8
McQuay IM 846
Operation
Fancoil Operation
Fancoil operation is either a 2-pipe or 4-pipe configuration
which is determined by jumper selection JP4 (see “Mounting
and Wiring the Thermostat” on page 3).
2-Pipe Operation
In this configuration, the thermostat permanently disables the
secondary output and disables the fan from cycling in an
invalid mode. The main output configuration is dependent on
the pipe sensor input. With a pipe sensor connected, the
thermostat automatically selects heating or cooling depending
on the sensed water temperature. If the pipe sensor input is
open (unconnected), the main output only cools. If the pipe
sensor input is shorted, the main output only heats. Normal
thermostat default is for 4-pipe operation. For 2-pipe
operation, install a jumper on JP4.
4-Pipe Operation
In this configuration, both the main output (COOL) and the
secondary output (HEAT) are available. These both cycle ON
depending on the mode of the thermostat. With a pipe sensor
connected, the thermostat automatically changes the main
output to heat and disables the secondary output if the main
coil water is hot. For 4-pipe operation, the JP4 jumper is
removed.
Pipe Sensor Operation
A pipe sensor can be connected when the thermostat is
configured for either 2-pipe or 4-pipe configuration. The pipe
sensor is used to determine the water temperature in the main
coil, which should be connected to the primary output.
Pipe sensor input—use a 10K Remote Probe or a standard ON-
Off Aquastat for summer/winter changeover.
Purge Cycle
With a pipe sensor connected, the thermostat initiates a purge
cycle if the sensed water temperature is ambiguous (not
adequately hot or cold). The sequence for purge cycle
operation is as follows:
1
The thermostat has a call for heat or cooling.
Note:
If at any time the call goes away, the thermostat
stops the purge cycle.
2
The thermostat checks the pipe sensor to verify water
temperature.
3
If the water temperature is beyond 15°F of the set point,
normal HVAC control occurs.
4
If the water temperature is not beyond 15°F of the set point,
the thermostat checks to see if the water temperature is
above 80°F or below 60°F.
If yes, normal HVAC control occurs.
If no, the thermostat opens the main output (COOL) for
three minutes.
5
After the three minute purge cycle occurs, the thermostat
checks to see if the water temperature is more than 15°F
from the set point, or above 80°F, or below 60°F.
If yes, normal HVAC operation occurs.
If no, the valve is left open and the thermostat continues to
search for a valid reading.
Note –
If at any time the demand goes away, the thermostat will
stop the purge cycle.
Table 4 shows output operation, depending on the sensed
water temperature.
Setback Operation
In this configuration, the low level switch input normally is
open. To enable setback operation, the JP3 jumper is removed
(see “Mounting and Wiring the Thermostat” on page 3). The
input will close and the thermostat heating and cooling setback
limits can then be used as temperature control points. Fan
operation in setback is cycled with demand. Press any
thermostat key to override setback for one hour. Setback
overrides any user setting except if the control is turned to
OFF.
Table 4: Output operation
2-Pipe/4-Pipe
selection
Water
temperature
Output operation
Main output
Secondary
output
2-pipe
JP4 ON
Cold
Cooling only
The fan does not cycle on
with a heat demand.
Disabled
Hot
Heating only
The fan does not cycle on
with a cool demand.
Disabled
4-pipe
JP4 OFF
Cold
Cooling
Heating
Hot
Heating only
The fan does not cycle on
with a cool demand.
Disabled
