Bryant T874 Manual

A SERVICE OF

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

60-2485—824

one controllin

g

the auxiliar

y

heat

)

makes onl

y

when the heat

pump alone cannot handle the load. That is how the strate

gy

of usin

g

the most economical heat first is implemented.

Cycling Rate

Normall

y

, thermostat anticipation is set to c

y

cle a fossil fuel

furnace about five or six times an hour

(

at 50 percent load

)

.

This provides a reasonable balance between comfort, stabilit

y

and econom

y

.

A heat pump, bein

g

a mechanical refri

g

eration s

y

stem, should

be c

y

cled at a rate of 2-1/2 to 3 times an hour. With a T874,

this is accomplished b

y

settin

g

the anticipator at the total

current draw of the controlled device

(

the contactor and

possibl

y

the chan

g

eover rela

y

or valve

)

. Most likel

y

, the

compressor operates from the first sta

g

e of the thermostat.

The second sta

g

e normall

y

controls the auxiliar

y

heaters. This

heat anticipator can be set for the current draw of the electric

heat primar

y

, and produces

g

ood control.

Voltage Anticipation

Althou

g

h not a specific re

q

uirement for heat pumps, several

models of the T874 have volta

g

e heat anticipation instead of

current anticipation.

With volta

g

e anticipation, the amount of heat added to the

thermostat is constant no matter which load is connected to it.

The anticipator realizes a constant volta

g

e and produces a

constant amount of heat durin

g

the ON c

y

cle. This allows the

c

y

clin

g

rate to be

designed into

the thermostat. The heat

pump manufacturer is assured of correct performance

independent of later thermostat ad

j

ustments.

Outdoor Reset

Outdoor reset, which a C815A Outdoor Thermistor makes

possible, is sometimes applied in heat pump s

y

stems to help

minimize the effect of the differential between sta

g

es. It takes

2°F

(

1.1°C

)

chan

g

e in temperature to brin

g

on the second

sta

g

e and full heatin

g

. Added to a certain amount of droop

under hi

g

h loads, there can be a lar

g

e offset between the

setpoint and room temperature.

Without some method to counteract this situation, the

temperature offset from the setpoint could be as much as 6°F

(

3.3°C

)

under hi

g

h load conditions. It is not likel

y

that the

occupants accept this variation without makin

g

fre

q

uent

thermostat settin

g

chan

g

es.

Outdoor reset has the effect of raisin

g

the setpoint at cold

outdoor temperatures. Then, even with a few de

g

rees droop,

the space temperature sta

y

s near the desired settin

g

.

This is done b

y

calibratin

g

the thermostat hi

g

h b

y

5°F

(

3°C

)

—

the amount of expected offset. This offset compensates for

differential and droop at hi

g

h load factors so the thermostat

switches at the setpoint. At warm temperatures, heat is added

to the thermostat to compensate for this offset; at cold

temperatures, the extra heat is automaticall

y

removed. The

effect is the same as raisin

g

the setpoint as the outdoor

temperature

g

oes down.

This is particularl

y

important on the 3-sta

g

e T874. The

additional sta

g

e means that more droop is introduced when

the s

y

stem is in operation. That is the reason the TRADELINE

T874W is sold in a packa

g

e with the outdoor sensor and the

correct subbase.

Outdoor Reset—How it Works

The heater in the thermostat is connected in series with a

thermistor sensor that measures outdoor temperature. See

Fi

g

. 34. In warm weather, the thermistor has ver

y

low

resistance, permittin

g

a lar

g

e current flow in the heater, which

g

enerates a relativel

y

lar

g

e amount of heat in the thermostat.

In cold weather, the outdoor sensor has hi

g

h resistance,

which results in low current flow and little or no heat bein

g

added to the thermostat. This raises the control point,

overcomin

g

the effects of droop and intersta

g

e differential.

Fig. 34. Location of thermistor.

Changeover

Chan

g

eover between heatin

g

and cooin

g

can be

accomplished either

automatically

or

manually

, dependin

g

on

the application. Chan

g

eover can occur in heatin

g

or coolin

g

.

The chan

g

eover valve is ener

g

ized as lon

g

as the subbase

function switch is in the appropriate position when

manual

chan

g

eover is used. If a s

y

stem manuall

y

chan

g

es over on

coolin

g

, the valve ener

g

izes whenever the subbase switch is

in the coolin

g

mode.

One of the mercur

y

bulb switches is used for control when the

thermostat has

automatic

chan

g

eover. When the

automatic

chan

g

eover is in coolin

g

, the chan

g

eover valve is ener

g

ized

when the first sta

g

e coolin

g

switch makes. The second sta

g

e

switch turns on the compressor.

It is more common with new e

q

uipment to find that

chan

g

eover occurs with coolin

g

; that is, the chan

g

eover valve

is ener

g

ized to produce coolin

g

.

Whether

automatic

or

manual

chan

g

eover is used and

whether the reversin

g

valve is ener

g

ized with coolin

g

or with

heatin

g

, a heat pump s

y

stem can be controlled b

y

the

appropriate T874 Multista

g

e Thermostat.

L1

(HOT)

L2

RESET

HEATER

C815A

OUTDOOR

THERMISTOR

SENSOR

M5826