Snap Acting Relay; Lockout Relay; Switching Relay - Honeywell Pneumatic Controls Engineering Manual

Switching Relay

A switching relay requires a two-position pilot signal and is
available with either single-pole, double-throw (spdt) or double-
pole, double-throw (dpdt) switching action. Pneumatic heating
and cooling control systems use relays to switch a valve or
damper actuator from one circuit to another or to positively
open or close a device. Both spdt and dpdt switching relays are
available with a variety of switching pressures.
Figure 35 shows a typical spdt switching relay application for
heating/cooling operation in which the thermostat controls the
heating/cooling coil valve. Seasonal mainline pressure changes
cause the action of the thermostat to be reversed. A discharge
low-limit control is switched into the control circuit for heating
and out of the circuit for cooling. The switching is done from
mainline pressure connected to the pilot port (P).
During the heating cycle, the 18 psi mainline pressure is above
the preset switching pressure. The common port (C) connects
to the normally disconnected port (X), connecting the low-limit
controller to the thermostat branchline to prevent discharge
temperatures below the controller setting. The normally
connected port (O) is blocked.
ROOM
THERMOSTAT
DA WINTER
RA SUMMER
M B
RESTRICTOR
M
M
VALVE
COIL
Fig. 35. Typical Switching Relay for Application.
During the cooling cycle, the 13 psi mainline pressure at the
pilot port (P) is below the minimum switching pressure of the
preset limits. The common port (C) connects to the normally
connected port (O), which is capped. The normally
disconnected port (X) is closed and removes the low-limit
controller from the system.
In a dpdt model, the common, normally connected, and
normally disconnected ports are duplicated in the second
switch section.
SWITCHING
RELAY
CAP
C O
P X
LIMIT
CONTROLLER
DISCHARGE
AIR
C2379

Snap Acting Relay

The snap acting relay is a spdt switch that provides two position
switching action from a modulating signal and has an adjustable
switching point. The switching differential is less than 1.0 psi.
The switching pressure is manually adjustable for 3 to 15 psi
operation.
Figure 36 shows a snap acting relay application. Operation is
similar to the switching relay. When the branchline pressure
from the outdoor air thermostat equals or exceeds the preset
switchover pressure, the relay connects the normally
disconnected port (X) and blocks the normally connected port
(O) to deliver main air to the normally open heating valve and
provide positive close off. When the outdoor air thermostat
pressure drops below the relay setpoint, the normally
disconnected port (X) is blocked and the normally connected
port (O) connects to the common port (C) to connect the valve
actuator to the room thermostat.
DA OUTDOOR AIR
THERMOSTAT
M
B
M P
C
SNAP ACTING
RELAY
N.O. HEATING
VALVE
Fig. 36. Typical Application for Snap Acting Relay.

Lockout Relay

The lockout relay is a three-port relay that closes off one
pressure signal when a second signal is higher. Figure 37
shows a typical application in which mixed air control becomes
disabled when outdoor air temperature is higher than return air
temperature. To prevent air from being trapped in the line
between the lockout relay and the snap acting relay, a small
bleed must be present either in the pilot chamber of the snap
acting relay or in the line.
MIXED AIR
TEMPERATURE
CONTROLLER
SNAP ACTING
M
B
RELAY
M X
O
EXH
RETURN AIR
THERMOSTAT
M B
M
Fig. 37. Lockout Relay in Economizer Cycle.
70-6925
Relays and Switches
DA ROOM
THERMOSTAT
X M M B
O
VAV TERMINAL UNIT
DAMPER ACTUATOR
C2360
OUTDOOR AIR
DAMPER ACTUATOR
C
P
OUTDOOR AIR
THERMOSTAT
BLEED
LOCKOUT
RELAY
M B
P 1
P 2 M
B
C2362
141