2-5
BOILER SYSTEMS AND OPERATION
SL 20-115 MODULATING GAS BOILER
the pump remains on for 5 minutes (adjustable) after the
heating
cycle ends.
if the
heating
cycle ends as a result of water temperature exceeding the load
specific band limit, the
postpurge
will continue as normal, but the boiler will enter
the
circulating
mode.
The cut off temperature levels are specific for each load; they are determined
using the preset target plus ½ of the supply differential entered. default values
for such differentials are 22°F (+/- 11°F) for space heating with outdoor reset and
20°F ( +/- 10°F) for dHW and other set point loads,
if there is a
Heating Enabled
signal any time during this cycle, the postpurge
ends, and a prepurge begins, otherwise the boiler will enter standby mode.
2.5.3 Ignition
after the prepurge, the gas valve opens for 4.0 seconds.
If no flame is detected at the end of the trial, the
interpurge
cycle is entered. if
ignition fails 3 times successively, the boiler locks out for a 1 hour long interval,
after which the ignition cycle is refreshed (e.g. for 3 further attempts). alternately,
the boiler can be powered down and restarted to accelerate the re-try sequence.
the homeowner should follow up persistent recurrence of the 1 hour reset
routine by contacting a qualified service technician, to evaluate the cause of such
ignition irregularity.
With a successful ignition, the output of the boiler decreases immediately to a low
level, and the heating cycle begins.
2.5.4 Heating
the heating cycle lasts until the
Heating Enabled
state ends (for all loads),
or until water temperature exceeds the target temperature by ½ of the supply
differential for the last served load and the throttle has fallen to the minimum
output. at the end of the heating cycle, the boiler enters a
postpurge.
During the heating cycle, the boiler addresses the multiple defined loads
in accordance with its Prioritization algorithm
(see Section 2.6.1)
. Boiler
output changes to meet heating demands as indicated by the temperature
characteristics of the boiler supply and return water. Boiler supply water
temperature targets vary according to the set up parameters entered, and are
subject to further variation for loads where outdoor reset has been selected.
Where thermal loads are within the boiler’s 20 to 115 MBH throttle range, the
control algorithm seeks to regulate the boiler supply temperature within ±3°F
of the target temperature. on/off differentials apply to loads below the 20 MBH
minimum input rating.
2.5.5 Circulating
this cycle operates when water temperature exceeds target by 1/2 of the supply
differential for that load at minimum output, and call for heat is still present. after
entering a
postpurge
, the system pump remains on until water temperature is
1/2 of the supply differential below setpoint temperature.
the boiler can also enter the Circulating mode during transition from a high
temperature load (e.g. DHW at 180°F) to a cool target (e.g. radiant floor on
outdoor reset, at perhaps 90°F). in the case in which the temperature is outside
the applicable differentials, the burner will shut off while the pump remains on