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10
Operation
(continued)
C. Supply (Outlet) Water Temperature
Regulation
1. Priority Demand
When more than one demand is present, the
higher priority demand is used to determine
boiler settings as shown in Table 10-1. For
example, when DHW priority is enabled, the
setpoint, difference above, difference below
and pump settings are taken from DHW
selections. Active priority is displayed on the
Home Screen (see Figure 10-3).
Table 10-1: Order of Priority
Priority Demand
The boiler is connected in a boiler-to-boiler network.
Sequencer
1
The boiler accepts demand from the Sequencer
Control
Master boiler.
DHW Demand is on and selected as the priority
demand. DHW Demand is always higher priority than
Domestic
Central Heat. DHW Demand also has higher priority
2
Hot Water
than the Sequencer Control when DHW priority is
enabled (Priority Time greater than zero) and DHW
Pump is set to Boiler Piped IWH.
Central
Enable/Disable is on and there is no DHW demand
3
Heat
or DHW priority time has expired.
Frost Protection is active and there is no other call
Frost
for heat. Frost protection will be a higher priority than
4
Protection
Sequencer Control if the Sequence Master has no
active call for heat.
Warm
WWSD is active and the boiler will not respond to
Weather
5
Enable/Disable. DHW Demand is not blocked by
Shutdown
WWSD.
(WWSD)
2. Setpoint Purpose
The control starts, stops, and modulates boiler
output from minimum to maximum to heat
water up to the active setpoint. Active setpoint
is determined by priority as shown in Table 10-
1.
3. Central Heat Setpoint
With Enable/Disable demand, the active
setpoint is either the Central Heat Setpoint,
Central Heat Time of Day Setpoint (if
thermostat is in Sleep or Away mode), Outdoor
Reset setpoint, or a value set by 4-20mA input
from an energy management system (EMS).
4. Outdoor Reset
If an outdoor sensor is connected to the boiler
and outdoor reset is enabled, the Central Heat
Setpoint will automatically adjust downwards
as the outdoor temperature increases. Room
air temperature overshoot is minimized since
water temperature is properly matched to
heating needs. Outdoor reset saves energy
by reducing room overheating, reducing
boiler temperature, increasing efficiency, and
reducing standby losses as boiler and system
piping cool down to ambient following a
78
Boiler Responding to:
APEX
Installation, Operating, & Service Instructions
heating cycle.
5. Boost Time
When the Central Heat Setpoint is decreased
by Outdoor Reset settings, the Boost function
can be enabled to increase the setpoint in the
event that central heat demand is not satisfied
for longer than the Boost Time minutes. The
Boost function increases the active setpoint
by 10°F for every 20 minutes (field adjustable)
the central heat demand continues unsatisfied.
This process continues until central heat
demand is satisfied, after which operating
setpoint reverts to value determined by
Outdoor Reset settings. Boost function is not
used if Boost Time is zero.
6. Domestic Hot Water (DHW) Setpoint
With DHW demand, the active setpoint is
either the DHW Setpoint or DHW Time of
Day Setpoint (if thermostat is in Sleep or
Away mode). The optimal value is based on
requirements of the specific indirect water
heater (if used).
7. Domestic Hot Water (DHW) Priority
If Domestic Hot Water Priority Time is greater
than zero and there is a DHW demand, the
system pump will be turned off (with System
Pump parameter set to Central Heat, No
Priority) and the DHW pump will be turned
on. Additionally, if Outdoor Reset is enabled,
the active setpoint is adjusted to the DHW
Setpoint. Priority protection is provided to
ensure Central Heat supply in event of an
excessively long DHW demand.
8. Time of Day (Setback) Setpoints
User-adjustable Time of Day Setpoints are
provided for both Central Heat and DHW
demands to save energy when a building is
unoccupied. Time of Day Setpoints are active
when an EnviraCOM thermostat is connected
to the boiler and is in Sleep or Away mode.
Example EnviraCOM thermostat: Honeywell
VisionPro IAQ, model TH9421C1004.
D. Boiler Protection Features
1. Supply Water Temperature High Limit
The boiler is equipped with a UL 353 listed
boiler control and UL 1434 listed high limit
sensor. Default response to supply temperature
is as follows.
• Supply exceeds 190°F (87.7°C) – output
(fan speed) reduced
• Supply exceeds 200°F (93.3°C) – recycle
• Supply exceeds 210°F (98.9°C) – manual
reset hard lockout
106308-05 - 3/23
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