Bryant Q674 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

27 60-2485—8

Fig. 39. Auxiliary heat in two-stage thermostat.

This method keeps the auxiliar

y

heat off until the heatin

g

load

is lar

g

e enou

g

h to demand 100 percent heat pump operation.

That demand is measured b

y

the thermostat and is the actual

heat re

q

uirement of the space. The two-sta

g

e thermostat

re

q

uires about a 2°F

(

1.1°C

)

room temperature drop to brin

g

on the second sta

g

e.

Stage or Time Modulated Control

There are two different wa

y

s to control auxiliar

y

heat, sta

g

ed

or time modulated control.

Outdoor thermostats

are used to switch additional increments

of electric heat into the thermostat circuit as the outdoor

temperature

g

ets lower. All increments of auxiliar

y

heat are

still controlled b

y

sta

g

e-two of the thermostat but onl

y

if the

outdoor temperature re

q

uires the additional capacit

y

. See

Fi

g

. 40.

Fig. 40. Outdoor thermostats for auxiliary heat.

The rationale for this s

y

stem is that it ver

y

closel

y

matches the

s

y

stem capacit

y

to the heatin

g

load of the buildin

g

. B

y

calculatin

g

the heat loss of a buildin

g

, the installin

g

contractor

can pick the theoretical outdoor temperature at which to

permit each additional unit of electric heat to be operated b

y

the second sta

g

e of the room thermostat.

The disadvanta

g

e of this s

y

stem is that it can re

q

uire several

outdoor thermostats. Also, since the unit is operatin

g

most

often at nearl

y

full capacit

y

, the thermostat is on lon

g

er which

results in more droop, with a little more offset from the

thermostat setpoint.

In addition, the capacit

y

is tied to theoretical load, based on heat

loss calculations. Normall

y

this is ver

y

close to the actual load,

but there are times where the second sta

g

e is switched on too

soon

(

no harm done

)

or too late

(

the buildin

g

g

ets cold

)

.

The T874 Thermostat produces

time modulated

control, which

is the on time of the thermostat chan

g

in

g

durin

g

each c

y

cle as

the load chan

g

es. The on time of the heatin

g

s

y

stem is

directl

y

proportional to the heatin

g

load of the buildin

g

. As an

example of the

q

ualit

y

of time modulated control, consider the

t

y

pical

g

as furnace. It can have a capacit

y

of 100,000 Btu

(

293 kW

)

, which is controlled On-Off. The output is 100,000 Btu

(

293 kW

)

or nothin

g

. A

q

ualit

y

thermostat controls so closel

y

that

the occupants rarel

y

detect chan

g

es in room temperature.

The same is true with 50,000 or 60,000 Btu

(

146.5 or

178.8 kW

)

of heat pump auxiliar

y

electric heat. The T874 can

provide the same hi

g

h

q

ualit

y

control.

This scheme also eliminates the need for outdoor thermostats

and thus offers the manufacturer an opportunit

y

to reduce unit

cost. One disadvanta

g

e is that a

thermostat jiggler

can be

demandin

g

more kW of strip heat when chill

y

.

The method an

y

particular manufacturer selects depends on

which ar

g

uments it finds persuasive and the opinions of its

distributors and dealers.

Three-Stage Thermostat

A definite improvement in

q

ualit

y

of control and econom

y

can

be achieved with a three-sta

g

e thermostat

(

T874W

)

, which is

especiall

y

true with heat pumps usin

g

dual compressors.

Three-sta

g

e control permits both the heat pump and the

auxiliar

y

heat to be tied directl

y

to the demand of the

controlled space. With a two-sta

g

e compressor, maximum

(

100 percent

)

heat pump operation is re

q

uired before auxiliar

y

heat is brou

g

ht on.

Emergency Heat

Some provision is

g

enerall

y

re

q

uired to back up the compressor

in the event of a failure durin

g

the heatin

g

season. In fact, it is

q

uite common to find that local buildin

g

codes or electric utilities

re

q

uire that a specified percent of the buildin

g

heatin

g

re

q

uirements be available from emer

g

enc

y

heat—a source

other than the heat pump compressor. Almost universall

y

, the

source is electric resistance heaters. T

y

picall

y

, the re

q

uirement

is for 70 or 80 percent of the buildin

g

heatin

g

needs.

Switching to Emergency Heat

Conventional heat pump control

(

thermostat lo

g

ic

)

usuall

y

includes a manual subbase switch to brin

g

on the emer

g

enc

y

heat.

In one control strate

gy

, the

emergency heat

rela

y

is turned on

b

y

the EM.HT. selector switch on the thermostat subbase. The

compressor is prevented from runnin

g

. The electric heaters

are c

y

cled as the

y

normall

y

would be b

y

the

second stage

of

M5836

COMPRESSOR

CONTRACTOR

THERMOSTAT

HEAT 2

HEAT 1

AUXILIARY

HEAT RELAY

CONTROLS

AT 65°F (18°C)

CONTROLS

AT 63°F (17°C)

M5834

COMPRESSOR

CONTACTOR

THERMOSTAT

HEAT 2

HEAT 1

AUXILIARY

HEAT RELAY 1

AUXILIARY

HEAT RELAY 2

AUXILIARY

HEAT RELAY 3

OUTDOOR

THERMOSTATS

20°F

(-7°C)

5°F

(-15°C)