Heat Transfer MODCON VWH 300 LP Installation & Maintenance page 53

PART 9. START-UP OPERATION
A. SINGLE MOD CON VWH BOILER
When power is applied to the MODCON VWH
boiler, the control first completes a power-up
systems check. During this time, the combus-
tion fan may run. The display will initially show
the current boiler outlet temperature. If a fault is
detected during the power-up test time, the con-
trol will display the appropriate fault code. If all
is well, the display will continue to show the
boiler outlet temperature and stand by waiting
for a demand for hot water. If the temperature of
the tank sensor falls to below the tank set point/
| d d| - tank differential | d h| temperature, a demand
for hot water is generated by the control. When
a demand for hot water is received, the control
begins the following the demand sequence. The
control will energize the pump output then wait
for adequate water flow to pass through the flow
switch. If the flow is not sufficient, the display
will show | F L| until the flow is sufficient. Once the
flow through the boiler is adequate, the control
will then measure the boiler sup ply temperature
and if it is below the set point temperature | d d| –
the supply hysteresis | S h| temperature, the con-
trol will ignite the burner. Once the boiler is lit,
the control will modulate the firing rate to con-
trol the supply water temperature to be at the
supply offset temperature (installer #4) above
the tank set point temperature | d d|. When the
tank temperature is equal to the tank set point
temperature, the control will extinguish the
burner and allow the combustion fan to run for
a time to purge gas from the combustion cham-
ber. The control will then be in stand-by waiting
to process the next demand for heat. In the
event that the control goes into lockout, it will
close the alarm contacts, show a diagnostic
code on the display, and illuminate the LED fault
indicator to assist in trouble shooting the failed
condition. If you have an alarm connected to the
alarm contacts it will be activated.
B. MULTIPLE MOD CON VWH BOILERS
If the MODCON VWH boiler is part of a cascad-
ed boiler system, then the operation is some-
what different. The control of each boiler in the
cascaded system completes its own power up
systems check as described above. One of the
boilers in the cascade system is designated as
the master boiler. After the master boiler com-
pletes its power up sequence, it checks the com-
GAS-FIRED HOT WATER SUPPLY BOILER
munication bus to see if any other boilers are
present and if there are boilers present, the mas-
ter control determines their boiler address. The
master boiler control will re-check the bus every
few seconds as long as it is powered up to
update the status of the connected boilers. The
control in the master boiler is the one that
processes all heat demands and dictates which
of the follower boilers should light and at what
firing rate the followers should run at. When the
master boiler receives a demand for heat, it
determines which boiler is the first in the firing
sequence and sends that boiler a command to
begin a demand sequence. That boiler will then
begin a demand sequence exactly as described
above. Once that boiler ignites, the master boil-
er control will increase the firing rate of that boil-
er until the system sensor temperature is at the
supply offset temperature (master boiler
installer #4) above the tank set point tempera-
ture or that boiler is at high firing rate. If the boil-
er gets to a high firing rate but the system sen-
sor is not at the desired temperature, the master
boiler control will then tell the next boiler in the
firing sequence to begin its demand sequence.
The second boiler will then begin to ramp up in
firing rate based on commands from the master
boiler control. This process will continue while
there is a demand, until either all boilers in the
cascade system are at high fire or the desired
temperature at the system sensor is reached.
When the tank temperature is equal to the set
point temperature the demand is complete and
the master boiler control will extinguish all boil-
ers that may be lit. If any boiler or multiple boil-
ers, including the master boiler in the cascade
system are in a lockout condition, the master
control will recognize the lockout condition and
skip over the boiler in the firing sequence. Each
boiler in the cascade system is responsible for
its own safety functions so if any individual boil-
er control senses an unsafe condition it will
extinguish the boiler and if necessary, put it in
lockout condition. In this way, every boiler in the
system has its individual safety controls intact
and operational even if the firing of the boiler is
under control of the master boiler. In the event
that the system sensor fails, all boilers in the
system will ignite simultaneously when there is
a demand and they will regulate their firing rate
individually to maintain the master set point
temperature (tank set point + master boiler
installer #4) at their individual supply sensors
Boiler Manual
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