Merge pull request #528 from NREL/slab-hvac-sizing

Updates HVAC sizing code for slab-on-grade foundation types

Author: shorowit

HPXMLtoOpenStudio/measure.xml

@@ -3,8 +3,8 @@
   <schema_version>3.0</schema_version>
   <name>hpxm_lto_openstudio</name>
   <uid>b1543b30-9465-45ff-ba04-1d1f85e763bc</uid>
-  <version_id>516faf23-4510-4e76-a9c0-5870d42526f9</version_id>
-  <version_modified>20201028T181202Z</version_modified>
+  <version_id>28973ccc-3461-4c07-b3a9-345fd52ee38b</version_id>
+  <version_modified>20201028T181823Z</version_modified>
   <xml_checksum>D8922A73</xml_checksum>
   <class_name>HPXMLtoOpenStudio</class_name>
   <display_name>HPXML to OpenStudio Translator</display_name>
@@ -429,12 +429,6 @@
       <usage_type>test</usage_type>
       <checksum>97E9A445</checksum>
     </file>
-    <file>
-      <filename>hvac_sizing.rb</filename>
-      <filetype>rb</filetype>
-      <usage_type>resource</usage_type>
-      <checksum>E50CBF7E</checksum>
-    </file>
     <file>
       <filename>test_hvac.rb</filename>
       <filetype>rb</filetype>
@@ -500,5 +494,17 @@
       <usage_type>resource</usage_type>
       <checksum>721A9C7B</checksum>
     </file>
+    <file>
+      <filename>hvac_sizing.rb</filename>
+      <filetype>rb</filetype>
+      <usage_type>resource</usage_type>
+      <checksum>51B1E37F</checksum>
+    </file>
+    <file>
+      <filename>test_hvac_sizing.rb</filename>
+      <filetype>rb</filetype>
+      <usage_type>test</usage_type>
+      <checksum>35C3F156</checksum>
+    </file>
   </files>
 </measure>

HPXMLtoOpenStudio/resources/hvac_sizing.rb

@@ -866,8 +866,8 @@ def self.process_load_floors(zone_loads, weather)
       next unless slab.is_thermal_boundary
 
       if slab.interior_adjacent_to == HPXML::LocationLivingSpace # Slab-on-grade
-        floor_ufactor = 0.1 # FIXME: Hard-coded
-        zone_loads.Heat_Floors += floor_ufactor * slab.area * (@heat_setpoint - weather.data.GroundMonthlyTemps[0])
+        f_value = calc_slab_f_value(slab)
+        zone_loads.Heat_Floors += f_value * slab.exposed_perimeter * @htd
       elsif slab.interior_adjacent_to == HPXML::LocationBasementConditioned
         # Based on MJ 8th Ed. A12-7 and ASHRAE HoF 2013 pg 18.31 Eq 40
         # FIXME: Assumes slab is uninsulated?
@@ -2980,6 +2980,70 @@ def self.get_foundation_wall_properties(foundation_wall)
     return u_wall_with_soil, u_wall_without_soil
   end
 
+  def self.calc_slab_f_value(slab)
+    # Calculation for the F-values in Table 4A for slab foundations.
+    # Important pages are the Table values (pg. 344-345) and the software protocols
+    # in Appendix 12 (pg. 517-518).
+    ins_rvalue = slab.under_slab_insulation_r_value + slab.perimeter_insulation_r_value
+    ins_rvalue_edge = slab.perimeter_insulation_r_value
+    edge_ins_rvalue =
+      if slab.under_slab_insulation_spans_entire_slab
+        ins_length = 1000.0
+      else
+        ins_length = 0
+        if slab.under_slab_insulation_r_value > 0
+          ins_length += slab.under_slab_insulation_width
+        end
+        if slab.perimeter_insulation_r_value > 0
+          ins_length += slab.perimeter_insulation_depth
+        end
+      end
+
+    soil_r_per_foot = Material.Soil(12.0).rvalue
+    slab_r_gravel_per_inch = 0.65 # Based on calibration by Tony Fontanini
+
+    # Because of uncertainty pertaining to the effective path radius, F-values are calculated
+    # for six radii (8, 9, 10, 11, 12, and 13 feet) and averaged.
+    f_values = []
+    for path_radius in 8..13
+      u_effective = []
+      for radius in 0..path_radius
+        spl = [Math::PI * radius - 1, 0].max # soil path length (SPL)
+
+        # Concrete, gravel, and insulation
+        if radius == 0
+          r_concrete = 0.0
+          r_gravel = 0.0 # No gravel on edge
+          r_ins = ins_rvalue_edge
+        else
+          r_concrete = Material.Concrete(slab.thickness).rvalue
+          r_gravel = [slab_r_gravel_per_inch * (12.0 - slab.thickness), 0].max
+          if radius <= ins_length
+            r_ins = ins_rvalue
+          else
+            r_ins = 0.0
+          end
+        end
+
+        # Air Films = Indoor Finish + Indoor Air Film + Exposed Air Film (Figure A12-6 pg. 517)
+        r_air_film = 0.05 + 0.92 + 0.17
+
+        # Soil
+        r_soil = soil_r_per_foot * spl # (h-F-ft2/BTU)
+
+        # Effective R-Value
+        r_air_to_air = r_concrete + r_gravel + r_ins + r_air_film + r_soil
+
+        # Effective U-Factor
+        u_effective << 1.0 / r_air_to_air
+      end
+
+      f_values << u_effective.inject(0, :+) # sum array
+    end
+
+    return f_values.sum() / f_values.size
+  end
+
   def self.get_feature(obj, feature, datatype, fail_on_error = true)
     val = nil
     if datatype == 'string'

HPXMLtoOpenStudio/tests/test_hvac_sizing.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+require_relative '../resources/minitest_helper'
+require 'openstudio'
+require 'openstudio/measure/ShowRunnerOutput'
+require 'minitest/autorun'
+require 'fileutils'
+require_relative '../measure.rb'
+require_relative '../resources/util.rb'
+
+class HPXMLtoOpenStudioHVACSizingTest < MiniTest::Test
+  def sample_files_dir
+    return File.join(File.dirname(__FILE__), '..', '..', 'workflow', 'sample_files')
+  end
+
+  def test_slab_f_factor
+    def get_unins_slab()
+      slab = HPXML::Slab.new(nil)
+      slab.thickness = 4.0 # in
+      slab.perimeter_insulation_depth = 0
+      slab.perimeter_insulation_r_value = 0
+      slab.under_slab_insulation_width = 0
+      slab.under_slab_insulation_spans_entire_slab = false
+      slab.under_slab_insulation_r_value = 0
+      return slab
+    end
+
+    # Uninsulated slab
+    slab = get_unins_slab()
+    f_factor = HVACSizing.calc_slab_f_value(slab)
+    assert_in_epsilon(1.41, f_factor, 0.01)
+
+    # R-10, 4ft under slab insulation
+    slab = get_unins_slab()
+    slab.under_slab_insulation_width = 4
+    slab.under_slab_insulation_r_value = 10
+    f_factor = HVACSizing.calc_slab_f_value(slab)
+    assert_in_epsilon(1.27, f_factor, 0.01)
+
+    # R-20, 4ft perimeter insulation
+    slab = get_unins_slab()
+    slab.perimeter_insulation_depth = 4
+    slab.perimeter_insulation_r_value = 20
+    f_factor = HVACSizing.calc_slab_f_value(slab)
+    assert_in_epsilon(0.39, f_factor, 0.01)
+
+    # R-40, whole slab insulation
+    slab = get_unins_slab()
+    slab.under_slab_insulation_spans_entire_slab = true
+    slab.under_slab_insulation_r_value = 40
+    f_factor = HVACSizing.calc_slab_f_value(slab)
+    assert_in_epsilon(1.04, f_factor, 0.01)
+  end
+end