Bryant T874 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

23 60-2485—8

Fig. 31. System switching for automatic changeover.

Automatic chan

g

eover is fre

q

uentl

y

used in areas where there

are lar

g

e temperature differences in 24 hours. Some uses for

the automatic chan

g

eover feature are in the southern states

(

cool in the mornin

g

and hot in the afternoon

)

and in

commercial applications.

Manual chan

g

eover re

q

uires a s

y

stem switch movement to

chan

g

e the mode. See Fi

g

. 32. Most manual chan

g

eover

switches consist of HEAT-OFF-COOL choices. For heatin

g

,

switch to HEAT. For coolin

g

, switch to COOL. It is not automatic.

Manual chan

g

eover is often used in areas where the

temperature is relativel

y

stable between da

y

and ni

g

ht.

Fig. 32. System switching for manual changeover.

Commercial Rooftop Application

The T874 can be used as part of a commercial rooftop

application to run mechanical coolin

g

.

An economizer is often used as part of a rooftop application to

save on air conditionin

g

. See Fi

g

. 33.

Fig. 33. Typical commercial rooftop economizer

application.

Enthalpy

is the temperature and humidit

y

of the air. A more

accurate anal

y

sis of the suitabilit

y

for free coolin

g

can be

made b

y

monitorin

g

the enthalp

y

.

When enthalp

y

is low enou

g

h, the thermostat and a sensor

tell the economizer damper to open to let in cool air from the

outside. When enthalp

y

is hi

g

h, the thermostat and sensor tell

the damper to close. No more outside air comes in, so the air

conditioner is used instead.

Heat Pump Application

A heat pump s

y

stem operates much like an air conditioner.

However, the heat pump is capable of movin

g

heat in two

directions; from inside the home to outdoors for coolin

g

, or

from outdoors to inside for heatin

g

.

The heat pump can be controlled b

y

separate rela

y

/contactor

components, which are ener

g

ized or de-ener

g

ized b

y

the T874.

Chan

g

eover between heatin

g

and coolin

g

for heat pump

circuits can be accomplished either manuall

y

or automaticall

y

.

Manual chan

g

eover control re

q

uires a s

y

stem switch

movement to chan

g

e the mode. The reversin

g

valve is

activated b

y

movin

g

the s

y

stem switch to COOL for coolin

g

or

to HEAT for heatin

g

operation.

Automatic chan

g

eover is accomplished b

y

one of the mercur

y

bulb switches in the thermostat. On a s

y

stem with automatic

chan

g

eover on coolin

g

, the chan

g

eover valve is ener

g

ized b

y

a mercur

y

switch that makes on a temperature rise. On

heatin

g

, the chan

g

eover valve is ener

g

ized b

y

a mercur

y

switch that makes on a temperature fall.

A description of the methods used to control heat pumps

follows.

Space Temperature Sensing Low Voltage

Controls

Control s

y

stems for heat pumps usuall

y

utilize low volta

g

e

control circuits. The variet

y

of control functions re

q

uired and

the relative complexit

y

re

q

uire a versatile and economical

method of control. There are several interlocks and indication

functions that would be more difficult and expensive to wire in

line volta

g

e circuits. Low volta

g

e controls have the precision

and flexibilit

y

needed.

The thermostat used is a low volta

g

e device that has

g

ood

temperature sensitivit

y

and several manual and automatic

switchin

g

capabilities.

Two-Stage Thermostat

In most installations, the heat pump itself re

q

uires auxiliar

y

heat, usuall

y

in the form of electric strip heaters, to carr

y

the

buildin

g

heatin

g

load at desi

g

n conditions. Since the heat

pump is usuall

y

more efficient, the control strate

gy

is to run

the heat pump

whenever possible

and the auxiliar

y

heat

only

when necessary—

durin

g

ver

y

cold weather or if the heat

pump fails. Usuall

y

the auxiliar

y

heat is used onl

y

below the

balance point

(

temperature at which the heat pump cannot

handle heatin

g

load b

y

itself

)

.

A two-sta

g

e thermostat makes this possible. The two sta

g

es

make

sequentially

as the temperature drops. There is a

de

g

ree or two between sta

g

es so that the second sta

g

e

(

the

OFF EM. HT. HEAT

AUTO COOL

M5828

OFF EM. HT. HEAT

COOL

M5829

M5827A

H205 OR H705

ENTHALPY

CONTROL

RETURN

AIR

DISCHARGE AIR SENSOR

DOWNSTREAM FROM THE

EVAPORATOR COIL

OUTDOOR

AIR