46
e. Outdoor Reset: Since heating loads are typically
lower when the outdoor temperature rises,
outdoor reset lets the installer reduce the boiler
target temperature as the outdoor temperature
increases. As mentioned above, this increases the
efficiency of the boiler. This is especially true with
condensing boilers because it helps to recover the
heat from the water vapor which, in conventional
boilers, carries valuable energy out with the
exhaust.
f. System Type Presets
: For convenience, the
P
UREFIRE
®
boiler control provides preset values for
the outdoor reset parameters based on the system
type. Table 8.19 shows the values that are applied
when different system preset types are selected. If
the system type, “User Defined” is chosen the
outdoor reset definition values may be adjusted. If
changes are made to the boiler design or mild
weather boiler temperatures, the system type is
automatically switched to, “User Defined”.
g. Warm Weather Shutdown
: If the boiler is set to
operate in CH Mode 1 or 2, the P
UREFIRE
®
control
is set by default to prevent the boiler from operating
to satisfy a central heat demand if the outdoor
temperature is above 70°F (21°C). This value can
be adjusted using the values shown below.
If the installer prefers to use custom values for the
outdoor reset parameters, the following provides
guidance.
h. Design Point
: The design point is defined by the
outdoor design temperature and the boiler
design temperature.
i. Boiler Design Temperature
: The boiler design
temperature is the temperature at which the boiler
is designed to operate in order to meet the load.
Copper finned tube radiators are typically rated at
180°F (82°C). The Table 8.21 shows typical boiler
design temperatures for different types of head
distribution units.
j. Outdoor Design Temperature
: The heat loss for the
structure is determined by considering the coldest
sustained outdoor temperature that is expected at
the site location. For a detailed list of outdoor
design temperatures by state, refer to the H-22
heat loss calculation guide published by AHRI.
Table 8.19: System Type Presets
System Type
Temperatures
Boiler
Design
Mild Weather
Boiler
1. Finned Tube Baseboard 180 140
2. Hydro Air Unit 190 140
3. Cast Iron Radiator 160 120
4. Cast Iron Baseboard 150 110
5. Low Mass Radiant 140 70
6. High Mass Radiant 120 70
7. user Defined (Default) 180 70
Table 8.20: Warm Weather Shutdown Range &
Default
Parameter Minimum Default Maximum
Warm Weather Shutdown 35°F (-18°C) 70°F (21°C) 100°F (38°C)
Figure 8.31: CH Settings – Warm Weather Shutdown
Figure 8.34: Outdoor Reset Operation
BOILER CONTROL: OPERATION
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