Bryant 50YEW Design Manual page 60

Water-to-Water System Design Guide
Part IV: Controls
Warm weather shutdown (WWSD): Radiant fl oor systems are the
most comfortable type of heating available today. However, they do
have one disadvantage – quickly switching from heating to cooling is
not possible due to the mass heat storage in the slab. For example,
in the spring or fall, there could be times where heating is required at
night, but cooling is required during the day. With a warm fl oor, the
cooling system has to work much harder to cool the space. WWSD
shuts down the water-to-water heat pump at a pre-determined
outdoor air temperature (adjustable at the user interface). When
a water-to-air heat pump is used for space cooling, this unit can be
enabled when WWSD is activate, allowing the water-to-air heat
pump to heat via forced air during the shoulder seasons, avoiding the
warm slab/cooling dilemma (see cooling enable, below). A normally
closed contact is provided in the 50YEW unit to de-energize the
heating system controls (e.g. radiant fl oor control panel) during
WWSD. WWSD does not affect DHW heating. In other words, the
water-to-water unit can still operate for generating DHW, even if the
heating distribution (e.g. radiant fl oor) system is disabled.
The WWSD activation (i.e. when the WWSD feature is enabled)
outdoor air temperature range is 40-100°F [4-38°C] with a default
value of 70°F [21°C]. The WWSD deactivation (i.e. when the
radiant heating returns to operating mode) temperature range is 35-
95°F [2-35°C] with a default value of 65°F [18°C] and a minimum
difference between activation and deactivation temperatures of
5°F [3°C]. If the outdoor air temperature (OAT) rises above
the activation temperature, the cooling enable signal (see below)
is enabled, and the control no longer controls the buffer tank
temperature. If the OAT falls below the deactivation temperature,
the control resumes monitoring the buffer tank temperature.
Cooling enable: Cooling enable is tied to the WWSD feature. If
desired, the water-to-air unit controls can be wired to the 50YEW
unit controls, which will allow the water-to-air unit to operate during
WWSD, but will disable the water-to-air unit when the 50YEW
unit is not in WWSD mode. When a heat pump thermostat
is connected to the water-to-air unit, forced air heating may be
used for the shoulder seasons, allowing quick heating to cooling
changeover. If this feature is used, the consumer will easily be
able to tell when WWSD is enabled because the water-to-air unit
thermostat will only be active during WWSD. Otherwise, the water-
to-air unit thermostat will be disabled, indicating that the consumer
should utilize the hydronic heating (e.g. radiant fl oor) thermostat.
Heat pump staging: For large capacity installations, multiple
50YEW units may be controlled by the fi rst heat pump via the
backup boiler function. The second unit simply needs a 24VAC
relay that is energized by the output of the fi rst unit. The third,
fourth, etc. units would be wired in the same manner.
Second stage heating (backup boiler): As discussed in part II of this
manual, optimal heat pump sizing may not include a water-to-water
heat pump that can handle 100% of the heating load. When a
backup boiler is used to supplement the heating capacity, a 24VAC
output from the 50YEW unit can energize the boiler. The boiler
control box simply needs a relay that can be used to interface with
the 50YEW unit.
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DHW priority: By default, DHW heating always takes priority over
space heating. Normally, the hot water load will be satisfi ed quickly,
and the unit can then switch back to space heating.
Time schedule: DHW temperatures may be adjusted
during occupied/unoccupied times via the user interface to save
energy costs.
Vacation mode: DHW generation may be disabled when the user
interface is placed in vacation mode. A return date and time may
be set to restore normal DHW temperatures.
Emergency DHW generation: If the 50YEW unit is locked out, a
24VAC signal can be sent to a contactor at the water heater to allow
the operation of the electric elements and associated thermostat.
Enhanced heat pump lockouts: Like any Bryant unit, the CXM
board locks out the compressor any time a lockout condition
occurs. The MPC reads the lockouts from the CXM, and reports
the condition to the user interface. The user interface changes
from a blue backlight to a red backlight, indicating a lockout. The
actual lockout is reported (e.g. High Pressure) at the interface. In
addition to the standard CXM faults, the MPC checks for bad
thermistors and high compressor discharge temperature, which
are also reported at the user interface.
Pump control: If the optional load and source pump(s)
are selected, the control energizes the pumps any time the
compressor is operating.
Variable speed fl oor pump (VSFP) output: Some radiant fl oor
systems utilize a variable speed pump on the fl oor system, which
changes fl ow based upon the number of zones open or closed.
Since the pump has built-in controls, only a power supply is needed.
An optional power terminal is available for VSFP applications.
Bryant: Whatever It Takes.