11
Marvair Wall Mount Heat Pump I&O Manual
11/2021 New
Electric Heat During Defrost (EHDD)
The board provides the option of running the electric heat (EH) during a defrost cycle. There are two
options for the EHDD, “Yes” and “No”. If “Yes” is selected, the board will energize the EH terminals
during a defrost cycle. If “No” is selected, the EH terminals will not be energized during a defrost cycle.
The factory setting for EHDD is “No”. If the S-circuit is in the “Yes” position, the EHDD function will
be disabled.
S-Circuit
The S-Circuit prevents the compressor and electric heat from operating simultaneously. The S-Circuit
is pin selectable using a jumper to select “Yes” to enable this feature or “No” to disable it. When “Yes”
is selected, the control will not allow the compressor to run when there is a call for electric heat at the
W2 input. It will turn the compressor off until the call for W2 is satisfied. The factory setting for the
S-Circuit is “No”.
Test Mode
A jumper selectable test mode can be used by technicians for trouble shooting purposes. The test mode
reduces the delay on make and anti-short cycle timers to ten seconds, and the LPS bypass becomes two
minutes.
Internal Head Pressure Control (Requires optional coil sensor)
The control has an internal, fixed set point head pressure control designed to maintain a constant head
pressure under low ambient conditions. It will monitor a coil sensor that is inserted into the condenser fan
coil and modulate the speed of the fan motor accordingly to maintain a constant coil temperature in turn
stabilizing the head pressure. The fixed set point that the control will maintain is 100°F. The motor will
have a minimum operating speed of 20%-30% of the full speed. It will also incorporate a fixed hard start
time of one second during which the condenser fan motor will turn on full speed for one second at start up.
High Pressure Switch (HPS)
The high pressure switch is mounted on the compressor liquid line. The HPS terminals are on the high
pressure switch input. This input monitors the status of the high pressure switch and determines when
this fault condition is present. If the HPS is open on the initial “Y” call, the control board will not allow
the compressor to operate. If the HPS opens while the compressor is running, the control board will turn
the compressor off, register the first HPS fault, and initiate the 3 minute anti-short cycle timer. If two
HPS faults occur within a one hour period, the control will enter a HPS lockout condition and energize
the alarm contacts. The RED status LED will blink once to indicate this condition. The lockout condition
will only be reset by manually cycling power to the control to prevent destructive short cycling. To reset
the switch, turn primary power off, then back on or turn thermostat system switch off, then back on.
The high pressure switch opens at 650 psig and closes at 450 psig.
Loss of Charge Switch (LPS)
The loss of charge switch is mounted on the compressor’s liquid line. The LPS terminals are the loss
of charge switch input. This input will be used to monitor the status of the loss of charge switch and
determine when a LPS fault is recognized. There is 3 minute loss of charge switch bypass timer. If the
LPS is open on the initial “Y” call or while the compressor is running, the control will ignore the status
of the LPS and not recognize a fault for 3 minutes. If the LPS is still open after 3 minutes, the control
will de-energize the compressor, register the first LPS fault, and initiate the 3 minute anti-short cycle
timer. If two LPS faults occur within a one hour period, the control will enter a LPS lockout condition
