diff --git a/routee/transit/thermal_energy.py b/routee/transit/thermal_energy.py
index 66d0968..9d252a5 100644
--- a/routee/transit/thermal_energy.py
+++ b/routee/transit/thermal_energy.py
@@ -213,7 +213,7 @@ def add_HVAC_energy(
         Trips on selected date and route, including deadhead trips.
     output_dir : Path or None
         Directory used to store downloaded TMY weather files (in a ``TMY/``
-        subdirectory). Must not be None.
+        subdirectory). If None, defaults to ``~/cache/routee-transit/TMY``.
 
     Returns
     -------
@@ -225,7 +225,7 @@ def add_HVAC_energy(
     if output_dir is not None:
         tmy_dir = output_dir / "TMY"
     else:
-        raise Exception("Must specify output_dir if downloading TMY data")
+        tmy_dir = Path.home() / "cache" / "routee-transit" / "TMY"
 
     # Based on gtfs stops data, get counties served
     df_stops = feed.stops
diff --git a/tests/test_thermal_energy.py b/tests/test_thermal_energy.py
index 3aa478a..901dbe6 100644
--- a/tests/test_thermal_energy.py
+++ b/tests/test_thermal_energy.py
@@ -36,21 +36,13 @@ def test_compute_HVAC_energy(self) -> None:
         # np.trapezoid on [0, ..., 0.99] with constant 10 gives 9.9.
         self.assertAlmostEqual(energy[0], 9.9, places=1)
 
-    @patch("routee.transit.thermal_energy.fetch_counties_gdf")
-    @patch("routee.transit.thermal_energy.download_tmy_files")
-    @patch("routee.transit.thermal_energy.get_hourly_temperature")
-    def test_add_HVAC_energy(
+    def _make_mock_feed_and_trips(
         self,
-        mock_get_hourly: MagicMock,
-        mock_download: MagicMock,
-        mock_fetch_counties: MagicMock,
-    ) -> None:
-        # Setup mock Feed
+    ) -> tuple[MagicMock, pd.DataFrame, gpd.GeoDataFrame, pd.DataFrame]:
         mock_feed = MagicMock()
         mock_feed.stops = pd.DataFrame(
             {"stop_id": ["S1"], "stop_lat": [40.0], "stop_lon": [-105.0]}
         )
-        # Mock stop_times for HVAC calculation (integration needs start/end)
         mock_feed.stop_times = pd.DataFrame(
             {
                 "trip_id": ["T1", "T1"],
@@ -58,26 +50,34 @@ def test_add_HVAC_energy(
                 "stop_id": ["S1", "S1"],
             }
         )
-
-        # Setup mock trips
         trips_df = pd.DataFrame({"trip_id": ["T1"]})
-
-        # Mock dependencies
         mock_county_gdf = gpd.GeoDataFrame(
             {
-                "county_id": ["G0800130"],  # Example FIPS
+                "county_id": ["G0800130"],
                 "STATEFP": ["08"],
                 "COUNTYFP": ["013"],
                 "geometry": [Point(-105.0, 40.0).buffer(1.0)],
             },
             crs="EPSG:4269",
         )
-        mock_fetch_counties.return_value = mock_county_gdf
-
-        # Mock hourly temperature and power mapping
         mock_hourly_temp = pd.DataFrame(
             {"hour": list(range(24)), "Dry Bulb Temperature [°C]": [20.0] * 24}
         )
+        return mock_feed, trips_df, mock_county_gdf, mock_hourly_temp
+
+    @patch("routee.transit.thermal_energy.fetch_counties_gdf")
+    @patch("routee.transit.thermal_energy.download_tmy_files")
+    @patch("routee.transit.thermal_energy.get_hourly_temperature")
+    def test_add_HVAC_energy(
+        self,
+        mock_get_hourly: MagicMock,
+        mock_download: MagicMock,
+        mock_fetch_counties: MagicMock,
+    ) -> None:
+        mock_feed, trips_df, mock_county_gdf, mock_hourly_temp = (
+            self._make_mock_feed_and_trips()
+        )
+        mock_fetch_counties.return_value = mock_county_gdf
         mock_get_hourly.return_value = mock_hourly_temp
 
         # use a temp directory for output
@@ -90,6 +90,29 @@ def test_add_HVAC_energy(
         # Should have results for 3 scenarios: summer, winter, median
         self.assertEqual(len(result), 3)
 
+    @patch("routee.transit.thermal_energy.fetch_counties_gdf")
+    @patch("routee.transit.thermal_energy.download_tmy_files")
+    @patch("routee.transit.thermal_energy.get_hourly_temperature")
+    def test_add_HVAC_energy_no_output_dir(
+        self,
+        mock_get_hourly: MagicMock,
+        mock_download: MagicMock,
+        mock_fetch_counties: MagicMock,
+    ) -> None:
+        """add_HVAC_energy should work without output_dir using a default cache path."""
+        mock_feed, trips_df, mock_county_gdf, mock_hourly_temp = (
+            self._make_mock_feed_and_trips()
+        )
+        mock_fetch_counties.return_value = mock_county_gdf
+        mock_get_hourly.return_value = mock_hourly_temp
+
+        # Call without specifying output_dir (previously raised an exception)
+        result = add_HVAC_energy(mock_feed, trips_df, output_dir=None)
+
+        self.assertIn("hvac_energy_kWh", result.columns)
+        self.assertIn("scenario", result.columns)
+        self.assertEqual(len(result), 3)
+
 
 if __name__ == "__main__":
     unittest.main()