chan

g

eover

)

.

Heat chan

g

eover valve—operates on heatin

g

. The revers-

in

g

valve or rela

y

is activated either b

y

movin

g

the s

y

s-

tem switch to HEAT

(

manual chan

g

eover

)

or b

y

a

mercur

y

switch that makes on a temperature fall

(

auto

chan

g

eover

)

.

S

y

stem monitor rela

y

—optional e

q

uipment on some heat

pumps includes an R4222P1065 or e

q

uivalent. This

s

y

stem monitor rela

y

detects a malfunction in the com-

pressor and indicates the malfunction b

y

activatin

g

the

EMERGENCY HEAT LED on the Q674 Switchin

g

Sub-

base. The s

y

stem monitor rela

y

is usuall

y

wired into the

L terminal on the Q674.

Each mercur

y

switch is identified b

y

function:

H1—Sta

g

e 1 Heatin

g

.

H2—Sta

g

e 2 Heatin

g

.

H3—Sta

g

e 3 Heatin

g

.

C1—Sta

g

e 1 Coolin

g

.

C2—Sta

g

e 2 Coolin

g

.

C3—Sta

g

e 3 Coolin

g

.

C/O—Chan

g

eover

(

heat pumps

)

.

Each anticipator is identified and each switch affected is

named

(

EX: H1 anticipator, C1 anticipator

)

.

All T874 Multista

g

e Thermostats use mercur

y

switches. Each

schematic indicates switch operation b

y

bein

g

drawn in the

open position with an arrow indicatin

g

operation with a

temperature RISE or FALL.

One circuit has been selected that is t

y

pical of various models

used with heat pumps. This circuit has been traced to

illustrate the functions performed b

y

these control s

y

stems.

Tracing Method:

1.

Alwa

y

s be

g

in at the s

y

stem transformer or R terminal.

You ma

y

want to draw the switch contacts in each

switch position to aid in tracin

g

. Colored pencils are

helpful when onl

y

one cop

y

of the circuit is available.

See Fi

g

. 16 for a description of the hookup s

y

mbols and

Fi

g

. 17 throu

g

h 25 for t

y

pical hookups.

2.

Completel

y

trace onl

y

one circuit at a time

(

for example:

heat or cool

)

.

3.

Connections are indicated b

y

small dots at the point of

intersection. If there is no dot, there is no connection.

4.

The left portion of the circuit

(

the thermostat

)

contains

the mercur

y

switches. The heat switches make on a

temperature fall, and the cool switches make on a tem-

perature rise. Fixed anticipation is represented b

y

a zi

g

-

za

g

line and ad

j

ustable anticipation is a zi

g

za

g

with an

arrow. The resistance of the fixed anticipator is so lar

g

e

it limits current so that a s

y

stem rela

y

cannot be pulled

in from a circuit path

g

oin

g

throu

g

h the fixed anticipator.

The rela

y

can be pulled in throu

g

h an ad

j

ustable antici-

pator because its resistance is

g

enerall

y

0 to 5 ohms.

5.

The center portion

(

the subbase

)

contains the switches.

The fan switch is above the s

y

stem switch. The small

circles on the switch represent the maximum possible

contacts available on the Q674 Subbase. The lar

g

er cir-

cles represent the switch positions available on this par-

ticular Q674, with the solid circle representin

g

where it

is actuall

y

switched on the dia

g

ram.

NOTE: Solid circles are not interconnected electricall

y

.

At the ri

g

ht, the rela

y

s and contactors are shown, attached to

the proper terminals. The terminals are represented b

y

lar

g

e

circles with terminal desi

g

nations in capital letters. See Table

3 for the meanin

g

of each lettered terminal.

Sometimes power for a fixed anticipator is brou

g

ht throu

g

h an

off s

y

stem rela

y

like the chan

g

eover rela

y

shown in Fi

g

. 20.

This current is kept low b

y

the hi

g

h resistance of the fixed

anticipator so that rela

y

does not pull in.

Fig. 16. Key to hookup symbols.

M5848

KEY TO HOOKUP SYMBOLS

TRANSFORMER

(24 VAC SECONDARY)

RELAY/CONTACTOR CONTACTS

LED

RELAY OR CONTACTOR COIL

MERCURY SWITCH

FIXED ANTICIPATOR

HIGH RESISTANCE

(TYPICALLY 5 KILOHMS)

ADJUSTABLE ANTICIPATOR

LOW RESISTANCE

(TYPICALLY 0 TO 5 OHMS)

TERMINAL

ODT

OUTDOOR THERMOSTAT

EHR

EMERGENCY HEAT RELAY

RTD

TIME DELAY RELAY

RD

DEFROST RELAY

CHP

PRESSURE SWITCH

LACO

LOW AMBIENT CUTOFF

B