DCN: 141-00940-09 11/02/07
21
HVAC SYSTEM TESTING
The TXB16 Control Unit connects to the HVAC system at the normal thermostat connections on the HVAC
unit. Standard thermostat control of HVAC systems consist of 24VAC contact closures in the thermostat.
You can verify that your HVAC system is working correctly by duplicating the contact closures by shorting
across the proper terminals at the HVAC system’s thermostat connection. Refer to the following HVAC
system example.
HVAC System Example
You can also verify the HVAC operation at the TXB16 Control Unit by placing
a wire jumper across the Control Units HVAC System terminals as follows
(with the HVAC system connected to theTXB16):
For Standard HVAC systems, jump across J4, HVAC System terminals as follows:
FAN operation: Jumper across R and G (Fan) terminals.
HEAT operation: Jumper across R and W (Heat) terminals (Fan is not usually necessary for gas furnaces).
COOL operation: Jumper across R and Y (Compressor) and G (Fan) terminals.
For Heat Pump systems with Changeover for cooling, jump across J4, HVAC System terminals as follows:
FAN operation: Jumper across R and G terminals.
HEAT Stage 1 operation: Jumper across R and Y (Compressor) and G (Fan) terminals.
HEAT Stage 2 operation: Jumper across R and W (Heat Strips) and G (Fan) terminals.
COOL operation: Jumper across R and Y (Compressor) and O (Change Over) and G (Fan) terminals.
If the Control Unit’s output LEDs are ON and you suspect you are not getting an output from the relays,
perform the above shorting test to verify that the HVAC system is working OK. If the HVAC system
responds correctly to the shorted terminals, then the output relay is suspect.
B 24VAC COMMON
R 24VAC HOT
G FAN
W HEAT
Y COMP
HVAC SYSTEM
STANDARD GAS/AC
FAN
RELAY
GAS
VALVE
COMP
RELAY
24VAC
Simplified diagram of an HVAC System and Thermostat.
The Thermostat operates like switches to control the
HVAC’s Heat, Cool and Fan functions.
FAN
HEAT
COMP
OUTDOOR
CONDENSING
UNIT
FURNACE AND BLOWER UNIT
TYPICAL THERMOSTAT
Thermostat
Connection
To verify the HVAC system is working properly you can
simulate the thermostat contacts by shorting across
the thermostat connections at the HVAC system as
follows:
R to G for FAN
R to W for Heat
R to Y for Compressor ( be sure to short FAN, R to G,
also if you are going to leave the compressor running
for longer than a few seconds to test)
