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Duct insulation air film #2

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Duct insulation air film #2

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47666b9
Update duct air film default R-values, and define air film and insula…
nathancoliver Jul 6, 2023
1427f07
Update supply and return duct R-values.
nathancoliver Jul 6, 2023
6aee1ae
Add definition to calculate duct air film r-value from nominal duct i…
nathancoliver Jul 10, 2023
9307a81
Remove duct air film r-values and update nominal duct insulation r-va…
nathancoliver Jul 10, 2023
24fc5ef
Perform duct air film calculation inside function when duct is uninsu…
nathancoliver Jul 11, 2023
84ea4db
Merge remote-tracking branch 'origin/master' into duct-insulation-air…
nathancoliver Mar 12, 2025
f915ba4
Update effective duct R-values according to 301-2022 Add. C.
nathancoliver Mar 13, 2025
21f83d1
Merge remote-tracking branch 'origin/master' into duct-insulation-air…
nathancoliver Aug 21, 2025
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28 changes: 22 additions & 6 deletions base.pxt
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Original file line number Diff line number Diff line change
Expand Up @@ -45,8 +45,10 @@
parameter "supply_duct_area_fraction", :default=>0 # fraction of conditioned floor area to set as supply duct area
parameter "return_duct_area_fraction", :default=>0 # fraction of conditioned floor area to set as return duct area
parameter "duct_zone", :default=>"Main Zone" # zone where supply and return ducts are located
parameter "duct_insulation_r_value", :default=>0 # R-value of duct insulation without air film
parameter "duct_air_film_r_value", :default=>1.5 # R-value of exterior air film for duct (or duct insulation)
parameter "duct_insulation_r_value_supply", :default=>0 # R-value of duct insulation without air film
parameter "duct_air_film_r_value_supply", :default=>1.7 # R-value of exterior air film for duct (or duct insulation)
parameter "duct_insulation_r_value_return", :default=>0 # R-value of duct insulation without air film
parameter "duct_air_film_r_value_return", :default=>1.7 # R-value of exterior air film for duct (or duct insulation)
parameter "duct_leakage_flow", :default=>0 # combined cfm airflow leaking out of supply + return ducts
parameter "duct_exterior_emittance", :default=>0 # Exposed (i.e. insulation) outside surface exterior long wave (thermal) emittance

Expand Down Expand Up @@ -75,6 +77,14 @@ def c_f(c)
return c*9/5.0 + 32.0
end

def calculate_duct_air_film_r_value(duct_insulation_r_value,duct_type)
if duct_type == "SUPPLY"
return 2.2438 - 0.4381 * duct_insulation_r_value
elsif duct_type == "RETURN"
return 2.0388 - 0.2947 * duct_insulation_r_value
end
end
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Did you test what happens if the air film is less than 1?

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@nathancoliver nathancoliver Jul 11, 2023 •
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I performed some tests, and it appears that negative air films close to zero produce unusual and unexpected heating and cooling energy use results. Other air film values, including positive numbers between 0 and 1 and negative numbers less than 1, do not produce strange results.


# Internal use
nsteps = 6

Expand Down Expand Up @@ -647,27 +657,33 @@ window_area = 0.0
rsVFPerTon = <%= fan_flow_per_cap %>

<% if supply_duct_area > 0 %>
<% if duct_insulation_r_value_supply > 0 %>
<% duct_air_film_r_value_supply = calculate_duct_air_film_r_value(duct_insulation_r_value_supply,"SUPPLY") %>
<% end %>
// Main Zone Supply Duct
DUCTSEG "Main Zone Supply Duct"
dsTy = SUPPLY
dsExArea = <%= supply_duct_area %>
dsExCnd = ADJZN
dsAdjZn = "<%= duct_zone %>"
dsInsulR = <%= duct_insulation_r_value %>
dsExH = <%= 1 / duct_air_film_r_value %>
dsInsulR = <%= duct_insulation_r_value_supply %>
dsExH = <%= 1 / duct_air_film_r_value_supply %>
dsLeakF = <%= duct_leakage_fraction %>
dsEpsLW = <%= duct_exterior_emittance %>
<% end %>

<% if return_duct_area > 0 %>
<% if duct_insulation_r_value_return > 0 %>
<% duct_air_film_r_value_return = calculate_duct_air_film_r_value(duct_insulation_r_value_return,"RETURN") %>
<% end %>
// Main Zone Return Duct
DUCTSEG "Main Zone Return Duct"
dsTy = Return
dsExArea = <%= return_duct_area %>
dsExCnd = ADJZN
dsAdjZn = "<%= duct_zone %>"
dsInsulR = <%= duct_insulation_r_value %>
dsExH = <%= 1 / duct_air_film_r_value %>
dsInsulR = <%= duct_insulation_r_value_return %>
dsExH = <%= 1 / duct_air_film_r_value_return %>
dsLeakF = <%= duct_leakage_fraction %>
dsEpsLW = <%= duct_exterior_emittance %>
<% end %>
Expand Down
4 changes: 2 additions & 2 deletions cases/dse/HVAC3c.pxv
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Original file line number Diff line number Diff line change
Expand Up @@ -8,5 +8,5 @@
:supply_duct_area_fraction=>0.2,
:return_duct_area_fraction=>0.05,
:duct_zone=>"Basement Zone",
:duct_insulation_r_value=>6,
:duct_air_film_r_value=>1.0
:duct_insulation_r_value_supply=>6,
:duct_insulation_r_value_return=>6
4 changes: 2 additions & 2 deletions cases/dse/HVAC3d.pxv
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,6 @@
:supply_duct_area_fraction=>0.2,
:return_duct_area_fraction=>0.05,
:duct_zone=>"Basement Zone",
:duct_insulation_r_value=>6,
:duct_air_film_r_value=>1.0,
:duct_insulation_r_value_supply=>6,
:duct_insulation_r_value_return=>6,
:duct_leakage_flow=>250
4 changes: 2 additions & 2 deletions cases/dse/HVAC3g.pxv
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Original file line number Diff line number Diff line change
Expand Up @@ -5,5 +5,5 @@
:supply_duct_area_fraction=>0.2,
:return_duct_area_fraction=>0.05,
:duct_zone=>"Attic Zone",
:duct_insulation_r_value=>6,
:duct_air_film_r_value=>1.0
:duct_insulation_r_value_supply=>6,
:duct_insulation_r_value_return=>6
4 changes: 2 additions & 2 deletions cases/dse/HVAC3h.pxv
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,6 @@
:supply_duct_area_fraction=>0.2,
:return_duct_area_fraction=>0.05,
:duct_zone=>"Attic Zone",
:duct_insulation_r_value=>6,
:duct_air_film_r_value=>1.0,
:duct_insulation_r_value_supply=>6,
:duct_insulation_r_value_return=>6,
:duct_leakage_flow=>250