Rheem Raypak Hi Delta P902C Installation & Operating Instructions Manual page 51

RESET RATIO (See Fig. 52)
The control uses the four following settings to determine the reset ratio:
Boiler Start (Boil START)
The Boil START temperature is the theoretical boiler supply water temperature that the heating system requires when the outdoor air
temperature equals the OUT START temperature setting. The Boil START is typically set to the desired building temperature.
Outdoor Start (OUT START)
The OUT START temperature is the outdoor air temperature at which the control provides the Boil START water temperature to
the system. The OUT START is typically set to the desired building temperature.
Outdoor Design (OUT DESIGN)
The OUT DESIGN is the outdoor air temperature that is the typical coldest annual temperature where the building is located. This
temperature is used when completing heat loss calculations for the building.
Boiler Design (Boil DESIGN)
The Boil DESIGN temperature is the water temperature required to heat the boiler zones when the outdoor air is as cold as
the OUT DESIGN temperature.
Warm Weather Shut Down (WWSD)
When the outdoor air temperature rises above the WWSD setting, the
control turns on the WWSD segment in the display. When the control is in
Warm Weather Shut Down, the Dem 1 segment is displayed if there is a
heat demand. However, the control does not operate the boiler to satisfy
this demand. The control continues to respond to DHW demands.
Reset Ratio
The controller uses the following four settings to cal-
culate the Reset Ratio (RR):
(OUTDOOR START – OUTDOOR DESIGN)
RESET RATIO =
(BOILER DESIGN – BOILER START)
For example, when using the default values, the RR
is:
RR = (70 - 10) / (180 - 70) = 0.55
Therefore, the RR is 0.55:1 (Outdoor:Water).
Boil
START
Decreasing Outdoor Temperature
Fig. 52: Reset Ratio
NOTE: The wiring diagrams in this manual show all
standard options. Refer to the large wiring diagram
provided with your heater for options installed on
your specific unit(s).
51
Boil DESIGN
OUT
DESIGN
OUT
START