Fixes possible ground-source heat pump sizing error if the heating or cooling design temperature differences are zero.

Author: shorowit

Changelog.md

@@ -30,6 +30,7 @@ __Bugfixes__
 - Fixes zero mech vent fan energy when CFIS system w/ `AdditionalRuntimeOperatingMode="air handler fan"` has the airflow rate set to zero.
 - Fixes requested EnergyPlus timeseries output variables/meters not displayed in DView if they don't have units.
 - Fixes possible errors when small water flow rates for variable-speed experimental ground-source heat pump model.
+- Fixes possible ground-source heat pump sizing error if the heating or cooling design temperature differences are zero.
 
 ## OpenStudio-HPXML v1.10.0
 

HPXMLtoOpenStudio/measure.xml

@@ -3,8 +3,8 @@
   <schema_version>3.1</schema_version>
   <name>hpxm_lto_openstudio</name>
   <uid>b1543b30-9465-45ff-ba04-1d1f85e763bc</uid>
-  <version_id>ebf347a8-a3b8-4096-a72c-078aa24e7d59</version_id>
-  <version_modified>2025-10-31T18:46:56Z</version_modified>
+  <version_id>a76321a6-1fe2-4a9b-8574-86c1fa95f899</version_id>
+  <version_modified>2025-10-31T20:47:24Z</version_modified>
   <xml_checksum>D8922A73</xml_checksum>
   <class_name>HPXMLtoOpenStudio</class_name>
   <display_name>HPXML to OpenStudio Translator</display_name>
@@ -420,7 +420,7 @@
       <filename>hvac_sizing.rb</filename>
       <filetype>rb</filetype>
       <usage_type>resource</usage_type>
-      <checksum>AFA48130</checksum>
+      <checksum>EE39F054</checksum>
     </file>
     <file>
       <filename>internal_gains.rb</filename>

HPXMLtoOpenStudio/resources/hvac_sizing.rb

@@ -3409,8 +3409,19 @@ def self.get_geothermal_loop_borefield_ft_per_ton(mj, hpxml_bldg, geothermal_loo
       cooling_month = 6 # July
     end
 
-    rtf_DesignMon_Heat = [0.25, (71.0 - weather.data.MonthlyAvgDrybulbs[heating_month]) / mj.htd].max
-    rtf_DesignMon_Cool = [0.25, (weather.data.MonthlyAvgDrybulbs[cooling_month] - 76.0) / mj.ctd].max
+    # Runtime fraction average in heating/cooling months, assumed
+    if mj.htd > 0
+      rtf_DesignMon_Heat = (mj.heat_setpoint - weather.data.MonthlyAvgDrybulbs[heating_month]) / mj.htd
+    else
+      rtf_DesignMon_Heat = 0
+    end
+    if mj.ctd > 0
+      rtf_DesignMon_Cool = (weather.data.MonthlyAvgDrybulbs[cooling_month] - mj.cool_setpoint) / mj.ctd
+    else
+      rtf_DesignMon_Cool = 0
+    end
+    rtf_DesignMon_Heat = [[rtf_DesignMon_Heat, 0.25].max, 1.0].min
+    rtf_DesignMon_Cool = [[rtf_DesignMon_Cool, 0.25].max, 1.0].min
 
     nom_length_heat = (1.0 - 1.0 / clg_ap.heat_rated_cops[0]) * (r_value_bore + r_value_ground * rtf_DesignMon_Heat) / (weather.data.DeepGroundAnnualTemp - (2.0 * clg_ap.design_hw - clg_ap.design_delta_t) / 2.0) * UnitConversions.convert(1.0, 'ton', 'Btu/hr')
     nom_length_cool = (1.0 + 1.0 / clg_ap.cool_rated_cops[0]) * (r_value_bore + r_value_ground * rtf_DesignMon_Cool) / ((2.0 * clg_ap.design_chw + clg_ap.design_delta_t) / 2.0 - weather.data.DeepGroundAnnualTemp) * UnitConversions.convert(1.0, 'ton', 'Btu/hr')