commit 5c684d053737433964e623999a010d8f9cfe3c9e
Author: Michael Housh <mhoush@houshhomeenergy.com>
Date:   Wed Mar 12 16:59:10 2025 -0400

    feat: Initial commit

diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..7cfbe01
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,7 @@
+root = true
+
+[*.swift]
+indent_style = space
+indent_size = 2
+tab_width = 2
+trim_trailing_whitespace = true
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..0023a53
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,8 @@
+.DS_Store
+/.build
+/Packages
+xcuserdata/
+DerivedData/
+.swiftpm/configuration/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc
diff --git a/.swiftformat b/.swiftformat
new file mode 100644
index 0000000..08f338e
--- /dev/null
+++ b/.swiftformat
@@ -0,0 +1,11 @@
+--self init-only
+--indent 2
+--ifdef indent
+--trimwhitespace always
+--wraparguments before-first
+--wrapparameters before-first
+--wrapcollections preserve
+--wrapconditions after-first
+--typeblanklines preserve
+--commas inline
+--stripunusedargs closure-only
diff --git a/.swiftlint.yml b/.swiftlint.yml
new file mode 100644
index 0000000..213129c
--- /dev/null
+++ b/.swiftlint.yml
@@ -0,0 +1,11 @@
+disabled_rules:
+  - closing_brace
+  - fuction_body_length
+  - opening_brace
+  - nesting
+
+included:
+  - Sources
+  - Tests
+
+ignore_multiline_statement_conditions: true
diff --git a/Package.resolved b/Package.resolved
new file mode 100644
index 0000000..2d6d94e
--- /dev/null
+++ b/Package.resolved
@@ -0,0 +1,78 @@
+{
+  "originHash" : "c1e2b47f4dcb7c57922f698b2ddfff2e29dfa400eb006d972bfea53df688f5d9",
+  "pins" : [
+    {
+      "identity" : "combine-schedulers",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/combine-schedulers",
+      "state" : {
+        "revision" : "5928286acce13def418ec36d05a001a9641086f2",
+        "version" : "1.0.3"
+      }
+    },
+    {
+      "identity" : "swift-case-paths",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/swift-case-paths.git",
+      "state" : {
+        "revision" : "19b7263bacb9751f151ec0c93ec816fe1ef67c7b",
+        "version" : "1.6.1"
+      }
+    },
+    {
+      "identity" : "swift-clocks",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/swift-clocks",
+      "state" : {
+        "revision" : "cc46202b53476d64e824e0b6612da09d84ffde8e",
+        "version" : "1.0.6"
+      }
+    },
+    {
+      "identity" : "swift-concurrency-extras",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/swift-concurrency-extras",
+      "state" : {
+        "revision" : "82a4ae7170d98d8538ec77238b7eb8e7199ef2e8",
+        "version" : "1.3.1"
+      }
+    },
+    {
+      "identity" : "swift-dependencies",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/swift-dependencies.git",
+      "state" : {
+        "revision" : "ec2862d1364536fc22ec56a3094e7a034bbc7da8",
+        "version" : "1.8.1"
+      }
+    },
+    {
+      "identity" : "swift-syntax",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/swiftlang/swift-syntax",
+      "state" : {
+        "revision" : "0687f71944021d616d34d922343dcef086855920",
+        "version" : "600.0.1"
+      }
+    },
+    {
+      "identity" : "swift-validations",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/m-housh/swift-validations.git",
+      "state" : {
+        "revision" : "95ea5d267e37f6cdb9f91c5c8a01e718b9299db6",
+        "version" : "0.3.4"
+      }
+    },
+    {
+      "identity" : "xctest-dynamic-overlay",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/xctest-dynamic-overlay",
+      "state" : {
+        "revision" : "39de59b2d47f7ef3ca88a039dff3084688fe27f4",
+        "version" : "1.5.2"
+      }
+    }
+  ],
+  "version" : 3
+}
diff --git a/Package.swift b/Package.swift
new file mode 100644
index 0000000..c88e347
--- /dev/null
+++ b/Package.swift
@@ -0,0 +1,36 @@
+// swift-tools-version: 6.1
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+  name: "swift-manual-s",
+  platforms: [
+    .macOS(.v10_15)
+  ],
+  products: [
+    .library(name: "ManualS", targets: ["ManualS"])
+  ],
+  dependencies: [
+    .package(url: "https://github.com/pointfreeco/swift-dependencies.git", from: "1.0.0"),
+    .package(url: "https://github.com/m-housh/swift-validations.git", from: "0.3.4")
+  ],
+  targets: [
+    .target(
+      name: "ManualS",
+      dependencies: [
+        "Models",
+        .product(name: "Dependencies", package: "swift-dependencies"),
+        .product(name: "DependenciesMacros", package: "swift-dependencies"),
+        .product(name: "Validations", package: "swift-validations")
+      ]
+    ),
+    .target(
+      name: "Models"
+    ),
+    .testTarget(
+      name: "ManualSTests",
+      dependencies: ["ManualS"]
+    )
+  ]
+)
diff --git a/Sources/ManualS/Exports.swift b/Sources/ManualS/Exports.swift
new file mode 100644
index 0000000..4de8be8
--- /dev/null
+++ b/Sources/ManualS/Exports.swift
@@ -0,0 +1,2 @@
+@_exported import Dependencies
+@_exported import Models
diff --git a/Sources/ManualS/Extensions/DesignInfo+validator.swift b/Sources/ManualS/Extensions/DesignInfo+validator.swift
new file mode 100644
index 0000000..92ce038
--- /dev/null
+++ b/Sources/ManualS/Extensions/DesignInfo+validator.swift
@@ -0,0 +1,19 @@
+import Models
+import Validations
+
+extension DesignInfo: AsyncValidatable {
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.validate(\.summer)
+  }
+}
+
+extension DesignInfo.Summer: AsyncValidatable {
+
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.outdoorTemperature, 0)
+      AsyncValidator.greaterThan(\.indoorTemperature, 0)
+      AsyncValidator.greaterThan(\.indoorHumidity, 0)
+    }
+  }
+}
diff --git a/Sources/ManualS/Extensions/HeatPumpCapacity+validator.swift b/Sources/ManualS/Extensions/HeatPumpCapacity+validator.swift
new file mode 100644
index 0000000..0cdfeca
--- /dev/null
+++ b/Sources/ManualS/Extensions/HeatPumpCapacity+validator.swift
@@ -0,0 +1,13 @@
+import Models
+import Validations
+
+extension Capacity.HeatPumpHeating: AsyncValidatable {
+
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.at47, 0)
+      AsyncValidator.greaterThan(\.at17, 0)
+      AsyncValidator.greaterThanOrEquals(\.at47, \.at17)
+    }
+  }
+}
diff --git a/Sources/ManualS/Extensions/HouseLoad+validator.swift b/Sources/ManualS/Extensions/HouseLoad+validator.swift
new file mode 100644
index 0000000..4bd21b4
--- /dev/null
+++ b/Sources/ManualS/Extensions/HouseLoad+validator.swift
@@ -0,0 +1,14 @@
+import Models
+import Validations
+
+extension HouseLoad: AsyncValidatable {
+
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.coolingTotal, 0)
+      AsyncValidator.greaterThan(\.coolingSensible, 0)
+      AsyncValidator.greaterThan(\.heating, 0)
+      AsyncValidator.greaterThanOrEquals(\.coolingTotal, \.coolingSensible)
+    }
+  }
+}
diff --git a/Sources/ManualS/Extensions/ManufacturersCoolingCapacity+validator.swift b/Sources/ManualS/Extensions/ManufacturersCoolingCapacity+validator.swift
new file mode 100644
index 0000000..38c1d46
--- /dev/null
+++ b/Sources/ManualS/Extensions/ManufacturersCoolingCapacity+validator.swift
@@ -0,0 +1,34 @@
+import Models
+import Validations
+
+extension Capacity.ManufacturersCooling: AsyncValidatable {
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.airflow, 0)
+      AsyncValidator.validate(\.capacity)
+      AsyncValidator.validate(\.otherCapacity, with: OptionalContainerValidator())
+    }
+  }
+}
+
+extension Capacity.ManufacturersCooling.Container: AsyncValidatable {
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.dryBulbTemperature, 0)
+      AsyncValidator.greaterThan(\.wetBulbTemperature, 0)
+      AsyncValidator.greaterThan(\.totalCapacity, 0)
+      AsyncValidator.greaterThan(\.sensibleCapacity, 0)
+      AsyncValidator.greaterThanOrEquals(\.totalCapacity, \.sensibleCapacity)
+    }
+  }
+}
+
+private struct OptionalContainerValidator: AsyncValidation {
+  typealias Value = Capacity.ManufacturersCooling.Container?
+
+  func validate(_ value: Capacity.ManufacturersCooling.Container?) async throws {
+    guard let value else { return }
+    try await value.validate()
+  }
+
+}
diff --git a/Sources/ManualS/Internal/BalancePoint.swift b/Sources/ManualS/Internal/BalancePoint.swift
new file mode 100644
index 0000000..0984707
--- /dev/null
+++ b/Sources/ManualS/Internal/BalancePoint.swift
@@ -0,0 +1,39 @@
+import CoreFoundation
+import Models
+import Validations
+
+extension BalancePoint.Request {
+
+  func respond() async throws -> BalancePoint.Response {
+    try await validate()
+    let balancePoint = await thermalBalancePoint(
+      heatLoss: Double(heatLoss),
+      at47: Double(heatPumpCapacity.at47),
+      at17: Double(heatPumpCapacity.at17),
+      designTemperature: Double(winterDesignTemperature)
+    )
+    return .init(balancePointTemperature: balancePoint)
+  }
+}
+
+extension BalancePoint.Request: AsyncValidatable {
+
+  @inlinable
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.heatLoss, 0)
+      AsyncValidator.validate(\.heatPumpCapacity)
+    }
+  }
+}
+
+private func thermalBalancePoint(
+  heatLoss: Double,
+  at47: Double,
+  at17: Double,
+  designTemperature: Double
+) async -> Double {
+  (30.0 * (((designTemperature - 65.0) * at47) + (65.0 * heatLoss))
+    - ((designTemperature - 65.0) * (at47 - at17) * 47.0))
+    / ((30.0 * heatLoss) - ((designTemperature - 65.0) * (at47 - at17)))
+}
diff --git a/Sources/ManualS/Internal/Derating.swift b/Sources/ManualS/Internal/Derating.swift
new file mode 100644
index 0000000..1e3d78e
--- /dev/null
+++ b/Sources/ManualS/Internal/Derating.swift
@@ -0,0 +1,101 @@
+import Models
+
+extension Derating.Request {
+  func respond() async throws -> Derating.Response {
+    switch systemType {
+    case .airToAir:
+      return .init(
+        coolingTotal: totalWetDerating(elevation: elevation),
+        coolingSensible: sensibleWetDerating(elevation: elevation),
+        heating: totalDryDerating(elevation: elevation)
+      )
+
+    case let .heatingOnly(type: type):
+      switch type {
+      case .boiler, .furnace:
+        return .init(heating: furnaceDerating(elevation: elevation))
+      case .electric:
+        return .init(heating: 1)
+      }
+    }
+  }
+}
+
+// swiftlint:disable cyclomatic_complexity
+private func furnaceDerating(elevation: Int) -> Double {
+  guard elevation > 0 else { return 1 }
+
+  if (0 ..< 1000).contains(elevation) { return 1 }
+  if (1000 ..< 2000).contains(elevation) { return 0.96 }
+  if (2000 ..< 3000).contains(elevation) { return 0.92 }
+  if (3000 ..< 4000).contains(elevation) { return 0.88 }
+  if (4000 ..< 5000).contains(elevation) { return 0.84 }
+  if (5000 ..< 6000).contains(elevation) { return 0.8 }
+  if (6000 ..< 7000).contains(elevation) { return 0.76 }
+  if (7000 ..< 8000).contains(elevation) { return 0.72 }
+  if (8000 ..< 9000).contains(elevation) { return 0.68 }
+  if (9000 ..< 10000).contains(elevation) { return 0.64 }
+  if (10000 ..< 11000).contains(elevation) { return 0.6 }
+  if (11000 ..< 12000).contains(elevation) { return 0.56 }
+  // greater than 12,000 feet in elevation.
+  return 0.52
+}
+
+private func totalWetDerating(elevation: Int) -> Double {
+  guard elevation > 0 else { return 1 }
+
+  if (0 ..< 1000).contains(elevation) { return 1 }
+  if (1000 ..< 2000).contains(elevation) { return 0.99 }
+  if (2000 ..< 3000).contains(elevation) { return 0.98 }
+  if (3000 ..< 4000).contains(elevation) { return 0.98 }
+  if (4000 ..< 5000).contains(elevation) { return 0.97 }
+  if (5000 ..< 6000).contains(elevation) { return 0.96 }
+  if (6000 ..< 7000).contains(elevation) { return 0.95 }
+  if (7000 ..< 8000).contains(elevation) { return 0.94 }
+  if (8000 ..< 9000).contains(elevation) { return 0.94 }
+  if (9000 ..< 10000).contains(elevation) { return 0.93 }
+  if (10000 ..< 11000).contains(elevation) { return 0.92 }
+  if (11000 ..< 12000).contains(elevation) { return 0.91 }
+  // greater than 12,000 feet in elevation.
+  return 0.9
+}
+
+private func sensibleWetDerating(elevation: Int) -> Double {
+  guard elevation > 0 else { return 1 }
+
+  if (0 ..< 1000).contains(elevation) { return 1 }
+  if (1000 ..< 2000).contains(elevation) { return 0.97 }
+  if (2000 ..< 3000).contains(elevation) { return 0.94 }
+  if (3000 ..< 4000).contains(elevation) { return 0.91 }
+  if (4000 ..< 5000).contains(elevation) { return 0.88 }
+  if (5000 ..< 6000).contains(elevation) { return 0.85 }
+  if (6000 ..< 7000).contains(elevation) { return 0.82 }
+  if (7000 ..< 8000).contains(elevation) { return 0.8 }
+  if (8000 ..< 9000).contains(elevation) { return 0.77 }
+  if (9000 ..< 10000).contains(elevation) { return 0.74 }
+  if (10000 ..< 11000).contains(elevation) { return 0.71 }
+  if (11000 ..< 12000).contains(elevation) { return 0.68 }
+  // greater than 12,000 feet in elevation.
+  return 0.65
+}
+
+private func totalDryDerating(elevation: Int) -> Double {
+  guard elevation > 0 else { return 1 }
+
+  if (0 ..< 1000).contains(elevation) { return 1 }
+  if (1000 ..< 2000).contains(elevation) { return 0.98 }
+  if (2000 ..< 3000).contains(elevation) { return 0.97 }
+  if (3000 ..< 4000).contains(elevation) { return 0.95 }
+  if (4000 ..< 5000).contains(elevation) { return 0.94 }
+  if (5000 ..< 6000).contains(elevation) { return 0.92 }
+  if (6000 ..< 7000).contains(elevation) { return 0.9 }
+  if (7000 ..< 8000).contains(elevation) { return 0.89 }
+  if (8000 ..< 9000).contains(elevation) { return 0.87 }
+  if (9000 ..< 10000).contains(elevation) { return 0.86 }
+  if (10000 ..< 11000).contains(elevation) { return 0.84 }
+  if (11000 ..< 12000).contains(elevation) { return 0.82 }
+  // greater than 12,000 feet in elevation.
+  return 0.81
+}
+
+// swiftlint:enable cyclomatic_complexity
diff --git a/Sources/ManualS/Internal/Interpolate.swift b/Sources/ManualS/Internal/Interpolate.swift
new file mode 100644
index 0000000..43358fe
--- /dev/null
+++ b/Sources/ManualS/Internal/Interpolate.swift
@@ -0,0 +1,104 @@
+import Models
+import Validations
+
+extension Interpolate.Request {
+  func respond() async throws -> Interpolate.Response {
+    try await validate()
+    fatalError()
+  }
+}
+
+// Basic validations of the request.
+extension Interpolate.Request: AsyncValidatable {
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.validate(\.designInfo)
+      AsyncValidator.validate(\.houseLoad)
+      AsyncValidator.validate(\.manufacturersCapacity)
+    }
+  }
+}
+
+private extension Interpolate.Request {
+
+  func parseInterpolationType() throws -> Interpolate.InterpolationType {
+    guard let otherCapacity = manufacturersCapacity.otherCapacity else {
+      let capacity = manufacturersCapacity.capacity
+      guard capacity.wetBulbTemperature == 63 else {
+        throw ValidationError(
+          message: "Expected manufacturers wet bulb temperature to be 63, but found: \(capacity.wetBulbTemperature)"
+        )
+      }
+      return .noInterpolation
+    }
+
+    // check if the
+    fatalError()
+  }
+
+  private func checkOneWayIndoorInterpolation(
+    _ capacity: Capacity.ManufacturersCooling.Container,
+    _ other: Capacity.ManufacturersCooling.Container
+  ) -> CapacityContainer? {
+    // ensure outdoor temperatures match, otherwise we are not a one way indoor interpolation.
+    guard capacity.outdoorTemperature == other.outdoorTemperature else {
+      return nil
+    }
+
+    // ensure indoor temperatures are not the same.
+    guard capacity.dryBulbTemperature != other.dryBulbTemperature else {
+      return nil
+    }
+
+    if capacity.dryBulbTemperature < other.dryBulbTemperature {
+      return .init(above: other, below: capacity)
+    }
+
+    return .init(above: capacity, below: other)
+  }
+
+  private func checkOneWayOutdoorInterpolation(
+    _ capacity: Capacity.ManufacturersCooling.Container,
+    _ other: Capacity.ManufacturersCooling.Container
+  ) -> CapacityContainer? {
+    // ensure outdoor temperatures match, otherwise we are not a one way indoor interpolation.
+    guard capacity.dryBulbTemperature == other.dryBulbTemperature else {
+      return nil
+    }
+
+    // ensure outdoor temperatures are not the same.
+    guard capacity.outdoorTemperature != other.outdoorTemperature else {
+      return nil
+    }
+
+    if capacity.outdoorTemperature < other.outdoorTemperature {
+      return .init(above: other, below: capacity)
+    }
+
+    return .init(above: capacity, below: other)
+  }
+
+  private func checkTwoWayInterpolation(
+    _ capacity: Capacity.ManufacturersCooling.Container,
+    _ other: Capacity.ManufacturersCooling.Container
+  ) -> CapacityContainer? {
+    // ensure outdoor temperatures do not match.
+    guard capacity.outdoorTemperature != other.outdoorTemperature else { return nil }
+    guard capacity.dryBulbTemperature != other.dryBulbTemperature else { return nil }
+
+    return nil
+  }
+}
+
+private struct CapacityContainer {
+  let above: Capacity.ManufacturersCooling.Container
+  let below: Capacity.ManufacturersCooling.Container
+}
+
+public struct ValidationError: Equatable, Error {
+  public let message: String
+
+  public init(message: String) {
+    self.message = message
+  }
+}
diff --git a/Sources/ManualS/Internal/RequiredKW.swift b/Sources/ManualS/Internal/RequiredKW.swift
new file mode 100644
index 0000000..bdfd875
--- /dev/null
+++ b/Sources/ManualS/Internal/RequiredKW.swift
@@ -0,0 +1,24 @@
+import Models
+import Validations
+
+extension RequiredKW.Request {
+  func respond() async throws -> RequiredKW.Response {
+    try await validate()
+    let capacityAtDesign = self.capacityAtDesign ?? 0
+    let requiredKW = (Double(heatLoss) - Double(capacityAtDesign)) / 3413
+    return .init(requiredKW: requiredKW)
+  }
+}
+
+extension RequiredKW.Request: AsyncValidatable {
+
+  @inlinable
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.heatLoss, 0)
+      AsyncValidator.validate(\.capacityAtDesign) {
+        AsyncValidator.greaterThan(0).optional()
+      }
+    }
+  }
+}
diff --git a/Sources/ManualS/Internal/SizingLimits.swift b/Sources/ManualS/Internal/SizingLimits.swift
new file mode 100644
index 0000000..c4cb328
--- /dev/null
+++ b/Sources/ManualS/Internal/SizingLimits.swift
@@ -0,0 +1,79 @@
+import CoreFoundation
+import Models
+
+extension SizingLimits.Request {
+  func respond() async throws -> SizingLimits.Response {
+    return try .init(
+      oversizing: .oversizingLimit(systemType: systemType, houseLoad: houseLoad),
+      undersizing: .undersizingLimits
+    )
+  }
+}
+
+private extension SizingLimits.Limits {
+  static let undersizingLimits = Self(
+    heating: 90,
+    coolingTotal: 90,
+    coolingSensible: 90,
+    coolingLatent: 90
+  )
+
+  static func oversizingLimit(
+    systemType: SystemType,
+    houseLoad: HouseLoad?
+  ) throws -> Self {
+    switch systemType {
+    case let .heatingOnly(type: type):
+      return .init(heating: type.oversizingLimit(), coolingTotal: 115)
+    case let .airToAir(type: _, compressor: compressor, climate: climate):
+      return try .init(
+        heating: 140,
+        coolingTotal: coolingTotalOversizingLimit(
+          houseLoad: houseLoad,
+          compressorType: compressor,
+          climateType: climate
+        ),
+        coolingSensible: nil,
+        coolingLatent: 150
+      )
+    }
+  }
+}
+
+private extension SystemType.HeatingOnlyType {
+
+  func oversizingLimit() -> Int {
+    switch self {
+    case .boiler, .furnace: return 140
+    case .electric: return 175
+    }
+  }
+}
+
+private func coolingTotalOversizingLimit(
+  houseLoad: HouseLoad?,
+  compressorType: SystemType.CompressorType,
+  climateType: SystemType.ClimateType
+) throws -> Int {
+  switch (compressorType, climateType) {
+  case (.singleSpeed, .mildWinterOrLatentLoad):
+    return 115
+  case (.multiSpeed, .mildWinterOrLatentLoad):
+    return 120
+  case (.variableSpeed, .mildWinterOrLatentLoad):
+    return 130
+  default:
+
+    guard let houseLoad else {
+      throw HouseLoadError()
+    }
+    let decimal = Double(houseLoad.coolingTotal + 15000) / Double(houseLoad.coolingTotal)
+    return Int(round(decimal * 100))
+  }
+}
+
+public struct HouseLoadError: Error, Equatable {
+  public let message = "House load not supplied."
+
+  public init() {}
+}
diff --git a/Sources/ManualS/ManualS.swift b/Sources/ManualS/ManualS.swift
new file mode 100644
index 0000000..ef7832c
--- /dev/null
+++ b/Sources/ManualS/ManualS.swift
@@ -0,0 +1,33 @@
+import Dependencies
+import DependenciesMacros
+import Models
+
+public extension DependencyValues {
+  var manualS: ManualS {
+    get { self[ManualS.self] }
+    set { self[ManualS.self] = newValue }
+  }
+}
+
+@DependencyClient
+public struct ManualS: Sendable {
+  public var balancePoint: @Sendable (BalancePoint.Request) async throws -> BalancePoint.Response
+  public var derating: @Sendable (Derating.Request) async throws -> Derating.Response
+  public var interpolate: @Sendable (Interpolate.Request) async throws -> Interpolate.Response
+  public var requiredKW: @Sendable (RequiredKW.Request) async throws -> RequiredKW.Response
+  public var sizingLimits: @Sendable (SizingLimits.Request) async throws -> SizingLimits.Response
+}
+
+extension ManualS: DependencyKey {
+  public static let liveValue = Self(
+    balancePoint: { try await $0.respond() },
+    derating: { try await $0.respond() },
+    interpolate: { try await $0.respond() },
+    requiredKW: { try await $0.respond() },
+    sizingLimits: { try await $0.respond() }
+  )
+}
+
+extension ManualS: TestDependencyKey {
+  public static let testValue: ManualS = Self()
+}
diff --git a/Sources/Models/BalancePoint.swift b/Sources/Models/BalancePoint.swift
new file mode 100644
index 0000000..2990a77
--- /dev/null
+++ b/Sources/Models/BalancePoint.swift
@@ -0,0 +1,24 @@
+public enum BalancePoint {
+
+  public struct Request: Codable, Equatable, Sendable {
+
+    public let winterDesignTemperature: Int
+    public let heatLoss: Int
+    public let heatPumpCapacity: Capacity.HeatPumpHeating
+
+    public init(winterDesignTemperature: Int, heatLoss: Int, heatPumpCapacity: Capacity.HeatPumpHeating) {
+      self.winterDesignTemperature = winterDesignTemperature
+      self.heatLoss = heatLoss
+      self.heatPumpCapacity = heatPumpCapacity
+    }
+  }
+
+  public struct Response: Codable, Equatable, Sendable {
+
+    public let balancePointTemperature: Double
+
+    public init(balancePointTemperature: Double) {
+      self.balancePointTemperature = balancePointTemperature
+    }
+  }
+}
diff --git a/Sources/Models/Core/AdjustmentMultiplier.swift b/Sources/Models/Core/AdjustmentMultiplier.swift
new file mode 100644
index 0000000..78407a2
--- /dev/null
+++ b/Sources/Models/Core/AdjustmentMultiplier.swift
@@ -0,0 +1,12 @@
+public struct AdjustmentMultiplier: Codable, Equatable, Sendable {
+
+  public let coolingTotal: Double?
+  public let coolingSensible: Double?
+  public let heating: Double
+
+  public init(coolingTotal: Double? = nil, coolingSensible: Double? = nil, heating: Double) {
+    self.coolingTotal = coolingTotal
+    self.coolingSensible = coolingSensible
+    self.heating = heating
+  }
+}
diff --git a/Sources/Models/Core/Capacity.swift b/Sources/Models/Core/Capacity.swift
new file mode 100644
index 0000000..412371e
--- /dev/null
+++ b/Sources/Models/Core/Capacity.swift
@@ -0,0 +1,104 @@
+// A container / namespace for different capacity containers.
+public enum Capacity {
+
+  public struct Cooling: Codable, Equatable, Sendable {
+
+    public let total: Int
+    public let sensible: Int
+    public var latent: Int
+
+    public init(
+      total: Int,
+      sensible: Int,
+      latent: Int? = nil
+    ) {
+      self.total = total
+      self.sensible = sensible
+      self.latent = latent ?? total - sensible
+    }
+  }
+
+  public struct Heating: Codable, Equatable, Sendable {
+    public let total: Int
+
+    public init(total: Int) {
+      self.total = total
+    }
+  }
+
+  public struct HeatPumpHeating: Codable, Equatable, Sendable {
+
+    public let at47: Int
+    public let at17: Int
+
+    public init(at47: Int, at17: Int) {
+      self.at47 = at47
+      self.at17 = at17
+    }
+  }
+
+  public struct ManufacturersCooling: Codable, Equatable, Sendable {
+
+    public let airflow: Int
+    public let capacity: Container
+    public let otherCapacity: Container?
+
+    public init(
+      airflow: Int,
+      capacity: Capacity.ManufacturersCooling.Container,
+      otherCapacity: Capacity.ManufacturersCooling.Container? = nil
+    ) {
+      self.airflow = airflow
+      self.capacity = capacity
+      self.otherCapacity = otherCapacity
+    }
+
+    public struct Container: Codable, Equatable, Sendable {
+
+      public let dryBulbTemperature: Int
+      public let wetBulbTemperature: Int
+      public let outdoorTemperature: Int
+      public let totalCapacity: Int
+      public let sensibleCapacity: Int
+      public let adjustmentMultipliers: AdjustmentMultiplier?
+
+      public var latentCapacity: Int { totalCapacity - sensibleCapacity }
+
+      public init(
+        dryBulbTemperature: Int,
+        wetBulbTemperature: Int,
+        outdoorTemperature: Int,
+        totalCapacity: Int,
+        sensibleCapacity: Int,
+        adjustmentMultipliers: AdjustmentMultiplier? = nil
+      ) {
+        self.dryBulbTemperature = dryBulbTemperature
+        self.wetBulbTemperature = wetBulbTemperature
+        self.outdoorTemperature = outdoorTemperature
+        self.totalCapacity = totalCapacity
+        self.sensibleCapacity = sensibleCapacity
+        self.adjustmentMultipliers = adjustmentMultipliers
+      }
+    }
+  }
+
+  public struct ManufacturersContainer: Codable, Equatable, Sendable {
+
+    public let wetBulb: Int
+    public let totalCapacity: Int
+    public let sensibleCapacity: Int
+    public let adjustmentMultipliers: AdjustmentMultiplier?
+
+    public init(
+      wetBulb: Int,
+      totalCapacity: Int,
+      sensibleCapacity: Int,
+      adjustmentMultipliers: AdjustmentMultiplier? = nil
+    ) {
+      self.wetBulb = wetBulb
+      self.totalCapacity = totalCapacity
+      self.sensibleCapacity = sensibleCapacity
+      self.adjustmentMultipliers = adjustmentMultipliers
+    }
+  }
+}
diff --git a/Sources/Models/Core/DesignInfo.swift b/Sources/Models/Core/DesignInfo.swift
new file mode 100644
index 0000000..e2ab4ae
--- /dev/null
+++ b/Sources/Models/Core/DesignInfo.swift
@@ -0,0 +1,45 @@
+import Foundation
+
+public struct DesignInfo: Codable, Equatable, Sendable {
+
+  public let summer: Summer
+  public let winter: Winter
+  public let elevation: Int
+
+  public init(
+    summer: DesignInfo.Summer = .init(),
+    winter: DesignInfo.Winter = .init(),
+    elevation: Int = 0
+  ) {
+    self.summer = summer
+    self.winter = winter
+    self.elevation = elevation
+  }
+}
+
+public extension DesignInfo {
+  struct Summer: Codable, Equatable, Sendable {
+
+    public let outdoorTemperature: Int
+    public let indoorTemperature: Int
+    public let indoorHumidity: Int
+
+    public init(
+      outdoorTemperature: Int = 90,
+      indoorTemperature: Int = 75,
+      indoorHumidity: Int = 50
+    ) {
+      self.outdoorTemperature = outdoorTemperature
+      self.indoorTemperature = indoorTemperature
+      self.indoorHumidity = indoorHumidity
+    }
+  }
+
+  struct Winter: Codable, Equatable, Sendable {
+    public let outdoorTemperature: Int
+
+    public init(outdoorTemperature: Int = 5) {
+      self.outdoorTemperature = outdoorTemperature
+    }
+  }
+}
diff --git a/Sources/Models/Core/HouseLoad.swift b/Sources/Models/Core/HouseLoad.swift
new file mode 100644
index 0000000..05093f7
--- /dev/null
+++ b/Sources/Models/Core/HouseLoad.swift
@@ -0,0 +1,18 @@
+public struct HouseLoad: Codable, Equatable, Sendable {
+
+  public let coolingTotal: Int
+  public let coolingSensible: Int
+  public let heating: Int
+
+  public var coolingLatent: Int { coolingTotal - coolingSensible }
+
+  public init(
+    coolingTotal: Int,
+    coolingSensible: Int,
+    heating: Int
+  ) {
+    self.coolingTotal = coolingTotal
+    self.coolingSensible = coolingSensible
+    self.heating = heating
+  }
+}
diff --git a/Sources/Models/Core/SystemType.swift b/Sources/Models/Core/SystemType.swift
new file mode 100644
index 0000000..bc0edfa
--- /dev/null
+++ b/Sources/Models/Core/SystemType.swift
@@ -0,0 +1,27 @@
+public enum SystemType: Codable, Equatable, Sendable {
+
+  case airToAir(type: EquipmentType, compressor: CompressorType, climate: ClimateType)
+  case heatingOnly(type: HeatingOnlyType)
+
+  public enum ClimateType: String, CaseIterable, Codable, Equatable, Sendable {
+    case mildWinterOrLatentLoad
+    case coldWinterOrNoLatentLoad
+  }
+
+  public enum CompressorType: String, CaseIterable, Codable, Equatable, Sendable {
+    case singleSpeed
+    case multiSpeed
+    case variableSpeed
+  }
+
+  public enum EquipmentType: String, CaseIterable, Codable, Equatable, Sendable {
+    case airConditioner
+    case heatPump
+  }
+
+  public enum HeatingOnlyType: String, CaseIterable, Codable, Equatable, Sendable {
+    case boiler
+    case electric
+    case furnace
+  }
+}
diff --git a/Sources/Models/Derating.swift b/Sources/Models/Derating.swift
new file mode 100644
index 0000000..2cf11f9
--- /dev/null
+++ b/Sources/Models/Derating.swift
@@ -0,0 +1,15 @@
+public enum Derating {
+
+  public struct Request: Codable, Equatable, Sendable {
+
+    public let elevation: Int
+    public let systemType: SystemType
+
+    public init(elevation: Int, systemType: SystemType) {
+      self.elevation = elevation
+      self.systemType = systemType
+    }
+  }
+
+  public typealias Response = AdjustmentMultiplier
+}
diff --git a/Sources/Models/Interpolate.swift b/Sources/Models/Interpolate.swift
new file mode 100644
index 0000000..5b2b020
--- /dev/null
+++ b/Sources/Models/Interpolate.swift
@@ -0,0 +1,148 @@
+public enum Interpolate {
+
+  public struct Request: Codable, Equatable, Sendable {
+
+    public let designInfo: DesignInfo
+    public let houseLoad: HouseLoad
+    public let systemType: SystemType
+    public let manufacturersCapacity: Capacity.ManufacturersCooling
+
+    public init(
+      designInfo: DesignInfo,
+      houseLoad: HouseLoad,
+      systemType: SystemType,
+      manufacturersCapacity: Capacity.ManufacturersCooling
+    ) {
+      self.designInfo = designInfo
+      self.houseLoad = houseLoad
+      self.systemType = systemType
+      self.manufacturersCapacity = manufacturersCapacity
+    }
+  }
+
+  public struct Response: Codable, Equatable, Sendable {
+
+    public let failed: Bool
+    public let failures: [String]?
+    public let interpolationType: InterpolationType
+    public let interpolatedCapacity: Capacity.Cooling
+    public let excessLatent: Int
+    public let finalCapacityAtDesign: Capacity.Cooling
+    public let altitudeDerating: AdjustmentMultiplier?
+    public let capacityAsPercentOfLoad: Capacity.Cooling
+    public let sizingLimits: SizingLimits.Response
+
+    public init(
+      failures: [String]? = nil,
+      interpolationType: InterpolationType,
+      interpolatedCapacity: Capacity.Cooling,
+      excessLatent: Int,
+      finalCapacityAtDesign: Capacity.Cooling,
+      altitudeDerating: AdjustmentMultiplier? = nil,
+      capacityAsPercentOfLoad: Capacity.Cooling,
+      sizingLimits: SizingLimits.Response
+    ) {
+      self.failed = failures != nil ? failures!.count > 0 : false
+      self.failures = failures
+      self.interpolationType = interpolationType
+      self.interpolatedCapacity = interpolatedCapacity
+      self.excessLatent = excessLatent
+      self.finalCapacityAtDesign = finalCapacityAtDesign
+      self.altitudeDerating = altitudeDerating
+      self.capacityAsPercentOfLoad = capacityAsPercentOfLoad
+      self.sizingLimits = sizingLimits
+    }
+  }
+
+  public enum InterpolationType2: Codable, Equatable, Sendable {
+    case noInterpolation(Capacity.ManufacturersCooling)
+    case oneWayIndoor(OneWayIndoor)
+    case oneWayOutdoor(OneWayOutdoor)
+  }
+
+  public struct OneWayIndoor: Codable, Equatable, Sendable {
+
+    public let airflow: Int
+    public let outdoorTemperature: Int
+    public let capacities: Capacities
+    public let adjustmentMultipliers: AdjustmentMultiplier?
+
+    public init(
+      airflow: Int,
+      outdoorTemperature: Int,
+      capacities: Interpolate.OneWayIndoor.Capacities,
+      adjustmentMultipliers: AdjustmentMultiplier? = nil
+    ) {
+      self.airflow = airflow
+      self.outdoorTemperature = outdoorTemperature
+      self.capacities = capacities
+      self.adjustmentMultipliers = adjustmentMultipliers
+    }
+
+    public struct Capacities: Codable, Equatable, Sendable {
+
+      public let aboveDewpoint: Capacity.ManufacturersContainer
+      public let belowDewpoint: Capacity.ManufacturersContainer
+
+      public init(aboveDewpoint: Capacity.ManufacturersContainer, belowDewpoint: Capacity.ManufacturersContainer) {
+        self.aboveDewpoint = aboveDewpoint
+        self.belowDewpoint = belowDewpoint
+      }
+    }
+  }
+
+  public struct OneWayOutdoor: Codable, Equatable, Sendable {
+
+    public let airflow: Int
+    public let wetBulb: Int
+    public let capacities: Capacities
+    public let adjustmentMultipliers: AdjustmentMultiplier?
+
+    public init(
+      airflow: Int,
+      wetBulb: Int,
+      capacities: Interpolate.OneWayOutdoor.Capacities,
+      adjustmentMultipliers: AdjustmentMultiplier? = nil
+    ) {
+      self.airflow = airflow
+      self.wetBulb = wetBulb
+      self.capacities = capacities
+      self.adjustmentMultipliers = adjustmentMultipliers
+    }
+
+    public struct Capacities: Codable, Equatable, Sendable {
+
+      public let aboveOutdoor: Capacity
+      public let belowOutdoor: Capacity
+
+      public init(
+        aboveOutdoor: Interpolate.OneWayOutdoor.Capacities.Capacity,
+        belowOutdoor: Interpolate.OneWayOutdoor.Capacities.Capacity
+      ) {
+        self.aboveOutdoor = aboveOutdoor
+        self.belowOutdoor = belowOutdoor
+      }
+
+      public struct Capacity: Codable, Equatable, Sendable {
+
+        public let outdoorTemperature: Int
+        public let totalCapacity: Int
+        public let sensibleCapacity: Int
+
+        public init(outdoorTemperature: Int, totalCapacity: Int, sensibleCapacity: Int) {
+          self.outdoorTemperature = outdoorTemperature
+          self.totalCapacity = totalCapacity
+          self.sensibleCapacity = sensibleCapacity
+        }
+      }
+    }
+
+  }
+
+  public enum InterpolationType: String, CaseIterable, Codable, Equatable, Sendable {
+    case noInterpolation
+    case oneWayIndoor
+    case oneWayOutdoor
+    case twoWay
+  }
+}
diff --git a/Sources/Models/RequiredKW.swift b/Sources/Models/RequiredKW.swift
new file mode 100644
index 0000000..77651e3
--- /dev/null
+++ b/Sources/Models/RequiredKW.swift
@@ -0,0 +1,22 @@
+public enum RequiredKW {
+
+  public struct Request: Codable, Equatable, Sendable {
+
+    public let capacityAtDesign: Int?
+    public let heatLoss: Int
+
+    public init(capacityAtDesign: Int? = nil, heatLoss: Int) {
+      self.capacityAtDesign = capacityAtDesign
+      self.heatLoss = heatLoss
+    }
+  }
+
+  public struct Response: Codable, Equatable, Sendable {
+
+    public let requiredKW: Double
+
+    public init(requiredKW: Double) {
+      self.requiredKW = requiredKW
+    }
+  }
+}
diff --git a/Sources/Models/SizingLimits.swift b/Sources/Models/SizingLimits.swift
new file mode 100644
index 0000000..b4a0f41
--- /dev/null
+++ b/Sources/Models/SizingLimits.swift
@@ -0,0 +1,44 @@
+public enum SizingLimits {
+
+  public struct Request: Codable, Equatable, Sendable {
+
+    public let systemType: SystemType
+    public let houseLoad: HouseLoad?
+
+    public init(systemType: SystemType, houseLoad: HouseLoad? = nil) {
+      self.systemType = systemType
+      self.houseLoad = houseLoad
+    }
+  }
+
+  public struct Response: Codable, Equatable, Sendable {
+
+    public let oversizing: SizingLimits.Limits
+    public let undersizing: SizingLimits.Limits
+
+    public init(oversizing: SizingLimits.Limits, undersizing: SizingLimits.Limits) {
+      self.oversizing = oversizing
+      self.undersizing = undersizing
+    }
+  }
+
+  public struct Limits: Codable, Equatable, Sendable {
+
+    public let heating: Int
+    public let coolingTotal: Int
+    public let coolingSensible: Int?
+    public let coolingLatent: Int?
+
+    public init(
+      heating: Int,
+      coolingTotal: Int,
+      coolingSensible: Int? = nil,
+      coolingLatent: Int? = nil
+    ) {
+      self.heating = heating
+      self.coolingTotal = coolingTotal
+      self.coolingSensible = coolingSensible
+      self.coolingLatent = coolingLatent
+    }
+  }
+}
diff --git a/Tests/ManualSTests/ManualSTests.swift b/Tests/ManualSTests/ManualSTests.swift
new file mode 100644
index 0000000..5b8ca9e
--- /dev/null
+++ b/Tests/ManualSTests/ManualSTests.swift
@@ -0,0 +1,213 @@
+import CoreFoundation
+import Dependencies
+import ManualS
+import Testing
+
+@Suite("ManualSTests")
+struct ManualSTests {
+
+  @Test
+  func balancePoint() async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+      let balancePoint = try await manualS.balancePoint(.init(
+        winterDesignTemperature: 5,
+        heatLoss: 49667,
+        heatPumpCapacity: .init(at47: 24600, at17: 15100)
+      ))
+      let rounded = round(balancePoint.balancePointTemperature * 10) / 10
+      #expect(rounded == 38.5)
+    }
+  }
+
+  @Test(
+    arguments: SystemType.makeAirToAirTestCases(climate: .mildWinterOrLatentLoad)
+  )
+  func mildWinterSizingLimits(system: SystemType) async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let limits = try await manualS.sizingLimits(.init(systemType: system))
+
+      #expect(limits.oversizing.coolingLatent == 150)
+      #expect(limits.oversizing.coolingTotal == system.compressorType!.mildWinterTotalLimit)
+      #expect(limits.undersizing.coolingTotal == 90)
+      #expect(limits.undersizing.coolingSensible == 90)
+      #expect(limits.undersizing.coolingLatent == 90)
+    }
+  }
+
+  @Test(
+    arguments: SystemType.makeAirToAirTestCases(climate: .coldWinterOrNoLatentLoad)
+  )
+  func coldWinterSizingLimits(system: SystemType) async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let limits = try await manualS.sizingLimits(.init(
+        systemType: system,
+        houseLoad: .init(coolingTotal: 17872, coolingSensible: 13894, heating: 49667)
+      ))
+
+      #expect(limits.oversizing.coolingLatent == 150)
+      #expect(limits.oversizing.coolingTotal == 184)
+      #expect(limits.undersizing.coolingTotal == 90)
+      #expect(limits.undersizing.coolingSensible == 90)
+      #expect(limits.undersizing.coolingLatent == 90)
+
+      await #expect(throws: HouseLoadError()) {
+        try await manualS.sizingLimits(.init(systemType: system))
+      }
+    }
+  }
+
+  @Test(
+    arguments: SystemType.HeatingOnlyType.allCases
+  )
+  func heatingOnlySizingLimits(heatingType: SystemType.HeatingOnlyType) async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let limits = try await manualS.sizingLimits(.init(
+        systemType: .heatingOnly(type: heatingType),
+      ))
+
+      #expect(limits.oversizing.heating == heatingType.oversizingLimit)
+      #expect(limits.undersizing.coolingTotal == 90)
+      #expect(limits.undersizing.heating == 90)
+      #expect(limits.undersizing.coolingSensible == 90)
+      #expect(limits.undersizing.coolingLatent == 90)
+    }
+  }
+
+  @Test(
+    arguments: [
+      (RequiredKW.Request(heatLoss: 49667), 14.55),
+      (RequiredKW.Request(capacityAtDesign: 11300, heatLoss: 49667), 11.24)
+    ]
+  )
+  func requiredKW(request: RequiredKW.Request, expected: Double) async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let requiredKW = try await manualS.requiredKW(request)
+      let rounded = round(requiredKW.requiredKW * 100) / 100
+      #expect(rounded == expected)
+    }
+  }
+
+  @Test(
+    arguments: [
+      (elevation: 0, expected: 1.0),
+      (elevation: 1000, expected: 0.96),
+      (elevation: 2000, expected: 0.92),
+      (elevation: 3000, expected: 0.88),
+      (elevation: 4000, expected: 0.84),
+      (elevation: 5000, expected: 0.8),
+      (elevation: 6000, expected: 0.76),
+      (elevation: 7000, expected: 0.72),
+      (elevation: 8000, expected: 0.68),
+      (elevation: 9000, expected: 0.64),
+      (elevation: 10000, expected: 0.6),
+      (elevation: 11000, expected: 0.56),
+      (elevation: 12000, expected: 0.52),
+      (elevation: 13000, expected: 0.52)
+    ]
+  )
+  func heatingDerating(elevation: Int, expected: Double) async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      for heatingType in [SystemType.HeatingOnlyType.boiler, .furnace] {
+        let derating = try await manualS.derating(.init(
+          elevation: elevation,
+          systemType: .heatingOnly(type: heatingType)
+        ))
+        #expect(derating.heating == expected)
+      }
+    }
+  }
+
+  @Test(
+    arguments: [
+      (elevation: 0, expected: AdjustmentMultiplier(coolingTotal: 1, coolingSensible: 1, heating: 1)),
+      (elevation: 1000, expected: AdjustmentMultiplier(coolingTotal: 0.99, coolingSensible: 0.97, heating: 0.98)),
+      (elevation: 2000, expected: AdjustmentMultiplier(coolingTotal: 0.98, coolingSensible: 0.94, heating: 0.97)),
+      (elevation: 3000, expected: AdjustmentMultiplier(coolingTotal: 0.98, coolingSensible: 0.91, heating: 0.95)),
+      (elevation: 4000, expected: AdjustmentMultiplier(coolingTotal: 0.97, coolingSensible: 0.88, heating: 0.94)),
+      (elevation: 5000, expected: AdjustmentMultiplier(coolingTotal: 0.96, coolingSensible: 0.85, heating: 0.92)),
+      (elevation: 6000, expected: AdjustmentMultiplier(coolingTotal: 0.95, coolingSensible: 0.82, heating: 0.9)),
+      (elevation: 7000, expected: AdjustmentMultiplier(coolingTotal: 0.94, coolingSensible: 0.8, heating: 0.89)),
+      (elevation: 8000, expected: AdjustmentMultiplier(coolingTotal: 0.94, coolingSensible: 0.77, heating: 0.87)),
+      (elevation: 9000, expected: AdjustmentMultiplier(coolingTotal: 0.93, coolingSensible: 0.74, heating: 0.86)),
+      (elevation: 10000, expected: AdjustmentMultiplier(coolingTotal: 0.92, coolingSensible: 0.71, heating: 0.84)),
+      (elevation: 11000, expected: AdjustmentMultiplier(coolingTotal: 0.91, coolingSensible: 0.68, heating: 0.82)),
+      (elevation: 12000, expected: AdjustmentMultiplier(coolingTotal: 0.9, coolingSensible: 0.65, heating: 0.81)),
+      (elevation: 13000, expected: AdjustmentMultiplier(coolingTotal: 0.9, coolingSensible: 0.65, heating: 0.81))
+    ]
+  )
+  func airToAirDerating(elevation: Int, expected: AdjustmentMultiplier) async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let derating = try await manualS.derating(.init(
+        elevation: elevation,
+        systemType: .airToAir(type: .airConditioner, compressor: .singleSpeed, climate: .mildWinterOrLatentLoad)
+      ))
+      #expect(derating == expected)
+    }
+  }
+}
+
+extension SystemType {
+
+  static func makeAirToAirTestCases(climate: SystemType.ClimateType) -> [Self] {
+    var items: [Self] = []
+    for compressor in SystemType.CompressorType.allCases {
+      for equipment in SystemType.EquipmentType.allCases {
+        items.append(.airToAir(type: equipment, compressor: compressor, climate: climate))
+      }
+    }
+    return items
+  }
+
+  var compressorType: SystemType.CompressorType? {
+    switch self {
+    case let .airToAir(type: _, compressor: compressor, climate: _): return compressor
+    case .heatingOnly: return nil
+    }
+  }
+}
+
+extension SystemType.CompressorType {
+  var mildWinterTotalLimit: Int {
+    switch self {
+    case .singleSpeed: return 115
+    case .multiSpeed: return 120
+    case .variableSpeed: return 130
+    }
+  }
+}
+
+extension SystemType.HeatingOnlyType {
+  var oversizingLimit: Int {
+    switch self {
+    case .boiler, .furnace: return 140
+    case .electric: return 175
+    }
+  }
+}