diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 7c6532a9..0ddb4baa 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -4143,6 +4143,59 @@ _LIQUID_FUEL_BOILER_SAP_MAIN_HEATING_CODES: Final[frozenset[int]] = (
     frozenset(range(120, 142))
 )
 
+# SAP 10.2 Table 4b gas-boiler code range (PDF p.168). Rows 101-119 are
+# "Gas boilers (including mains gas, LPG and biogas)" — 101-109 are
+# 1998-or-later, 110-114 pre-1998 fan-assisted flue, 115-119 pre-1998
+# balanced/open flue. The code identifies the boiler TYPE/efficiency, not
+# the specific carrier: the same row applies to mains gas, bulk/bottled
+# LPG and biogas alike. The older Elmhurst export lodged §14.0 "Fuel
+# Type: Mains gas" explicitly, but the newer form leaves §14.0 "Fuel
+# Type" empty and lodges only the SAP code (e.g. 104 condensing combi,
+# EES "BGW"). For these, §15.0 "Water Heating Fuel Type" names the
+# carrier — a combi/boiler heats space + water from the one appliance —
+# so it disambiguates mains-gas-vs-LPG. Codes 120-141 (CPSU + range
+# cookers) are already covered by
+# `_LIQUID_FUEL_BOILER_SAP_MAIN_HEATING_CODES`.
+_GAS_BOILER_SAP_MAIN_HEATING_CODES: Final[frozenset[int]] = (
+    frozenset(range(101, 120))
+)
+
+# SAP10 main-fuel codes in the gas / LPG family — the only carriers a
+# Table 4b gas-boiler row (101-119) can have (mains gas, mains gas
+# community, bottled/bulk/special-condition LPG). Per
+# `_ELMHURST_MAIN_FUEL_TO_SAP10`: mains gas = 26, mains gas community =
+# 1, LPG bottled/bulk/special = 5/6/7, "Bulk LPG" = 27. The §15.0
+# water-heating-fuel derivation is gated on the resolved fuel being one
+# of these so it can't mis-assign electricity from a separate immersion
+# (where §15.0 lodges the immersion's fuel, not the boiler's) — that
+# case still strict-raises `MissingMainFuelType` to force a mapper fix.
+_GAS_LPG_MAIN_FUEL_CODES: Final[frozenset[int]] = frozenset({1, 5, 6, 7, 26, 27})
+
+
+def _elmhurst_gas_boiler_main_fuel(
+    sap_main_heating_code: Optional[int],
+    water_heating_fuel_code: Optional[int],
+) -> Optional[int]:
+    """Derive a gas/LPG main-fuel code for a Table 4b gas boiler whose
+    §14.0 "Fuel Type" string is absent (newer Elmhurst export form).
+
+    Returns the §15.0 water-heating fuel code when, and only when, the
+    SAP main-heating code is a Table 4b gas-boiler row (101-119) AND the
+    §15.0 fuel resolves to a gas/LPG carrier — the same combi/boiler
+    heats space + water, so §15.0 names the boiler's carrier. Returns
+    None otherwise (non-gas-boiler code, or §15.0 lodges a non-gas fuel
+    such as an electric immersion), leaving the caller to strict-raise.
+
+    Spec: SAP 10.2 Table 4b "Seasonal efficiency for gas and liquid fuel
+    boilers" (PDF p.168) — rows 101-119 are gas-family boilers.
+    """
+    if (
+        sap_main_heating_code in _GAS_BOILER_SAP_MAIN_HEATING_CODES
+        and water_heating_fuel_code in _GAS_LPG_MAIN_FUEL_CODES
+    ):
+        return water_heating_fuel_code
+    return None
+
 
 # Elmhurst §14.0 "Main Heating EES Code" → Table 32 main fuel code.
 # Empirically derived from the heating-systems corpus at
@@ -4770,6 +4823,19 @@ def _map_elmhurst_sap_heating(survey: ElmhurstSiteNotes) -> SapHeating:
         and mh.main_heating_sap_code in _LIQUID_FUEL_BOILER_SAP_MAIN_HEATING_CODES
     ):
         main_fuel_int = water_heating_fuel
+    # Gas / LPG boilers: SAP 10.2 Table 4b codes 101-119 (PDF p.168)
+    # identify a gas-family boiler but not the specific carrier (mains
+    # gas vs LPG vs biogas). The newer Elmhurst export leaves §14.0
+    # "Fuel Type" empty and lodges only the SAP code (e.g. 104 condensing
+    # combi, EES "BGW"); the §15.0 "Water Heating Fuel Type" names the
+    # carrier because the same combi/boiler heats space + water. Adopt it
+    # only when it resolves to a gas/LPG fuel, so a regular boiler paired
+    # with an electric immersion (where §15.0 lodges "Electricity") still
+    # strict-raises rather than mis-billing the gas boiler as electric.
+    if main_fuel_int is None:
+        main_fuel_int = _elmhurst_gas_boiler_main_fuel(
+            mh.main_heating_sap_code, water_heating_fuel,
+        )
     # Solid-fuel main heating: SAP code rows 150-160 (open / closed
     # room heaters with boiler) and 600-636 (independent solid-fuel
     # boilers) cover multiple distinct fuels under a single Table 4a
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index c8b92602..40ba7aff 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -2120,6 +2120,37 @@ def test_elmhurst_main_fuel_to_sap10_maps_bio_liquid_water_heating_labels() -> N
     assert _ELMHURST_MAIN_FUEL_TO_SAP10["Bio-liquid HVO from used cooking oil"] == 71
 
 
+def test_elmhurst_gas_boiler_main_fuel_derives_carrier_from_water_heating() -> None:
+    # Arrange — SAP 10.2 Table 4b (PDF p.168) rows 101-119 are "Gas
+    # boilers (including mains gas, LPG and biogas)". The code identifies
+    # the boiler type/efficiency, NOT the carrier. The newer Elmhurst
+    # export leaves §14.0 "Fuel Type" empty and lodges only the SAP code
+    # (e.g. 104 condensing combi); the §15.0 "Water Heating Fuel Type"
+    # names the carrier because the same combi/boiler heats space + water.
+    from datatypes.epc.domain.mapper import (
+        _elmhurst_gas_boiler_main_fuel,  # pyright: ignore[reportPrivateUsage]
+    )
+
+    # Act / Assert — combi (104) + §15.0 mains gas (26) → mains gas.
+    assert _elmhurst_gas_boiler_main_fuel(104, 26) == 26
+    # Regular condensing (102) + §15.0 bulk LPG (27) → bulk LPG.
+    assert _elmhurst_gas_boiler_main_fuel(102, 27) == 27
+    # Boundary codes of the 101-119 gas-boiler range resolve too.
+    assert _elmhurst_gas_boiler_main_fuel(101, 26) == 26
+    assert _elmhurst_gas_boiler_main_fuel(119, 5) == 5  # bottled LPG
+    # §15.0 lodges a separate electric immersion's fuel (30), NOT the
+    # gas boiler's carrier → no derivation; caller strict-raises.
+    assert _elmhurst_gas_boiler_main_fuel(104, 30) is None
+    # Non-gas-boiler SAP code (224 = air-source heat pump) → None even
+    # when §15.0 names a gas fuel (the HP doesn't burn it).
+    assert _elmhurst_gas_boiler_main_fuel(224, 26) is None
+    # Liquid-fuel boiler range (120-141) is owned by the separate
+    # `_LIQUID_FUEL_BOILER_SAP_MAIN_HEATING_CODES` branch → None here.
+    assert _elmhurst_gas_boiler_main_fuel(120, 26) is None
+    # No SAP code lodged → None.
+    assert _elmhurst_gas_boiler_main_fuel(None, 26) is None
+
+
 def test_elmhurst_main_heating_ees_maps_no_system_code_to_electricity() -> None:
     # Arrange — SAP 10.2 §A.2.2 (PDF p.189 area) "When no main heating
     # system is identified, the calculation is for the assumed system