Control System Operation; General - Smith XWH 150 Instruction Manual

GENERAL

HOW THE WATER HEATER OPERATES
The XP water heater uses an advanced stainless steel heat
exchanger and electronic control module that allows fully
condensing operation. The blower pulls in air and pushes flue
products out of the water heater through the heat exchanger and
flue piping. The control module regulates blower speed to control
the water heater firing rate. The gas valve senses the amount of
air flowing into the water heater and allows only the right amount
of gas to flow.
CONTROL INPUTS AND OUTPUTS
Tank thermostat / tank sensor
Either of these inputs will tell the water heater to provide heat to
the water tank. If a tank sensor is connected, the Control Module
will ignore the tank thermostat.
0 - 10VDC input (set point or power)
The XP water heater can be controlled by a Building Management
System (BMS) using a 0 - 10 VDC signal. The control can be
configured by the installer to use this signal to either control set
point or firing rate. This signal can be sent to the heater through
ModBus as well.
TEMPERATURE CONTROL
Modulation
The XP water heater is capable of modulating its firing rate from
a minimum of 20% to a maximum of 100%. The firing rate is
dictated by the hot water draw and various other temperature
limitations.
Setpoint Adjustment (Economy Mode/Clock)
The controller may be programmed to reduce the DHW set point
during certain times each week. Seven different start and stop
times may be programmed for the DHW Setpoint Adjustment.
Flame current support
To prevent nuisance shutdowns when the water heater is firing
at minimum rates, the control will increase fan speed when the
flame signal drops too low.
PROTECTION FEATURES
Outlet temperature, flue temperature, and temperature rise
limiting
The outlet temperature is monitored by the water heater outlet
temperature sensor. When the outlet temperature exceeds 185°F,
the unit will reduce the fan speed. If the outlet water temperature
exceeds 195°F the control will shut the unit down until it cools off.
The control module monitors the flue temperature by a sensor
located in the flue exhaust. If the flue temperature exceeds
215°F the control will reduce the maximum fan speed. If the flue
temperature exceeds 240°F the control will shut the unit down.
The unit will restart automatically once the flue temperature
drops 25°F and the minimum off time has expired.
The control monitors the temperature difference between the
inlet and the outlet sensor. On Models 150 - 500 if this difference
is between 25 - 30°F the control will reduce the fan speed. If the
temperature difference exceeds 30°F the control will shut the unit
down.
On Models 600 - 800 if the temperature difference is between
30 - 35°F the control will reduce the maximum fan speed. If the
temperature difference exceeds 35°F the control will shut the unit
down.

CONTROL SYSTEM OPERATION

The unit will restart automatically once the temperature difference
has dropped below 25°F (Models 150 - 500) or 30°F (Models 600
- 800) and the minimum off time has expired.
Freeze protection
DO NOT install the water heater in a room likely to freeze.
The following integral feature of the Control Module provides
some protection for the water heater.
• The Control Module provides freeze-up protection as
follows when the water heater water temperature drops
below 45°F:
• Below 45°F, the water heater pump operates constantly.
• Below 37°F, the water heater turns on.
• The water heater and pump will turn off if water heater
water temperature rises above 45°F.
This feature of the Control Module does not eliminate the
possibility of freezing. The installation must still use recognized
design, installation and maintenance practice to prevent freeze
potential for the water heater.
Monitor external limits
Connections are provided on the connection board for external
limits such as flow switch, gas pressure switches, and a damper
proving switch. The Control Module will shut off the burner and
inhibit relighting whenever any of these external limits open.
Runtime and alarm outputs
The water heater provides dry contacts for indicating when the
water heater is running, and when it is unable to operate.
Runtime and cycle counting
The control uses a timer to monitor the total hours of burner
operation. The timer monitors the time the water heater is firing.
The control uses two (2) ignition counters to monitor the amount
of water heater cycles. The first counter counts all ignitions of the
control. The second counter counts only ignition attempts that
have failed.
Service
This unit can be programmed to access the Contact Name and
Contact Phone parameter for service and maintenance.
WATER HEATER TEMPERATURE REGULATION
Operating temperature (target)
The Control Module senses water temperature and regulates
water heater firing and firing rate to achieve a target temperature.
The target temperature can be set between 60°F and 190°F.
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