Bryant 50YEW Design Manual page 59

then be reduced by the outdoor temperature reset function,
depending on the current outdoor air temperature (OAT) value.
The valid range for the buffer tank heating setpoint is 70-140°F
[21-60°C], with a default value of 100°F [38°C]. The valid range for
the heating differential value (HTD) is 4-20°F [2-11°C], adjustable
in 2°F [1°C] increments, with a default value of 6°F [3°C].
There are four outdoor reset variables used for reducing the
buffer tank setpoint. The outdoor design temperature (ODT) is
the OAT above which setpoint reduction begins. The valid range
for ODT is –40°F to 50°F [-40°C to 10°C], with a default value
of 0°F [-18°C]. The maximum design buffer tank temperature
(MaxBT) is the maximum desired buffer tank setpoint at the
outdoor design temperature. The valid range for MaxBT is 80-
140°F [27-60°C], with a default value of 130°F [54°F]. The building
balance point temperature (the temperature at which heating
is no longer needed) is the OAT at which maximum setpoint
(MaxBT) reduction will occur. The valid range for building balance
point is 50-70°F [10-21°C], with a default value of 60°F [16°F].
The minimum design water temperature is the minimum desired
buffer tank setpoint at the building balance point temperature. The
valid range for minimum buffer tank temperature is 70°F-120°F
[21-49°C], with a default value of 70°F [21°c]. If an OAT sensor
is not detected (or if a thermistor erro has occurred), the buffer
tank setpoint will not be reduced based on the OAT value (i.e.
the controller will use the buffer tank setpoint as described in the
previous paragraph).
Figure 4-3: Example Outdoor Temperature Reset
55
[131]
50
[122]
45
[113]
40
[104] Minimum
35
Buffer Tank
[95]
Temperature
30
[86]
25
[77]
20
[68] 20C [68F]
15
[59]
10
[50]
30 25
[86] [77]
Bryant Geothermal Heat Pump Systems
Building
Balance
Point
Temperature
20 15 10 5
[68] [59] [50] [41]
Outdoor Temperature, °C [°F]
Water-to-Water System Design Guide
Figure 4-3 shows an example outdoor temperature reset curve for
a climate that has an outdoor design temperature of -4°F [-20°C].
At design temperature, the radiant fl oor system needs 126°F
[52°C] water. However, when the outdoor temperature is 68°F
[20°C], the home needs no heating (building balance point). In
between -4°F and 68°F [-20°C and 20°C], the water temperature
in the buffer tank is adjusted accordingly. For homes that are well
insulated and tightly sealed, the building balance point may be
55°F [13°C] or lower, so the slope of the line changes based upon
settings at the user interface. The radiant fl oor design temperature
will also change the slope of the line. If tighter pipe spacing is
used, for example, the water temperature at the outdoor design
temperature may only be 100°F [38°C]. Again, as the settings are
changed at the user interface, the slope of the line will change.
As mentioned earlier, the lower the heating water temperature
at design conditions, the higher the effi ciency (COP) of the heat
pump. The combination of a lower design temperature and
outdoor temperature reset can result in a signifi cant impact on
operating costs.
Outdoor
Design
Temperature
0 -5 -10 -15
[32] [23] [14] [5]
Part IV: Controls
52C [126F]
Maximum
Buffer Tank
Temperature
-20 -25 -30
[-4] [-13] [-22]
57