fix(elmhurst): raise on unmapped fuel-fired secondary room-heater code

The Elmhurst Summary lodges only the secondary heating SAP code (Table 4a
Category 10), never its fuel. `_elmhurst_secondary_fuel_from_sap_code`
mapped the gas block (601-613 → mains gas) and solid block (631-634 →
house coal) to their modal defaults, but returned None for any OTHER
Category-10 code — and None makes the cascade SILENTLY bill the secondary
as electricity (13.19 p/kWh). For a fuel-fired heater (e.g. 621-625
liquid-fuel oil/bioethanol) that is a large, invisible mis-price.

Per the UnmappedElmhurstLabel strict-raise pattern (mirrors the wall_type
/ glazing label raises), a fuel-fired Category-10 code (601-699) outside
the mapped gas/solid blocks now RAISES instead of guessing. Electric room
heaters (691-699) keep returning None — electricity IS their fuel.

The gas block 601-613 still resolves to the modal default mains gas: the
Summary cannot distinguish mains gas from LPG/biogas, so an LPG or biogas
live-effect fire (worksheet "simulated case 37" used biogas at 7.60 p/kWh
vs our 3.48 p/kWh mains-gas default, a +7 SAP gap) is not recoverable from
the Summary export — that is a data-availability limit, not a guess we can
fix here. This commit closes the genuinely-silent-wrong path; the gas
sub-fuel remains the documented modal default.

Worksheet harness 47/47, 0 raised. 3 AAA tests, pyright net-zero,
regression clean, corpus gauge unchanged (Elmhurst-path only; the API path
lodges the secondary fuel explicitly).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

datatypes/epc/domain/mapper.py

@@ -5347,7 +5347,7 @@ def _elmhurst_secondary_fuel_from_sap_code(
                column B for LPG. Cohort default is mains gas
                (`_ELMHURST_MAIN_FUEL_TO_SAP10["Mains gas"] = 26`).
       621-625: Liquid fuel room heaters (oil / bioethanol). Cohort
-               not yet exercised; deferred until a fixture surfaces.
+               not yet exercised — NOT mapped, so they RAISE (see below).
       631-634: Solid fuel room heaters (open fire, closed room
                heater with/without boiler). House coal is the modal
                default per Table 12 secondary rate (3.67 p/kWh).
@@ -5358,6 +5358,21 @@ def _elmhurst_secondary_fuel_from_sap_code(
     in grate") path — pre-slice the cascade defaulted to electricity
     at 13.19 p/kWh, over-charging secondary by ~£340/yr and pushing
     SAP -15.81 below the worksheet's 63.87.
+
+    RAISE-don't-guess: a Category-10 room-heater code (601-699) that is
+    fuel-fired but NOT in the mapped gas/solid blocks (e.g. 621-625 liquid
+    fuel) used to return None → the cascade silently billed it as
+    electricity (13.19 p/kWh), a large mis-price for an oil/LPG heater.
+    Per the `UnmappedElmhurstLabel` strict-raise pattern these now raise
+    so the gap surfaces instead of producing a wrong SAP. Electric room
+    heaters (691-699) keep returning None — electricity IS their fuel.
+
+    NOTE the gas block 601-613 still resolves to the MODAL default mains
+    gas: the Summary lodges only the SAP code, never the gas sub-fuel, so
+    an LPG or biogas live-effect fire (worksheet "simulated case 37" used
+    biogas at 7.60 p/kWh) is indistinguishable here from mains gas — the
+    cohort default is correct for the common case and the rarer sub-fuels
+    are not recoverable from the Summary export.
     """
     if sap_code is None:
         return None
@@ -5365,6 +5380,12 @@ def _elmhurst_secondary_fuel_from_sap_code(
         return 26  # Mains gas, matching `_ELMHURST_MAIN_FUEL_TO_SAP10`
     if 631 <= sap_code <= 634:
         return 11  # House coal (Coal in `_ELMHURST_MAIN_FUEL_TO_SAP10`)
+    if 691 <= sap_code <= 699:
+        return None  # Electric room heaters → cascade electricity default
+    if 601 <= sap_code <= 699:
+        # Fuel-fired Category-10 room heater we do not yet map — raise
+        # rather than let the cascade silently bill it as electricity.
+        raise UnmappedElmhurstLabel("secondary_heating.sap_code", str(sap_code))
     return None
 
 

datatypes/epc/domain/tests/test_from_site_notes.py

@@ -713,3 +713,52 @@ class TestUnmeasurableWallThickness:
 
     def test_wall_thickness_mm_is_none(self, result: EpcPropertyData) -> None:
         assert result.sap_building_parts[0].wall_thickness_mm is None
+
+
+class TestElmhurstSecondaryFuelFromSapCode:
+    """`_elmhurst_secondary_fuel_from_sap_code` must RAISE on an unmapped
+    fuel-fired Table 4a Category-10 room-heater code rather than return
+    None and let the cascade silently bill it as electricity (13.19
+    p/kWh). The Summary lodges only the secondary SAP code, not its fuel.
+    """
+
+    def test_gas_room_heater_resolves_to_mains_gas_modal_default(self) -> None:
+        # Arrange — SAP code 605 (flush-fitting live-effect gas fire), in
+        # the 601-613 gas block. The Summary cannot distinguish mains gas
+        # from LPG/biogas, so the modal default is mains gas (26).
+        from datatypes.epc.domain.mapper import (
+            _elmhurst_secondary_fuel_from_sap_code,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act
+        fuel = _elmhurst_secondary_fuel_from_sap_code(605)
+
+        # Assert
+        assert fuel == 26
+
+    def test_electric_room_heater_returns_none_for_electricity_default(self) -> None:
+        # Arrange — SAP code 693 (electric room heater); electricity IS its
+        # fuel, so None (→ cascade electricity default) is correct, NOT a
+        # silent mis-fuel.
+        from datatypes.epc.domain.mapper import (
+            _elmhurst_secondary_fuel_from_sap_code,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act
+        fuel = _elmhurst_secondary_fuel_from_sap_code(693)
+
+        # Assert
+        assert fuel is None
+
+    def test_unmapped_liquid_fuel_room_heater_raises(self) -> None:
+        # Arrange — SAP code 621 (liquid-fuel room heater) is fuel-fired but
+        # not in the mapped gas/solid blocks; returning None would silently
+        # bill it as electricity. It must raise instead.
+        from datatypes.epc.domain.mapper import (
+            UnmappedElmhurstLabel,
+            _elmhurst_secondary_fuel_from_sap_code,  # pyright: ignore[reportPrivateUsage]
+        )
+
+        # Act / Assert
+        with pytest.raises(UnmappedElmhurstLabel):
+            _elmhurst_secondary_fuel_from_sap_code(621)