Install Instructions

3. Outdoor Reset Operation:
To maximize fuel savings, P
UREFIRE
®
boilers are factory
equipped for outdoor reset operation. The following
describes the operating principal of this feature.
Operating Principle:
• Heat Loss decreases as Outdoor Temperatures
increases.
• Heat distribution system is sized for Design
Conditions (Coldest Outdoor Temperature).
• Boilers operate more efficiently at lower water
temperatures.
• Condensing boilers have no minimum return
water temperature.
The integrated control system on the P
UREFIRE
®
boiler
adjusts the boiler supply setpoint based on outdoor
temperature. In the installer menu, under “CH
Settings” there are two parameters that define the
design point of the boiler. The design point represents
the conditions for which the boiler was sized.
Figure 8.3 shows a graphical representation of Outdoor
Reset operation. The bold line shows the target supply
temperature under the conditions indicated.
Boiler Design Temperature
: The boiler design
temperature required to heat the boiler under a design
load is called Boiler Design Temperature. Figure 8.3
shows this at 160°F. This indicates that at any outdoor
temperature below the design temperature the control
will target 160°F unless the Boiler Maximum
(described later) is set lower than this temperature.
The factory default value for Boiler Design
Temperature is 180°F (82°C).
Outdoor Design Temperature
: The Outdoor Design
Temperature is the outdoor temperature used to
calculate the heat loss from the building. Figure 8.3
shows this at 10°F.
Commonly used values for this can be found in Table
8.8: Outdoor Design Temperature for Many North
American Cities. The factory default for this value is
0°F (-18°C).
Mild Weather Boiler Temperature
: This is the
temperature required to heat the space under a
minimum load. Table 8.7: System Type Presets, shows
recommended values for this temperature setting based
on the heat distribution system type. Figure 8.3 shows
this temperature at its factory default of 70°F (21°C).
Note that, depending on the type of heat distribution
used, this value should be set higher. For example, if
the boiler targets 70°F when a hydro air heater is
used, relatively cold air is likely to be supplied to the
space if there is no thermostat to keep it from running.
Mild Weather Outdoor Temperature
: This is the
outdoor temperature at which the boiler should
operate at its minimum heating load. Figure 8.3
shows this value at the factory default value of 70°F
(21°C). At any outdoor temperature above this value,
the boiler will target the Mild Weather Boiler
Temperature until the outdoor temperature exceeds
the Warm Weather Shutdown Temperature (described
later). Note that this is limited by the Boiler Minimum
Temperature (described later).
System Type Presets
: The PUREFIRE
®
Control allows
the installer to select preset values for Boiler Design
Temperature and Mild Weather Boiler Temperature
based on the type of heat distribution used. Table 8.7
shows the system types and the corresponding boiler
temperatures.
Type #7 is designated user defined and shows the
default boiler temperatures. Once this choice is
selected, the installer can change the numbers as
desired.
Boiler Minimum
: This value is the lower limit of the
boiler target temperature. Figure 8.3 shows that a
Boiler Minimum temperature of 80°F will not allow
the target to drop below this value.
This is useful if the boiler is used in conjunction with a
fan coil unit, kick space heater, unit heaters or other
instance where a low boiler supply temperature can
cause uncomfortable conditions. The default value for
Boiler Minimum is OFF.
Boiler Maximum
: If the boiler is installed in an
installation where high water temperatures can cause
damage to tubing or floors, this will limit the
maximum temperature that can be achieved. Figure
8.3 shows this value at 150°F, which is below the
Boiler Design Temperature. Notice that the target
temperature will not exceed the Boiler Maximum. The
default value for Boiler Maximum is 195°F (91°C).
Boost Function
: The boost function increases the boiler
supply target temperature by the value, “Temp”, if the
demand is not satisfied in the value, “Time.” Both of
these values are selected by the installer.
The default for these values are as follows:
Temp = 18°F and Time = 20 minutes.
This function is designed to help the boiler recover
from a setback condition. It will also help if too low of
a value is chosen for the boiler design and/or mild
weather temperature.
This value will continue to increase by the Temp value
if the demand continues through consecutive Time
periods.
The value is allowed to exceed the design temperature
but can never exceed the boiler maximum temperature.
44
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 8.7: System Type Presets
BOILER CONTROL: INTERNAL WIRING & OPERATION