43
General
How the appliance operates
The VF Series uses a copper finned tube heat exchanger to
transfer heat from the flue products to the water. An electronic
control module monitors various inputs to initiate a call for
heat. The blower provides both primary and secondary air to
the burner and forces the flue products out of the combustion
chamber and into the vent system. The control module
regulates the blower speed to control the firing rate of the unit.
The modulating gas valve monitors the amount of combustion
air being pulled into the blower and regulates the amount of
gas supplied, which then mixes with the combustion air and is
supplied to the burner.
0 - 10V input (set point or power)
The VF Series 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.
Temperature control
Modulation
The VF Series is capable of modulating its firing rate from a
minimum of 25% to a maximum of 100%. The firing rate
is dictated by the call for heat (i.e., domestic hot water), the
heating load, ramp delay (if enabled), and various other
temperature limitations.
Gradient limiting
If during operation of the heater the outlet water temperature
is rising too quickly, the control will reduce the firing rate to its
lowest setting.
Night setback
The controller may be programmed to reduce the tank (water
heaters) set points during a certain time each day. A start and
stop time can be programmed for each day of the week.
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
(85°C), the unit will reduce the fan speed. If the outlet water
temperature exceeds 195° (90.6°C) 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
400°F (204°C) the control will reduce the maximum fan speed.
If the flue temperature exceeds 450°F (232°C) the control will
shut the unit down. The unit will restart automatically once the
flue temperature drops 100°F (56°C) and the minimum off time
has expired.
The control monitors the temperature difference between
the inlet and the outlet sensor. If this difference exceeds 55°F
(31°C) the control will reduce the fan speed. If the temperature
difference exceeds 65°F (36°C) the control will shut the unit
down. The unit will restart automatically once the temperature
difference has dropped 10°F (6°C) and the minimum off time
has expired.
NOTICE
If an inline high gas pressure regulator is
used, it MUST BE of the lockup type and
be located a minimum of 10 feet from
the appliance. Failure to do so may result
in insufficient gas volume supplied to the
appliance.
NOTICE
If a pressure drop of more than 2” water
column occurs between Standby (static)
Mode and Operating (dynamic) Mode, a
gas volume problem exists. Contact the gas
utility, gas supplier, qualified installer, or
service agency to determine the necessary
steps to provide the proper gas volume to
the appliance.
How the control module operates
The control module receives input from appliance sensors and
external devices. The control module activates and controls the
blower and gas valve to regulate heat input and switches the
system pumps on and off as needed. The user programs the
module to meet system needs by adjusting control parameters.
These parameters set operating temperatures and appliance
operating modes. Water heater operation can be based on a
tank sensor or a tank thermostat.
Control inputs and outputs
Tank thermostat (field provided)
This input tells the water heater to provide water for heating
a domestic hot water tank (see FIG. 5-4 on page 38 of this
manual).
8
Operating information
Installation & Operation Manual
