Specifications

Heating Cooling
LAT = EAT + HC
CFM x 1.08
LAT (DB) = EAT (DB) - SC
CFM x 1.08
LWT = EWT - HE
GPM x 500
LWT = EWT + HR
GPM x 500
LC = TC - SC
Heating & Cooling Calculations
Performance Data Notes
1. Capacity data includes water pumping watts and is based upon 15% (by volume) propylene glycol antifreeze solution.
2. Desuperheater capacity is based upon 0.4 GPM Flow per nominal ton at 90°F entering hot water temperature.
3. Interpolation between above categories is permissible; extrapolation is not.
4. See Flow Rate Selection above for proper application.
5
RXT Series Catalog - August 2010 Roth
Calculations, Water Flow Selection, & Performance Data Notes
Water Flow Selection
Proper ow rate is crucial for reliable operation of geothermal heat
pumps. The performance data shows three ow rates for each entering
water temperature (EWT column). The general “rule of thumb” when
selecting ow rates is the following:
Top ow rate: Open loop systems (1.5 to 2.0 gpm per ton)
Middle ow rate: Minimum closed loop system ow rate
(2.25 to 2.50 gpm/ton)
Bottom ow rate: Nominal (optimum) closed loop system ow rate
(3.0 gpm/ton)
Although the “rule of thumb” is adequate in most areas of North
America, it is important to consider the application type before applying
this “rule of thumb.” Antifreeze is generally required for all closed loop
(geothermal) applications. Extreme Southern U.S. locations are the
only exception. Open loop (well water) systems cannot use antifreeze,
and must have enough ow rate in order to avoid freezing conditions at
the Leaving Source Water Temperature (LWT) connection.
Calculations must be made for all systems without antifreeze to deter-
mine if the top ow rate is adequate to prevent LWT at or near freezing
conditions. The following steps should taken in making this calculation:
Determine minimum EWT based upon your geographical area.
Go to the performance data table for the heat pump model selected and
look up the the Heat of Extraction (HE) at the “rule of thumb” water ow
rate (GPM) and at the design Entering Air Temperature (EAT).
Calculate the temperature dierence (TD) based upon the HE and GPM
of the model (step 4).
TD = HE / (GPM x 500).
Calculate the LWT (step 6).
LWT = EWT - TD.
If the LWT is below 35-38°F, there is potential for freezing conditions if
the ow rate or water temperature is less than ideal conditions, and the
ow rate must be increased.
Example 1:
EWT = 50°F.
Model RXT048, high capacity. Flow rate = 6 GPM.
Air Flow = 1500 CFM. HE = 36,700 Btuh.
TD = 36,700 / (6 x 500) = 12.2°F
LWT = 50 - 12.2 = 37.8°F
Since the water ow is leaving at approximately 38°F, the ow rate
is acceptable.
Example 2:
EWT = 40°F.
Model RXT048, high capacity. Flow rate = 6 GPM.
Air Flow = 1500 CFM. HE = 36,700 Btuh.
TD = 36,700 / (6 x 500) = 12.2°F
LWT = 40 - 12.2 = 27.8°F
Water ow rate must be increased to avoid freezing.