Specifications

Optional temperature
control
Sequence of operation
Johnson Metasys
Mammoth’s 100% Outdoor Air heat pump units offer
three choices of temperature control:
1. An optional Mammoth DDC control system for
complete unit control. Wiring diagrams and startup
information for the MDDC controller is not provided
in this document. Each MDCC controller has an
individual Installation and Operation Manual
created for it. Contact Mammoth for more details.
2. A Johnson Metasys DDC controller that can be
used as a stand-alone controller or with a Johnson
Metasys BMS.
Several typical wiring diagrams have been
provided for this option (Figure 8 and Figure 9).
Startup information for the Johnson Metasys
controller is provided on page 15.
3. Temperature control by others which includes a
24-volt terminal strip for connection of a controller
to operate the compressors, fan motor, reversing
valve and reheat valve.
Several typical wiring diagrams have been
provided for this option (Figure 6 and Figure 7).
Consult the factory for a specific diagram. Startup
for controls by others is not provided in this
document. It is not common but it is possible to
operate a 100% OA unit using a thermostat with
humidity control (by others). Startup instructions
are provided on page 15.
14 U-5 IOM
The Metasys control system is can be tied in to a
Metasys Building Automation System or it can be an
independent, stand-alone control system. The
system includes:
● Outdoor air temperature sensor
● Mixed air temperature sensor
● Outdoor air humidity sensor
● Discharge air temperature sensor
● Cooling control from outdoor air and humidity
sensors
● Heating control from discharge air sensor
● Reheat control for discharge air
● Neutral discharge air temperature
● Fan operation from damper end switch
● Adjustable temperature/humidity set points
● Adjustable differential for heating, cooling
● Spring return outdoor damper actuator with end
switch
● Air proving switch
● Optional Zone Terminal for field modification of set
points on stand-alone controls.
On a call for unit operation, the outdoor air damper
will open. The controller will first examine the mixed
air temperature. If the outdoor air is below the set
point of the mixed air sensor, the preheat will be
energized to increase the air temperature. When the
damper is fully open, the fan will run continuously.
After the need for preheat is examined, the controller
then looks at the need for compressor cooling. If the
outdoor air is above the first stage cooling set point,
then the first stage compressor will operate in the
cooling mode. If the outdoor air humidity is above the
set point, the controller will energize stage one
cooling operation. On a further increase in outdoor
temperature, the second stage-cooling compressor
will be energized.
If the discharge air temperature is below the set point
(55 F and adjustable), the hot gas reheat valve will
be activated to provide hot refrigerant gas to the
reheat coil to increase the discharge air temperature.
If the discharge air temperature rises above the set
point of the discharge air temperature, the hot gas
reheat valve will be de-activated. If the outdoor air
temperature falls below the second stage cooling
control set point, the second stage cooling will be de-
activated.
If the outdoor air humidity falls below the outdoor air
humidity set point, first stage cooling will remain on
until the outdoor air temperature is below the first
stage cooling set point. The first stage cooling will
remain on until the outdoor air humidity and
temperature are below their set points.
If the outdoor air temperature falls below the first
stage heating set point, the compressor will operate
in the heating mode. If the mixed air temperature falls
below the preheat set point (40 F and adjustable),
the optional preheat coil will be activated to provide
heating. If the mixed air temperature rises above the
set point, the preheat will be de-activated. If the
discharge temperature rises above the set point, the
compressor heating will be de-activated.
Figure 10. Metasys control points
and unit schematic
Outdoor Air Temp. Sensor
Damper Section
Fan/Motor Section
Airflow
Airflow
Mixed Air Temp. Sensor
Outdoor Air Humidity Sensor
Discharge Air Temp. Sensor
T
H
M
D
Preheat
DX Coil
Hot Gas Reheat
Filters