2-7
BOILER SYSTEMS AND OPERATION
SL 20-115 MODULATING GAS BOILER
running of a radiant floor. The large throttle turndown range coupled with outdoor
reset means that several loads can receive heat in-turn, with less on/off duty
cycling than would occur with other heat sources.
the boiler is shipped with default values for each type of declarable load (e.g.
DHW, radiant floor). The preset variables for DHW (a value of 80 on a 20-90
scale) give it substantial priority, which declines over time if another load is
calling. spacing heating loads (with outdoor reset) are given a default value of
35, while other setPoint loads start with 50 points. over time, such points are
transferred between the loads in accordance with the system rules. the boiler
will automatically shift between loads once the points totals reach an upset value.
upon a load transfer, the burner will modulate down to its base level and will
then throttle up as required for the new load. if competing loads have not been
satisfied at the load switch stage, the control will register the firing rate and will
return rapidly to such rate on the next rotation.
Heat-apportioning can be altered using the keypad in the password protected
Installer
input section. For instance if Baseboards (set up with outdoor reset as
load 3) are not providing the desired heat, the on-time of the baseboards can
be increased by increasing the Prioritization value relative to the other declared
channels (e.g. moving the load 3 preset value from 50 to 60 or reducing another
load’s value). temperature targets for the under-served load could also be raised
in this example. the basic function is that the the difference between settings
equals the run time (in minutes, for the initial cycle) of the higher load setting
before switch-over to the simultaneously calling load, e.g. dHW with priority set
to 80 will run for approx. 35 min. before switching to a clashing load set with
priority 45. Following such initial cycle, the clashing loads will switch back and
forth after further 10 min. intervals pending satisfaction of one or both loads.
to cause repeated unequal run times (for example, constant 35 vs 5 min. runs) it
is necessary to integrate an external load removing relay or timer on one of the
heat calls (contact the Factory for details).do not set each load to the top value,
and it is generally better to avoid equal ratings. note: the priority scale values do
not need to add up to 100 – they are relative only to each other.
2.6.2 Load Pairing
With software versions 3.10.0 and higher, the controller allows two loads of
compatible temperature settings to be run together.
See IBC Technical Note:
Load Pairing for further details.
2.6.3 Reset Heating
the boiler offers outdoor reset control as standard equipment; this coordinates
the control of boiler supply water temperature for space heating with the outdoor
temperature. Outdoor Reset offers enhanced home comfort and efficiency by
using variable water temperatures to compensate for the differing rates of heat
loss that a building faces as outdoor temperature changes. at any outdoor
temperature
above the coldest day expected
, it automatically uses lower
heating temperatures than would otherwise be used. this saves energy without
sacrificing comfort.
When applied within a condensing boiler, outdoor reset offers direct and
significant combustion efficiency benefits by allowing generally lower circulating
water temperature. this provides cooler return water to the boiler, promoting
more condensation (= energy capture).
NOTE
US law effective Sept 2012
requires residential heating
boilers under 300,000 btu/
hr to be equipped with
Automatic Means of boiler
water temperature adjustment
in relation to heat demand.
This boiler offers such control
but also a means to override
it. See the inside back cover
of this Manual for a message
in respect of the use of such
override.
