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VAV-SVX02B-EN
Thermostat Selection
and Location
In general, both one- and two-pipe
thermostats have a gain of 2.5 psi per
degree Fahrenheit, unless special
thermostats are used that provide a
different gain. The major difference
between them is that a one-pipe
thermostat is considered a low-
capacity thermostat and a two-pipe
thermostat is considered a high-
capacity thermostat.
When selecting between one-pipe and
two-pipe thermostats consider the
following:
Use one-pipe thermostat when:
tubing runs are less than 50 feet from
thermostat to controller.
a single thermostat will control no
more than three controllers.
slower response is desired from the
thermostat.
Use two-pipe thermostat when:
tubing runs exceed 50 feet from
thermostat to controller.
a single thermostat will control four or
more controllers.
high capacity air is required due to
restrictions in the air line.
Thermostat Piping
The two-pipe thermostat (Figure 1)
includes a restrictor (internally). Attach
supply air line and output pressure
(branch) line to the thermostat.
The one-pipe thermostat (Figure 2)
requires an external restrictor tee.
Connect main air supply to restricted
leg of restrictor tee. Connect branch line
from restrictor tee to the thermostat.
Figure 1–Two-Pipe Remote Thermostat (Reverse-Acting)
Thermostat
Figure 2–Typical One-Pipe Thermostat (Reverse-Acting)
2
Tee
20
(137.9)
15
(103.4)
S
Two-Pipe
Remote-Mounted
T-Stat
(Reverse-Acting)
(137.9)
20
S
Tee
Restrictor
Tee
Restricted Leg
Fan
P.E.
Switch–1
(N.O.)
9 PSI
(62.06 kPa)
One-Pipe
Remote-Mounted
T-Stat
(Reverse-Acting)