diff --git a/scripts/backfill_boiler_efficiency_band.py b/scripts/backfill_boiler_efficiency_band.py
index ff083e626..01271d694 100644
--- a/scripts/backfill_boiler_efficiency_band.py
+++ b/scripts/backfill_boiler_efficiency_band.py
@@ -7,12 +7,14 @@ override rows for the existing stock by re-parsing that text with the SAME guard
 live classifier uses (``boiler_efficiency_band_guard``), so the backfill and the
 forward path cannot drift.
 
-One ``boiler_efficiency_band`` row per ``main_heating_system`` row whose description
-carries a band (``A``–``G``), keyed to the same ``(property_id, building_part)``.
-Descriptions with no band (a plain boiler, a non-boiler heating system) get no row.
-The modelling gate (gas/oil boilers only, ADR-0068) is applied at overlay time, not
-here — so a band parsed off an electric ``Boiler: A rated NA`` is stored for
-fidelity but ignored by the calculator, exactly as the live classifier stores it.
+One ``boiler_efficiency_band`` row per ``main_heating_system`` row, keyed to the
+same ``(property_id, building_part)`` — the parseable band (``A``–``G``) where the
+description carries one, else an explicit ``Unknown``. This mirrors the finaliser
+(which records Unknown rather than skipping), so backfilled historical data matches
+go-forward data. The modelling gate (gas/oil boilers only, ADR-0068) is applied at
+overlay time, not here — so a band parsed off an electric ``Boiler: A rated NA`` is
+stored for fidelity but ignored by the calculator, and ``Unknown`` (incl. on a
+non-boiler system) is inert to the calculator, exactly as the live path stores them.
 
 GATED: the ``boiler_efficiency_band`` value of the FE-owned ``override_component``
 pgEnum must exist first (the Drizzle migration in the assessment-model repo). Until
@@ -63,24 +65,30 @@ class BandBackfillRow:
 
 
 def band_backfill_rows(rows: Iterable[MainHeatingRow]) -> list[BandBackfillRow]:
-    """The ``boiler_efficiency_band`` rows to write for a set of
-    ``main_heating_system`` rows — one per row whose description carries a
-    parseable SEDBUK band, keyed to the same property + building part. Rows with
-    no band (or the ``UNKNOWN`` sentinel) produce nothing.
+    """The ``boiler_efficiency_band`` row to write for **every**
+    ``main_heating_system`` row, keyed to the same property + building part — the
+    parseable SEDBUK band (``A``–``G``) where the description carries one, else an
+    explicit ``Unknown``. This mirrors the finaliser (which records Unknown rather
+    than skipping), so backfilled historical data matches go-forward data;
+    ``Unknown`` is inert to modelling (no efficiency anchor) and fine on a
+    non-boiler system.
 
     Pure and DB-free, so the parse/decide logic is unit-tested without a
     database; the connection wrapper below just feeds it rows and upserts."""
     backfill: list[BandBackfillRow] = []
     for row in rows:
         band = boiler_efficiency_band_guard(row.original_spreadsheet_description)
-        if band is None or band is BoilerEfficiencyBand.UNKNOWN:
-            continue
+        value = (
+            band.value
+            if band is not None and band is not BoilerEfficiencyBand.UNKNOWN
+            else BoilerEfficiencyBand.UNKNOWN.value
+        )
         backfill.append(
             BandBackfillRow(
                 property_id=row.property_id,
                 portfolio_id=row.portfolio_id,
                 building_part=row.building_part,
-                override_value=band.value,
+                override_value=value,
                 original_spreadsheet_description=row.original_spreadsheet_description,
             )
         )
diff --git a/tests/scripts/test_backfill_boiler_efficiency_band.py b/tests/scripts/test_backfill_boiler_efficiency_band.py
index c94220201..9319564af 100644
--- a/tests/scripts/test_backfill_boiler_efficiency_band.py
+++ b/tests/scripts/test_backfill_boiler_efficiency_band.py
@@ -30,14 +30,22 @@ def test_backfills_a_band_row_per_boiler_with_a_parseable_band() -> None:
     ]
 
 
-def test_leaves_descriptions_without_a_band_untouched() -> None:
+def test_records_unknown_for_descriptions_without_a_band() -> None:
+    # Aligned with the finaliser: every main_heating_system row gets a band row —
+    # a description with no determinable band (plain boiler, non-boiler system) is
+    # recorded as an explicit Unknown, so historical + go-forward data match.
+    # Unknown is fine for non-boilers (inert to modelling).
     rows = [
-        _row("Gas boiler"),
-        _row("Community Heating Systems: Community boilers only (RdSAP)"),
-        _row(""),
+        _row("Gas boiler", property_id=1),
+        _row("Community Heating Systems: Community boilers only (RdSAP)", property_id=2),
     ]
 
-    assert band_backfill_rows(rows) == []
+    result = band_backfill_rows(rows)
+
+    assert [(r.property_id, r.override_value) for r in result] == [
+        (1, "Unknown"),
+        (2, "Unknown"),
+    ]
 
 
 def test_uses_the_same_guard_as_the_live_path_for_multi_system_and_electric() -> None: