diff --git a/Changelog.md b/Changelog.md
index 6b45149864..5900c773c8 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -50,6 +50,7 @@ __Bugfixes__
 - Fixes possible error for a combi boiler system.
 - Fixes error if modeling a ground-to-air heat pump with a separate backup heating system.
 - Fixes default CFIS fan power during ventilation only mode.
+- Fixes a bug that potentially oversizes heat pumps when detailed performance capacity fractions are provided.
 
 ## OpenStudio-HPXML v1.8.1
 
diff --git a/HPXMLtoOpenStudio/measure.xml b/HPXMLtoOpenStudio/measure.xml
index f834f1337d..819304b94d 100644
--- a/HPXMLtoOpenStudio/measure.xml
+++ b/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>aab4ee31-48b7-4f7a-bfc0-1d8edf16961d</version_id>
-  <version_modified>2024-10-22T03:59:34Z</version_modified>
+  <version_id>e7d819f6-fda2-4bf2-b6c3-cd1ff3adb569</version_id>
+  <version_modified>2024-10-22T17:43:11Z</version_modified>
   <xml_checksum>D8922A73</xml_checksum>
   <class_name>HPXMLtoOpenStudio</class_name>
   <display_name>HPXML to OpenStudio Translator</display_name>
@@ -393,7 +393,7 @@
       <filename>hvac_sizing.rb</filename>
       <filetype>rb</filetype>
       <usage_type>resource</usage_type>
-      <checksum>BEF6DAA5</checksum>
+      <checksum>CD77F9C7</checksum>
     </file>
     <file>
       <filename>internal_gains.rb</filename>
@@ -693,7 +693,7 @@
       <filename>test_hvac_sizing.rb</filename>
       <filetype>rb</filetype>
       <usage_type>test</usage_type>
-      <checksum>822ACA5D</checksum>
+      <checksum>C88CFFFC</checksum>
     </file>
     <file>
       <filename>test_lighting.rb</filename>
diff --git a/HPXMLtoOpenStudio/resources/hvac_sizing.rb b/HPXMLtoOpenStudio/resources/hvac_sizing.rb
index 6acc08d3cf..d35a98c61d 100644
--- a/HPXMLtoOpenStudio/resources/hvac_sizing.rb
+++ b/HPXMLtoOpenStudio/resources/hvac_sizing.rb
@@ -2530,11 +2530,14 @@ def self.adjust_outdoor_condition_var_speed(outdoor_temp, hvac_sys, mode)
       max_rated_dp = detailed_performance_data.find { |dp| dp.outdoor_temperature == rated_odb && dp.capacity_description == HPXML::CapacityDescriptionMaximum }
       if max_rated_dp.capacity.nil?
         property = :capacity_fraction_of_nominal
+        # Should use nominal instead of maximum
+        capacity_nominal = 1.0
       else
         property = :capacity
+        # Should use nominal instead of maximum
+        capacity_nominal = (mode == :clg) ? hvac_sys.cooling_capacity : hvac_sys.heating_capacity
       end
-      capacity_max = detailed_performance_data.find { |dp| dp.outdoor_temperature == rated_odb && dp.capacity_description == HPXML::CapacityDescriptionMaximum }.send(property)
-      odb_adj = HVAC.interpolate_to_odb_table_point(detailed_performance_data, HPXML::CapacityDescriptionMaximum, outdoor_temp, property) / capacity_max
+      odb_adj = HVAC.interpolate_to_odb_table_point(detailed_performance_data, HPXML::CapacityDescriptionMaximum, outdoor_temp, property) / capacity_nominal
     end
     return odb_adj
   end
diff --git a/HPXMLtoOpenStudio/tests/test_hvac_sizing.rb b/HPXMLtoOpenStudio/tests/test_hvac_sizing.rb
index 44deb9f4bf..c1317f888d 100644
--- a/HPXMLtoOpenStudio/tests/test_hvac_sizing.rb
+++ b/HPXMLtoOpenStudio/tests/test_hvac_sizing.rb
@@ -1153,6 +1153,7 @@ def test_autosizing_factors
         htg_cap_orig = hpxml_bldg.heat_pumps[0].heating_capacity
         clg_cap_orig = hpxml_bldg.heat_pumps[0].cooling_capacity
         backup_htg_cap_orig = hpxml_bldg.heat_pumps[0].backup_heating_capacity
+
         # apply autosizing factor
         hpxml, hpxml_bldg = _create_hpxml('base-hvac-air-to-air-heat-pump-var-speed-detailed-performance-autosize.xml')
         hpxml_bldg.heat_pumps[0].backup_heating_capacity = nil
@@ -1345,6 +1346,35 @@ def test_autosizing_limits
     end
   end
 
+  def test_detailed_performance_autosizing
+    args_hash = {}
+    args_hash['hpxml_path'] = File.absolute_path(@tmp_hpxml_path)
+
+    # Test heat pump w/ detailed performance
+    hpxml, hpxml_bldg = _create_hpxml('base-hvac-air-to-air-heat-pump-var-speed-detailed-performance-autosize.xml')
+    hpxml_bldg.header.heat_pump_sizing_methodology = HPXML::HeatPumpSizingMaxLoad
+    hpxml_bldg.heat_pumps[0].backup_heating_capacity = nil
+    XMLHelper.write_file(hpxml.to_doc, @tmp_hpxml_path)
+    _model, _hpxml, hpxml_bldg = _test_measure(args_hash)
+    htg_cap_orig = hpxml_bldg.heat_pumps[0].heating_capacity
+    clg_cap_orig = hpxml_bldg.heat_pumps[0].cooling_capacity
+    backup_htg_cap_orig = hpxml_bldg.heat_pumps[0].backup_heating_capacity
+
+    # Test heat pump w/ detailed performance when maximum fractions are over 1.0 at rated temperature
+    hpxml, hpxml_bldg = _create_hpxml('base-hvac-air-to-air-heat-pump-var-speed-detailed-performance-autosize.xml')
+    hpxml_bldg.header.heat_pump_sizing_methodology = HPXML::HeatPumpSizingMaxLoad
+    hpxml_bldg.heat_pumps[0].backup_heating_capacity = nil
+    hpxml_bldg.heat_pumps[0].heating_capacity = nil
+    hpxml_bldg.heat_pumps[0].cooling_capacity = nil
+    hpxml_bldg.heat_pumps[0].heating_detailed_performance_data.find { |dp| dp.outdoor_temperature == HVAC::AirSourceHeatRatedODB && dp.capacity_description == HPXML::CapacityDescriptionMaximum }.capacity_fraction_of_nominal = 1.2
+    hpxml_bldg.heat_pumps[0].cooling_detailed_performance_data.find { |dp| dp.outdoor_temperature == HVAC::AirSourceCoolRatedODB && dp.capacity_description == HPXML::CapacityDescriptionMaximum }.capacity_fraction_of_nominal = 1.1
+    XMLHelper.write_file(hpxml.to_doc, @tmp_hpxml_path)
+    _model, _hpxml, hpxml_bldg = _test_measure(args_hash)
+    assert_equal(hpxml_bldg.heat_pumps[0].heating_capacity, htg_cap_orig)
+    assert_equal(hpxml_bldg.heat_pumps[0].cooling_capacity, clg_cap_orig)
+    assert_equal(hpxml_bldg.heat_pumps[0].backup_heating_capacity, backup_htg_cap_orig)
+  end
+
   def test_manual_j_detailed_sizing_inputs
     # Run base
     args_hash = {}