diff --git a/Sources/ManualS/Extensions/NormalizePercentage.swift b/Sources/ManualS/Extensions/NormalizePercentage.swift
new file mode 100644
index 0000000..a431f22
--- /dev/null
+++ b/Sources/ManualS/Extensions/NormalizePercentage.swift
@@ -0,0 +1,9 @@
+import CoreFoundation
+
+func normalizePercentage(
+  _ lhs: Int,
+  _ rhs: Int
+) -> Int {
+  let value = Double(lhs) / Double(rhs)
+  return Int((value * 1000).rounded() / 10.0)
+}
diff --git a/Sources/ManualS/Internal/CoolingInterpolation.swift b/Sources/ManualS/Internal/CoolingInterpolation.swift
index ae0ca2b..c911c16 100644
--- a/Sources/ManualS/Internal/CoolingInterpolation.swift
+++ b/Sources/ManualS/Internal/CoolingInterpolation.swift
@@ -129,14 +129,6 @@ private extension SizingLimits.Response {
   }
 }
 
-private func normalizePercentage(
-  _ lhs: Int,
-  _ rhs: Int
-) -> Int {
-  let value = Double(lhs) / Double(rhs)
-  return Int((value * 1000).rounded() / 10.0)
-}
-
 private extension CoolingInterpolation.InterpolationRequest {
 
   func interpolatedCapacity(outdoorDesignTemperature: Int) async -> Capacity.Cooling {
diff --git a/Sources/ManualS/Internal/FurnaceInterpolation.swift b/Sources/ManualS/Internal/FurnaceInterpolation.swift
new file mode 100644
index 0000000..6b8e0c6
--- /dev/null
+++ b/Sources/ManualS/Internal/FurnaceInterpolation.swift
@@ -0,0 +1,55 @@
+import Models
+import Validations
+
+extension FurnaceInterpolation.Request {
+  private static let undersizingLimit = 90
+  private static let oversizingLimit = 140
+
+  func respond() async throws -> FurnaceInterpolation.Response {
+    try await validate()
+    let altitudeDerating = try await altitudeDerating()
+    let outputCapacity = Int(
+      (Double(inputRating) * (altitudeDerating ?? 1.0))
+        * (afue / 100)
+    )
+    let capacityAsPercentOfLoad = normalizePercentage(outputCapacity, heatLoss)
+
+    var failures = [String]()
+    if capacityAsPercentOfLoad < Self.undersizingLimit {
+      failures.append("Undersizing failure.")
+    }
+    if capacityAsPercentOfLoad > Self.oversizingLimit {
+      failures.append("Oversizing failure.")
+    }
+
+    return .init(
+      failures: failures.isEmpty ? nil : failures,
+      altitudeAdjustmentMultiplier: altitudeDerating,
+      capacityAsPercentOfLoad: capacityAsPercentOfLoad,
+      oversizingLimit: Self.oversizingLimit,
+      undersizingLimit: Self.undersizingLimit,
+      outputCapacity: outputCapacity
+    )
+  }
+
+  func altitudeDerating() async throws -> Double? {
+    guard let elevation else { return nil }
+    let response = try await Derating.Request(
+      elevation: elevation,
+      systemType: .heatingOnly(type: .furnace)
+    ).respond()
+    return response.heating
+  }
+
+}
+
+extension FurnaceInterpolation.Request: AsyncValidatable {
+  public var body: some AsyncValidation<Self> {
+    AsyncValidator.accumulating {
+      AsyncValidator.greaterThan(\.heatLoss, 0)
+      AsyncValidator.greaterThan(\.inputRating, 0)
+      AsyncValidator.greaterThan(\.afue, 0)
+      AsyncValidator.lessThan(\.afue, 100)
+    }
+  }
+}
diff --git a/Sources/ManualS/ManualS.swift b/Sources/ManualS/ManualS.swift
index 69f2d5a..77d4243 100644
--- a/Sources/ManualS/ManualS.swift
+++ b/Sources/ManualS/ManualS.swift
@@ -12,13 +12,20 @@ public extension DependencyValues {
 @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 coolingInterpolation:
     @Sendable (CoolingInterpolation.Request) async throws -> CoolingInterpolation.Response
+
+  public var furnaceInterpolation:
+    @Sendable (FurnaceInterpolation.Request) async throws -> FurnaceInterpolation.Response
+
   public var heatPumpHeatingInterpolation:
     @Sendable (HeatPumpHeatingInterpolation.Request) async throws -> HeatPumpHeatingInterpolation.Response
 
   public var requiredKW: @Sendable (RequiredKW.Request) async throws -> RequiredKW.Response
+
   public var sizingLimits: @Sendable (SizingLimits.Request) async throws -> SizingLimits.Response
 }
 
@@ -27,6 +34,7 @@ extension ManualS: DependencyKey {
     balancePoint: { try await $0.respond() },
     derating: { try await $0.respond() },
     coolingInterpolation: { try await $0.respond() },
+    furnaceInterpolation: { try await $0.respond() },
     heatPumpHeatingInterpolation: { try await $0.respond() },
     requiredKW: { try await $0.respond() },
     sizingLimits: { try await $0.respond() }
diff --git a/Sources/Models/FurnaceInterpolation.swift b/Sources/Models/FurnaceInterpolation.swift
new file mode 100644
index 0000000..1aee7d9
--- /dev/null
+++ b/Sources/Models/FurnaceInterpolation.swift
@@ -0,0 +1,53 @@
+public enum FurnaceInterpolation {
+
+  public struct Request: Codable, Equatable, Sendable {
+
+    public let elevation: Int?
+    public let winterDesignTemperature: Int
+    public let heatLoss: Int
+    public let inputRating: Int
+    public let afue: Double
+
+    public init(
+      elevation: Int? = nil,
+      winterDesignTemperature: Int,
+      heatLoss: Int,
+      inputRating: Int,
+      afue: Double
+    ) {
+      self.elevation = elevation
+      self.winterDesignTemperature = winterDesignTemperature
+      self.heatLoss = heatLoss
+      self.inputRating = inputRating
+      self.afue = afue
+    }
+  }
+
+  public struct Response: Codable, Equatable, Sendable {
+
+    public let failed: Bool
+    public let failures: [String]?
+    public let altitudeAdjustmentMultiplier: Double?
+    public let capacityAsPercentOfLoad: Int
+    public let oversizingLimit: Int
+    public let undersizingLimit: Int
+    public let outputCapacity: Int
+
+    public init(
+      failures: [String]? = nil,
+      altitudeAdjustmentMultiplier: Double? = nil,
+      capacityAsPercentOfLoad: Int,
+      oversizingLimit: Int,
+      undersizingLimit: Int,
+      outputCapacity: Int
+    ) {
+      self.failed = failures == nil ? false : !failures!.isEmpty
+      self.failures = failures
+      self.altitudeAdjustmentMultiplier = altitudeAdjustmentMultiplier
+      self.capacityAsPercentOfLoad = capacityAsPercentOfLoad
+      self.oversizingLimit = oversizingLimit
+      self.undersizingLimit = undersizingLimit
+      self.outputCapacity = outputCapacity
+    }
+  }
+}
diff --git a/Tests/ManualSTests/ManualSTests.swift b/Tests/ManualSTests/ManualSTests.swift
index fa1a564..3a1200b 100644
--- a/Tests/ManualSTests/ManualSTests.swift
+++ b/Tests/ManualSTests/ManualSTests.swift
@@ -159,6 +159,51 @@ struct ManualSTests {
     }
   }
 
+  @Test
+  func furnaceInterpolation() async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let response = try await manualS.furnaceInterpolation(
+        .init(
+          elevation: nil,
+          winterDesignTemperature: 5,
+          heatLoss: 49667,
+          inputRating: 60000,
+          afue: 96
+        )
+      )
+
+      #expect(response.failed == false)
+      #expect(response.capacityAsPercentOfLoad == 116)
+      #expect(response.outputCapacity == 57600)
+    }
+  }
+
+  @Test
+  func furnaceInterpolation_with_oversizedFurnace() async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let response = try await manualS.furnaceInterpolation(
+        .init(
+          elevation: nil,
+          winterDesignTemperature: 5,
+          heatLoss: 49667,
+          inputRating: 160_000,
+          afue: 96
+        )
+      )
+
+      #expect(response.failed)
+      #expect(response.failures == ["Oversizing failure."])
+    }
+  }
+
   @Test
   func balancePoint() async throws {
     try await withDependencies {