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. HEATCOOL
M5829
M5827A
H205 OR H705
ENTHALPY
CONTROL
RETURN
AIR
DISCHARGE AIR SENSOR
DOWNSTREAM FROM THE
EVAPORATOR COIL
OUTDOOR
AIR