Unit Startup
RT-SVX24J-EN
103
components will return to the state they were in prior to
entering purge.
During dehumidification control an evaporator frost
control function designed specifically for reheat modes
will be active.This function will reduce refrigeration circuit
capacity to 50% (1st compressor on each circuit remaining
on) when the Entering Evaporator Temp drops below a
non-adjustable limit of 35° F for 10 continuous minutes.
Once capacity is reduced, it will remain reduced until the
current cycle of dehumidification is terminated or a purge
cycle occurs.
If the Entering EvaporatorTemp remains below 35° F for an
additional 10 minutes both circuits will be de-activated and
remain off until the Entering EvaporatorTemp rises above
45°F. Even though all compressors have been de-activated
the unit will remain in dehumidification mode and re-
enable compressors up to 50% capacity when the Entering
Evaporator Temp rises to 45°F or greater.
Energy Recovery Sequence of Operation
The primary components of the energy recovery system
are the energy recovery wheel, exhaust air bypass
damper, outdoor air bypass damper, and the energy
recovery preheat output. See
Figure 67, p. 104 Figure 68,
p. 105
A filter is also placed between the wheel and the
outdoor air damper and an indicator scheme similar to that
for final filters is provided to notify the user when that filter
needs to be changed.
The energy recovery wheel will only be energized when
both the Supply Fan and Exhaust Fan are requested on by
the various functions that control them. Energy recovery is
a passive function and can not request fan operation.
Once the required airflow is present the wheel will be
commanded on if the indoor vs. outdoor conditions are
such that energy can be recovered. This is assessed
differently in cooling and heating modes.
In cooling mode, wheel activation conditions are assessed
based on indoor (return air) vs. outdoor enthalpy. Indoor
and outdoor enthalpy values are calculated using the
same sensors as used for comparative enthalpy. If the
outdoor enthalpy is 3 BTU/lb. greater than indoor enthalpy
the wheel is activated to remove energy from the
incoming outdoor air. In heating mode the wheel is
activated based on indoor vs. outdoor dry bulb
temperature. If the outdoor temperature is 5° F less than
the indoor temperature. the wheel is activated to recover
heat energy from the exhaust air.
In cooling mode the exhaust air bypass damper is held
closed, providing 100% energy recovery capacity during
cooling modes of operation. In heating modes, including
CV heating, VAV Heating, CV Supply Air Tempering, VAV
Supply Air Tempering, Morning Warm-up, and Daytime
Warm-up the exhaust air bypass damper is controlled to
discharge air temperature. The damper is modulated to
keep the supply air temp at the Supply Air Heating setpoint
for VAV control, or for CV control, supply air temp will be
controlled to a calculated Supply Air Heat Setpoint based
on conditions in the space.
If the wheel is active, supplemental heat (electric,
hydronic, gas) control algorithms will be disabled until the
exhaust air bypass damper is fully closed (maximum
heating capacity from the wheel). At this point,
supplemental heat algorithms are released to calculate
supplemental heat capacity requests using standard
setpoints until the setpoints are satisfied. In VAV occupied
modes the energy recovery wheel will remain active after
termination of supplemental heat above heating setpoint
until the exhaust air bypass damper is opened fully for 3
minutes (indicating minimal capacity requested from the
wheel). In CV occupied heating mode the wheel will
remain active after termination of a heating cycle until the
zone temp rises above the Occupied Zone Heating
Se 1.0°F and the exhaust air bypass damper is fully
open. The wheel will remain active if these conditions
persist continuously until the expiration of a HI-adjustable
time-out period or until the zone temp rises above the
Occupied Zone Cooling Setpoint - 0.5°F.
During active Economizing control the energy wheel will
be disabled but the outdoor air bypass damper will open
an amount that tracks the opening of the OA damper
proportionally from minimum position to fully open.
To protect the wheel from frost build-up in heating modes
a frost avoidance function is included. This feature will
energize the energy recovery preheat output (if
configured) and modulate the outdoor air bypass damper
open (to reduce incident cold outdoor air on the wheel) as
necessary when the Leaving RecoveryTemp Sensor value
is less than the Recovery Frost Avoidance Setpoint. The
Leaving Recovery Temp Sensor is installed in the leaving
air stream on the exhaust-fan side of the energy wheel.
Figure 66
provides the exhaust air temperature setpoint
for 70ºF return air at various percents of relative humidity.
Where variable effectiveness / outside air bypass is not
enough to prevent frosting conditions, the energy
recovery wheel shuts off. Turning the wheel off during
frost conditions is a reliable method of preventing the
wheel from frosting, however, energy is not being
recovered and the extreme heating load must be handled
otherwise. Extreme winter design condition for energy
recovery units may require return air preheat.
An energy recovery wheel proving function is also
provided to indicate when the wheel is not turning after it
has been commanded on.
WARNING
Toxic Hazards!
Do not use an energy wheel in an application where the
exhaust air is contaminated with harmful toxins or
biohazards. Failure to follow this instruction could
result in death or serious injury.
