fix(water-heating): 18-/24-hour immersion DHW bills 100% low-rate (Table 12a scope)

SAP 10.2 Table 12a (PDF p.191) is titled "High-rate fractions for systems
using 7-hour and 10-hour tariffs"; its "Immersion water heater" row lists
the tariff as "7-hour or 10-hour" only, routing to Table 13. An 18-hour or
24-hour tariff is OUTSIDE the table's scope — it provides at least 18
hours/day at the low rate, more than enough to heat any immersion cylinder
off-peak, so the high-rate fraction is 0 (all DHW billed at the low rate).

`electric_dhw_high_rate_fraction` previously mapped 18-/24-hour to the
10-hour equations (returning ~0.10 for a 110 L dual immersion) on an
over-literal reading of Table 13 Note 1 ("at least 10 hours"). The Elmhurst
dr87 worksheet for solid fuel 5 (cert 001431: 18-hour meter, 110 L dual
immersion, WHC 903) refutes that: HW (245) high-rate = 0.0 kWh, (246)
low-rate = 100%. Table 12a's title bounds the table to the two named
tariffs; 18-/24-hour fall outside it.

Resolves the Table-13 blocker on the immersion-extractor fix: once the
Summary extractor captures the dual immersion, the 18-hour solid-fuel
corpus certs stay at high_frac=0 (matching their worksheets) instead of
regressing to the 10-hour-column 0.10.

API SAP eval unchanged: 57.6% within 0.5, mean|err| 1.185, signed -0.165
(the cached sample has no 18-hour WHC-903 certs; one 24-hour cert shifts
sub-threshold). Regression gate green (3 pre-existing fails unrelated).

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

domain/sap10_calculator/tables/table_13.py

@@ -22,12 +22,24 @@ is needed:
       Single: [(14530 - 762 N) / (1.5 V) - 80 + 10 N] / 100
 
 where V is the cylinder volume (litres) and N is the assumed occupancy
-(Appendix J Table 1b). Per Note 2 the result is clamped to [0, 1]. Per
-Note 1 the 10-hour equations apply to any tariff providing at least 10
-hours/day at the low rate (so 18-hour and 24-hour use the 10-hour
-column). Heat pumps providing water heating only are treated as dual
-immersion (Note 1) — out of scope of this helper (callers route those
-via Table 12a).
+(Appendix J Table 1b). Per Note 2 the result is clamped to [0, 1].
+
+Table 12a (PDF p.191) — whose title reads "High-rate fractions for
+systems using 7-hour and 10-hour tariffs" — routes the "Immersion water
+heater" row to Table 13 for the tariff "7-hour or 10-hour" ONLY. An
+18-hour or 24-hour tariff is outside Table 12a/13's scope: it provides
+at least 18 hours/day at the low rate, more than enough to heat any
+immersion cylinder off-peak, so the high-rate fraction is 0 (all DHW
+billed at the low rate). The Elmhurst dr87 worksheet for solid fuel 5
+(cert 001431: 18-hour meter, 110 L dual immersion, WHC 903) confirms
+this — HW (245) high-rate = 0.0 kWh, (246) low-rate = 100%. (An earlier
+reading mapped 18-/24-hour to the 10-hour column via Note 1's "at least
+10 hours"; the worksheet refutes it — Table 12a's title bounds the table
+to the two named tariffs.)
+
+Heat pumps providing water heating only are treated as dual immersion
+(Note 1) — out of scope of this helper (callers route those via Table
+12a).
 """
 
 from __future__ import annotations
@@ -47,13 +59,19 @@ def electric_dhw_high_rate_fraction(
 
     `single_immersion` selects the single- vs dual-immersion equation
     (RdSAP10 §10.5 p.54: an immersion is assumed dual on a dual meter).
-    The 7-hour tariff uses the 7-hour equations; every other off-peak
-    tariff (10/18/24-hour, all >= 10 hours low-rate per Note 1) uses the
-    10-hour equations. STANDARD has no off-peak split and is rejected —
-    callers must early-return before this fires.
+    The 7-hour tariff uses the 7-hour equations; the 10-hour tariff uses
+    the 10-hour equations. The 18-hour and 24-hour tariffs are outside
+    Table 12a/13's "7-hour and 10-hour" scope (PDF p.191 title) — they
+    provide >= 18 hours/day at the low rate, so the high-rate fraction is
+    0. STANDARD has no off-peak split and is rejected — callers must
+    early-return before this fires.
     """
     if tariff is Tariff.STANDARD:
         raise ValueError("Table 13 high-rate fraction is undefined for STANDARD")
+    if tariff in (Tariff.EIGHTEEN_HOUR, Tariff.TWENTY_FOUR_HOUR):
+        # Outside Table 12a's 7-hour/10-hour scope — >= 18 h/day low rate
+        # heats the cylinder entirely off-peak (high-rate fraction 0).
+        return 0.0
     v = cylinder_volume_l
     n = occupancy_n
     if tariff is Tariff.SEVEN_HOUR:
@@ -62,7 +80,7 @@ def electric_dhw_high_rate_fraction(
         else:
             fraction = ((6.8 - 0.024 * v) * n + 14 - 0.07 * v) / 100
     else:
-        # >= 10 hours/day at the low rate (10/18/24-hour) — Note 1.
+        # 10-hour tariff (18-/24-hour handled above as out-of-scope).
         if single_immersion:
             fraction = ((14530 - 762 * n) / (1.5 * v) - 80 + 10 * n) / 100
         else:

tests/domain/sap10_calculator/test_table_13.py

@@ -83,21 +83,28 @@ def test_table_13_large_cylinder_single_immersion_clamps_to_zero() -> None:
     assert fraction == 0.0
 
 
-def test_table_13_eighteen_hour_uses_ten_hour_column() -> None:
-    # Arrange — SAP 10.2 Table 13 Note 1 (PDF p.197): the table applies
-    # "for tariffs providing at least 10 hours ... at the low rate", so an
-    # 18-hour tariff resolves to the 10-hour equations, not a separate
-    # column.
+def test_table_13_eighteen_and_twenty_four_hour_bill_full_low_rate() -> None:
+    # Arrange — SAP 10.2 Table 12a (PDF p.191) is titled "High-rate
+    # fractions for systems using 7-hour and 10-hour tariffs"; its
+    # "Immersion water heater" row lists the tariff as "7-hour or 10-hour"
+    # only, routing to Table 13. An 18-hour / 24-hour tariff is OUTSIDE the
+    # table's scope: it provides at least 18 hours/day at the low rate, more
+    # than enough to heat any immersion cylinder off-peak, so the high-rate
+    # fraction is 0 (100% billed at the low rate). The Elmhurst dr87
+    # worksheet for solid fuel 5 (cert 001431: 18-hour meter, 110 L dual
+    # immersion, WHC 903) bills HW (245) high-rate = 0.0 kWh, (246) low-rate
+    # = 100% — confirming high_frac = 0 for an 18-hour immersion DHW.
 
     # Act
     eighteen = electric_dhw_high_rate_fraction(
         cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
         single_immersion=False, tariff=Tariff.EIGHTEEN_HOUR,
     )
-    ten = electric_dhw_high_rate_fraction(
+    twenty_four = electric_dhw_high_rate_fraction(
         cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
-        single_immersion=False, tariff=Tariff.TEN_HOUR,
+        single_immersion=False, tariff=Tariff.TWENTY_FOUR_HOUR,
     )
 
     # Assert
-    assert abs(eighteen - ten) <= 1e-9
+    assert eighteen == 0.0
+    assert twenty_four == 0.0