diff --git a/HPXMLtoOpenStudio/resources/hpxml_schematron/EPvalidator.xml b/HPXMLtoOpenStudio/resources/hpxml_schematron/EPvalidator.xml
index 2cb6b07991..4f6c78ab44 100644
--- a/HPXMLtoOpenStudio/resources/hpxml_schematron/EPvalidator.xml
+++ b/HPXMLtoOpenStudio/resources/hpxml_schematron/EPvalidator.xml
@@ -338,7 +338,7 @@
     <sch:title>[HVACSizingControl]</sch:title>
     <sch:rule context='/h:HPXML/h:Building/h:BuildingDetails/h:BuildingSummary/h:extension/h:HVACSizingControl'>
       <sch:assert role='ERROR' test='count(h:HeatPumpSizingMethodology) &lt;= 1'>Expected 0 or 1 element(s) for xpath: HeatPumpSizingMethodology</sch:assert>
-      <sch:assert role='ERROR' test='h:HeatPumpSizingMethodology[text()="ACCA" or text()="HERS" or text()="MaxLoad"] or not(h:HeatPumpSizingMethodology)'>Expected HeatPumpSizingMethodology to be 'ACCA' or 'HERS' or 'MaxLoad'</sch:assert>
+      <sch:assert role='ERROR' test='h:HeatPumpSizingMethodology[text()="ACCA2023" or text()="ACCA" or text()="HERS" or text()="MaxLoad"] or not(h:HeatPumpSizingMethodology)'>Expected HeatPumpSizingMethodology to be 'ACCA2023' or 'ACCA' or 'HERS' or 'MaxLoad'</sch:assert>
       <sch:assert role='ERROR' test='count(h:HeatPumpBackupSizingMethodology) &lt;= 1'>Expected 0 or 1 element(s) for xpath: HeatPumpBackupSizingMethodology</sch:assert>
       <sch:assert role='ERROR' test='h:HeatPumpBackupSizingMethodology[text()="emergency" or text()="supplemental"] or not(h:HeatPumpBackupSizingMethodology)'>Expected HeatPumpBackupSizingMethodology to be 'emergency' or 'supplemental'</sch:assert>
       <sch:assert role='ERROR' test='count(h:AllowIncreasedFixedCapacities) &lt;= 1'>Expected 0 or 1 element(s) for xpath: AllowIncreasedFixedCapacities</sch:assert>
diff --git a/HPXMLtoOpenStudio/resources/hvac_sizing.rb b/HPXMLtoOpenStudio/resources/hvac_sizing.rb
index 832db9cec8..a6724ba4d6 100644
--- a/HPXMLtoOpenStudio/resources/hvac_sizing.rb
+++ b/HPXMLtoOpenStudio/resources/hvac_sizing.rb
@@ -1711,26 +1711,105 @@ def self.apply_fractions_load_served(hvac_heating, hvac_loads, frac_zone_heat_lo
   end
 
   # Returns the ACCA Manual S sizing allowances for a given type of HVAC equipment.
-  # These sizing allowances are used in the logic that determines how to convert heating/cooling
+  # These sizing allowances are used in the logic that determines how to convert heating/cooling 
   # design loads into corresponding equipment capacities.
-  # 
-  # @param hvac_cooling [HPXML::CoolingSystem or HPXML::HeatPump] The cooling portion of the current HPXML HVAC system
-  # @return [Array<Double, Double, Double>] Oversize fraction (frac), oversize delta (Btu/hr), undersize fraction (frac)
-  def self.get_hvac_size_limits(hvac_cooling)
-    oversize_limit = 1.15
-    oversize_delta = 15000.0
-    undersize_limit = 0.9
 
-    if not hvac_cooling.nil?
-      if hvac_cooling.compressor_type == HPXML::HVACCompressorTypeTwoStage
-        oversize_limit = 1.2
-      elsif hvac_cooling.compressor_type == HPXML::HVACCompressorTypeVariableSpeed
-        oversize_limit = 1.3
-      end
-    end
+  # New ACCA Manual S specifies different size limits for clg only/HPs depending on 
+  # the sizing condition (Standard, Dry, Variable Speed, etc.)
+  # and total cooling load (24,000 BTU/hr cutoff)
 
-    return oversize_limit, oversize_delta, undersize_limit
-  end
+  # get_hvac_size_limits() does not address section/table N2.3.3 Two-Speed Heat Pump Sizing Condition 
+  # or section/table N2.3.4 Variable-Capacity Equipment Sizing Condition
+  # since these size conditions also include heating size factors and minimum compressor heating size factors
+  # To maintain consistency w/ previous implementation in ACCA Man. S 2014, only cooling size limits are returned.
+  # Therefore, only sections/tables N2.3.1 and N2.3.2 from ACCA Man. S 2023 are addressed in this method.
+
+  # @param hvac_cooling [HPXML::CoolingSystem or HPXML::HeatPump] The cooling portion of the current HPXML HVAC system 
+  # @param hvac_sizings [HVACSizingValues] Object with sizing values for a given HVAC system
+  # @param hpxml_bldg [HPXML::Building] HPXML Building object representing an individual dwelling unit
+  # @param weather [WeatherFile] Weather object containing EPW information
+  # @return [Array<Double, Double, Double, Double>] total clg oversize limit (frac), 
+  # total clg undersize limit (frac), sens. clg undersize limit (frac), lat. clg undersize limit (frac)
+
+  def self.get_hvac_size_limits(hvac_cooling, hvac_sizings, hpxml_bldg, weather)
+
+    load_shr = hvac_sizings.Cool_Load_Sens / hvac_sizings.Cool_Load_Tot
+    # calculate Climate JSHR to determine standard vs dry sizing condition (new ACCA)
+
+    if hpxml_bldg.header.heat_pump_sizing_methodology == HPXML::HeatPumpSizingACCA2023
+      if not hvac_cooling.nil?
+        if load_shr < 0.95
+        # larger latent load
+        # Section N.2.3.1 ACCA Man. S 2023, Standard Sizing Condition
+          total_clg_undersize_limit = 0.90
+          sens_clg_undersize_limit = 0.90
+          lat_clg_undersize_limit = 1.00
+          if hvac_cooling.compressor_type == HPXML::HVACCompressorTypeSingleStage && hvac_sizings.Cool_Load_Tot <= 24000 # BTU/hr
+            total_clg_oversize_limit = 1.20
+          elsif hvac_cooling.compressor_type == HPXML::HVACCompressorTypeSingleStage && hvac_sizings.Cool_Load_Tot > 24000 # BTU/hr
+            total_clg_oversize_limit = 1.15
+          elsif hvac_cooling.compressor_type == HPXML::HVACCompressorTypeTwoStage
+            total_clg_oversize_limit = 1.25
+          end
+        elsif load_shr >= 0.95 # Section N.2.3.2 ACCA Man. S 2023, Dry Sizing Condition
+          if hvac_cooling.compressor_type == HPXML::HVACCompressorTypeSingleStage
+            # Total Cooling Capacity / (Total Cooling Load + 6 kBtuh) <= 1.00
+            # 1.00 = non-load adjusted oversize limit
+            # goal --> determine "load adjusted" oversize limit
+            # Total Cooling Capacity <= 1.00 * (Total Cooling Load + 6 kBtuh)
+            # Total Cooling Capacity <= 1.00 * Total Cooling Load + 1.00 * 6 kBtuh 
+            # Total Cooling Capacity / Total Cooling Load <= 1.00 + (1.00 * 6 kBtuh) / Total Cooling Load
+            # Total Cooling Capacity Factor <= 1.00 + 6 kBtuh / Total Cooling Load
+            # Effective Total Cooling Oversize Limit = 1.00 + 6 kBtuh / Total Cooling Load
+            total_clg_oversize_limit = 1 + 6000 / hvac_sizings.Cool_Load_Tot
+            total_clg_undersize_limit = 0.90
+            sens_clg_undersize_limit = 0.90
+            lat_clg_undersize_limit = 1.00
+          elsif hvac_cooling.compressor_type == HPXML::HVACCompressorTypeTwoStage
+            total_clg_oversize_limit = 1.15 # minimum compressor speed! TODO: incorporate minimum speed logic elsewhere
+            total_clg_undersize_limit = 0.90 # total clg undersize limit for dry climate, 2-stage compressor is as specified in ACCA 2014
+            # ACCA 2023 Table N2.3.2 does not specify total cooling undersize limit.
+            sens_clg_undersize_limit = 0.90
+            lat_clg_undersize_limit = 1.00
+          end
+        end
+      end
+    elsif hpxml_bldg.header.heat_pump_sizing_methodology == HPXML::HeatPumpSizingACCA
+    # References "Overview of Size Limits for Residential HVAC Equipment" from Man. S 2014
+      if ((weather.data.HDD65F / weather.data.CDD50F) < 2.0) || (load_shr < 0.95)
+        # mild winter or has latent cooling load
+        if not hvac_cooling.nil?
+          total_clg_undersize_limit = 0.90
+          sens_clg_undersize_limit = 0.90
+          lat_clg_undersize_limit = 1.00
+          if hvac_cooling.compressor_type == HPXML::HVACCompressorTypeSingleStage
+            total_clg_oversize_limit = 1.15
+          elsif hvac_cooling.compressor_type == HPXML::HVACCompressorTypeTwoStage
+            total_clg_oversize_limit = 1.20
+          elsif hvac_cooling.compressor_type == HPXML::HVACCompressorTypeVariableSpeed
+            total_clg_oversize_limit = 1.30
+          end
+        end
+      elsif ((weather.data.HDD65F / weather.data.CDD50F) >= 2.0) && (load_shr >= 0.95)
+        # cold winter and no latent cooling load
+        # Manual S 2023 doesn't specify latent/sensible capacity undersize limits in this situation.
+        # from software perspective, could either return zero, nil, or the same latent/sensible
+        # undersize limits as the situation with latent cooling load/mild winter.
+        total_clg_undersize_limit = 0.90
+        sens_clg_undersize_limit = nil
+        lat_clg_undersize_limit = nil
+        # Max Total Cooling Capacity = Total Cooling Load + 15 kBtuh
+        # i.e. Total Cooling Capacity <= Total Cooling Load + 15 kBtuh
+        # <= means solve for oversize limit, size factor (capacity/load) <= oversize limit
+        # Total Cooling Capacity / Total Cooling Load <= (Total Cooling Load + 15 kBtuh) / Total Cooling Load
+        # Total Cooling Oversize Limit = (Total Cooling Load + 15 kBtuh) / Total Cooling Load
+        total_clg_oversize_limit = (hvac_sizings.Cool_Load_Tot + 15000) / hvac_sizings.Cool_Load_Tot
+      end
+    end
+    
+    return total_clg_oversize_limit, total_clg_undersize_limit, sens_clg_undersize_limit, lat_clg_undersize_limit 
+  
+  end 
 
   # Transfers the design load totals from the HVAC loads object to the HVAC sizings object.
   #
@@ -2108,7 +2187,7 @@ def self.apply_hvac_equipment_adjustments(mj, runner, hvac_sizings, weather, hva
       hvac_cooling_ap = hvac_cooling.additional_properties
       is_ducted = !hvac_cooling.distribution_system.nil?
       cooling_delta_t = mj.cool_setpoint - hvac_cooling_ap.leaving_air_temp
-      oversize_limit, oversize_delta, undersize_limit = get_hvac_size_limits(hvac_cooling)
+      tot_clg_oversize_limit, tot_clg_undersize_limit, sens_clg_undersize_limit, lat_clg_undersize_limit = get_hvac_size_limits(hvac_cooling)
     end
 
     if hvac_sizings.Cool_Load_Tot <= 0
@@ -2218,11 +2297,11 @@ def self.apply_hvac_equipment_adjustments(mj, runner, hvac_sizings, weather, hva
           # If the adjusted equipment size is negative (occurs at altitude), use oversize limit (the adjustment
           # almost always hits the oversize limit in this case, making this a safe assumption)
           if (cool_cap_design < 0) || (cool_sens_cap_design < 0)
-            cool_cap_design = oversize_limit * hvac_sizings.Cool_Load_Tot
+            cool_cap_design = tot_clg_oversize_limit * hvac_sizings.Cool_Load_Tot
           end
 
           # Limit total capacity to oversize limit
-          cool_cap_design = [cool_cap_design, oversize_limit * hvac_sizings.Cool_Load_Tot].min
+          cool_cap_design = [cool_cap_design, tot_clg_oversize_limit * hvac_sizings.Cool_Load_Tot].min
 
           # Determine rated capacities
           hvac_sizings.Cool_Capacity = cool_cap_design / total_cap_curve_value
@@ -2231,7 +2310,7 @@ def self.apply_hvac_equipment_adjustments(mj, runner, hvac_sizings, weather, hva
           # Determine the final sensible capacity at design using the SHR
           cool_sens_cap_design = cool_cap_design * design_shr
 
-        elsif cool_sens_cap_design < undersize_limit * hvac_sizings.Cool_Load_Sens
+        elsif cool_sens_cap_design < sens_undersize_limit * hvac_sizings.Cool_Load_Sens
           # Size by MJ8 Sensible Load, return to rated conditions, find rated sensible capacity with SHRRated. Limit total
           # capacity to oversizing limit.
 
@@ -2241,7 +2320,7 @@ def self.apply_hvac_equipment_adjustments(mj, runner, hvac_sizings, weather, hva
           cool_cap_design = cool_sens_cap_design / design_shr
 
           # Limit total capacity to oversize limit
-          cool_cap_design = [cool_cap_design, oversize_limit * hvac_sizings.Cool_Load_Tot].min
+          cool_cap_design = [cool_cap_design, tot_clg_oversize_limit * hvac_sizings.Cool_Load_Tot].min
 
           # rated capacities
           hvac_sizings.Cool_Capacity = cool_cap_design / total_cap_curve_value
@@ -2344,7 +2423,7 @@ def self.apply_hvac_equipment_adjustments(mj, runner, hvac_sizings, weather, hva
         cool_cap_design = cool_load_sens_cap_design + cool_load_lat_cap_design
 
         # Limit total capacity via oversizing limit
-        cool_cap_design = [cool_cap_design, oversize_limit * hvac_sizings.Cool_Load_Tot].min
+        cool_cap_design = [cool_cap_design, tot_clg_oversize_limit * hvac_sizings.Cool_Load_Tot].min
         hvac_sizings.Cool_Capacity = cool_cap_design / total_cap_curve_value
         hvac_sizings.Cool_Capacity_Sens = hvac_sizings.Cool_Capacity * hvac_cooling_shr
       end
@@ -3168,8 +3247,10 @@ def self.calculate_heat_pump_adj_factor_at_outdoor_temperature(mj, hvac_heating,
   # @return [Double] Heat pump backup load (Btu/hr)
   def self.calculate_heat_pump_backup_load(mj, hvac_heating, heating_load, hp_nominal_heating_capacity, hvac_heating_speed, hpxml_bldg)
     if hpxml_bldg.header.heat_pump_backup_sizing_methodology == HPXML::HeatPumpBackupSizingEmergency
-      # Size backup to meet full design load in case heat pump fails
-      return heating_load
+      # Size backup to meet 85% of design load in case heat pump fails 
+      # New ACCA Man S (2024)--> emergency heating load is 85% of heating load 
+      # See Table N1.16.3.2 Electric Resistance Emergency Heat 
+      return 0.85*heating_load
     elsif hpxml_bldg.header.heat_pump_backup_sizing_methodology == HPXML::HeatPumpBackupSizingSupplemental
       if not hvac_heating.backup_heating_switchover_temp.nil?
         min_compressor_temp = hvac_heating.backup_heating_switchover_temp