Slice 25b: 000487 §4 closure (7/8) — has_electric_shower routes Nbath

Closes §4 LINE_43 + LINE_44/45/46/61/62/64 for 000487 (7 of 8 fails).
LINE_65 still fails — needs Appendix J step 8 (electric-shower kWh
derivation from cert) to land before LINE_65 heat gains close.

Spec citation: SAP10.2 Appendix J (p.81) step 2a: `Nbath = 0.13N + 0.19
if shower also present; = 0.35N + 0.50 if no shower present`. The
"shower also present" branch fires when ANY shower is lodged — mixer OR
electric — per the implicit reading that step 1a's Noutlets includes
electric showers in the count.

Changes:
- SapHeating gains `electric_shower_count` + `mixer_shower_count`.
- `water_heating_from_cert` gains `has_electric_shower: bool = False`;
  combined with mixer-flow-rate presence to drive `has_shower`.
- `_mixer_shower_flow_rates_from_cert` honors `mixer_shower_count`
  (default 1 vented when unlodged — preserves legacy behaviour).
- `_has_electric_shower_from_cert` new helper.
- `water_heating_section_from_cert` plumbs `has_electric_shower`
  through bootstrap + final call (and the internal cert_to_inputs path).
- 000487 fixture: `electric_shower_count=1, mixer_shower_count=0`.

§4 per-fixture:
  fixture | LINE_42 | LINE_43 | LINE_44-46 | LINE_61-65
  000474  |   ✓     |   ✓     |    ✓       |   ✓ (9/9)
  000477  |   ✓     |   ✓     |    ✓       |   ✗ LINE_61/62/64/65 (slice 25c)
  000480  |   ✓     |   ✓     |    ✓       |   ✓ (9/9)
  000487  |   ✓     |   ✓     |    ✓       |   ✓ except LINE_65 (8/9)
  000490  |   ✓     |   ✓     |    ✓       |   ✓ (9/9)
  000516  |   ✓     |   ✓     |    ✓       |   ✓ (9/9)

Scoreboard:
  section_cascade_pins: 279 → 286 PASS (+7)
  e2e SapResult:         32 →  32 PASS (unchanged — LINE_65 cascade still
    open, blocks downstream §5 LINE_72/73 + §6 LINE_84 + §7 + downstream)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

datatypes/epc/domain/epc_property_data.py

@@ -141,6 +141,18 @@ class SapHeating:
     # SAP10 hot-water demand inputs from sap_heating.
     number_baths: Optional[int] = None
     number_baths_wwhrs: Optional[int] = None
+    # Per SAP10.2 Appendix J (p.81) step 1a: Noutlets includes electric
+    # showers in the count for Nshower; step 2a routes Nbath through the
+    # "shower also present" branch (0.13N + 0.19) when ANY shower is
+    # lodged — including electric. Modelled separately from mixer outlets
+    # because electric showers don't draw warm water from the system.
+    electric_shower_count: Optional[int] = None
+    # PCDF mixer-shower lodgement (count of outlets that DO draw warm
+    # water from the main HW system). When set, overrides the heuristic
+    # default of 1 vented outlet @ 7 L/min used by `_mixer_shower_flow_
+    # rates_from_cert`. Most certs lodge only count; the standard
+    # vented-system flow rate from Table J4 (7 L/min) is the default.
+    mixer_shower_count: Optional[int] = None
 
 
 @dataclass

docs/sap-spec/HANDOVER_NEXT.md

@@ -133,14 +133,15 @@ Two test files contain the strict pins:
 Total: **169 PASS / 83 FAIL** across the strict pins. 4 of 6 fixtures fully
 close §1+§2+§4. 000487 is the worst (RR fixture defect propagates everywhere).
 
-(Post-slice-25a: section_cascade_pins 279 PASS / 33 FAIL, e2e SapResult
-32 PASS / 40 FAIL. §3 + §5 + §6 + §7 (mostly) pinned. §3 NOW FULLY
-CLOSES for all 6 fixtures (24/24) — slice 25a closed 000487 by lodging
-detailed RR surfaces + adding gable_wall_external + Ext1 alt U
-override + RdSAP §3.8 roof-area-as-max rule + half-up rounding.
-Remaining failures: §4 monthly on 000477+487 (slice 25b), §5 LINE_72/73
-+ §6 LINE_84 on 000477/487 (cascade from §4), §7 LINE_92/93 marginal
-on 000474/477/480/490 (precision artefact), §7 on 000487 (cascade).)
+(Post-slice-25b: section_cascade_pins 286 PASS / 26 FAIL, e2e SapResult
+32 PASS / 40 FAIL. §3 fully closes for all 6 fixtures (24/24). §4 closes
+8 of 9 for 000487 — only LINE_65 (heat gains from WH) still fails
+because the §4 cascade doesn't yet derive (64a) electric-shower kWh
+from the cert (Appendix J step 8). Remaining cascade failures: §4 on
+000477 (combi loss precision, slice 25c) + §4 LINE_65 on 000487
+(electric shower derivation), §5/§6 LINE_72/73/84 on 000477+487
+(cascade from §4), §7 LINE_92/93 marginal on 000474/477/480/490
+(precision artefact), §7 on 000487 (cascade from §4 LINE_65).)
 
 ### B.2 SapResult pin matrix (post-slice-22/23)
 
@@ -201,6 +202,7 @@ fixture | section §4 pin status
 ### B.5 Recent slices (in reverse order — newest first)
 
 ```
+Slice 25b: 000487 §4 closure (7/8) — has_electric_shower + mixer/electric counts on SapHeating, Appendix J step 2a fix
 Slice 25a: 000487 §3 closure — detailed RR + gable_wall_external + Ext1 alt U=1.9 + §3.8 max-floor roof + half-up rounding
 Slice 26c: §7 mean internal temp cascade pin (60 cases, 44 PASS) — LINE_85..94
 Slice 26b: §6 solar gains cascade pin (12 cases, 10 PASS) + SapRoofWindow solar attrs + plumb to §6 cascade

packages/domain/src/domain/ml/tests/_fixtures.py

@@ -100,6 +100,8 @@ def make_sap_heating(
     secondary_fuel_type: Optional[int] = None,
     secondary_heating_type: Optional[int] = None,
     number_baths: Optional[int] = None,
+    electric_shower_count: Optional[int] = None,
+    mixer_shower_count: Optional[int] = None,
 ) -> SapHeating:
     """Build a SapHeating with SAP10 API defaults."""
     return SapHeating(
@@ -115,6 +117,8 @@ def make_sap_heating(
         secondary_fuel_type=secondary_fuel_type,
         secondary_heating_type=secondary_heating_type,
         number_baths=number_baths,
+        electric_shower_count=electric_shower_count,
+        mixer_shower_count=mixer_shower_count,
     )
 
 

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -718,12 +718,14 @@ def water_heating_section_from_cert(
         if main is not None and main.main_heating_index_number is not None
         else None
     )
+    has_electric_shower = _has_electric_shower_from_cert(epc)
     bootstrap = water_heating_from_cert(
         epc=epc,
         mixer_shower_flow_rates_l_per_min=_mixer_shower_flow_rates_from_cert(epc),
         has_bath=_has_bath_from_cert(epc),
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
+        has_electric_shower=has_electric_shower,
     )
     combi_loss_override = pcdb_combi_loss_override(
         pcdb_main,
@@ -737,6 +739,7 @@ def water_heating_section_from_cert(
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
         combi_loss_monthly_kwh_override=combi_loss_override,
+        has_electric_shower=has_electric_shower,
     )
 
 
@@ -950,16 +953,30 @@ def _mixer_shower_flow_rates_from_cert(
 ) -> tuple[float, ...]:
     """Pull mixer-shower flow rates from the cert.
 
-    The cert lodges flow rate per shower outlet (Elmhurst worksheets
-    show "Vented hot water system, 7.00"). The domain model doesn't
-    surface that field yet; until it does, every cert defaults to the
-    SAP10.2 Table J4 "Vented hot water system" row at 7 L/min — the
-    existing-dwelling minimum and what both validation fixtures
-    (000474 + 000490) actually lodge. Combi-pumped showers (11 L/min)
-    or electric showers (handled via (64a)m, not here) will need
-    proper shower-outlet-type plumbing in a later slice.
+    When `sap_heating.mixer_shower_count` is lodged, use that count
+    of vented mixers @ Table J4's 7 L/min row. When None, default to a
+    single vented outlet — the modal RdSAP lodging. Combi-pumped
+    showers (11 L/min) need a richer cert surface in a future slice.
     """
-    return (_SHOWER_FLOW_VENTED_L_PER_MIN,)
+    count = (
+        epc.sap_heating.mixer_shower_count
+        if epc.sap_heating is not None
+        else None
+    )
+    if count is None:
+        count = 1
+    return tuple(_SHOWER_FLOW_VENTED_L_PER_MIN for _ in range(max(0, count)))
+
+
+def _has_electric_shower_from_cert(epc: EpcPropertyData) -> bool:
+    """True iff cert lodges ≥ 1 instantaneous electric shower.
+
+    Electric showers don't draw warm water from the main HW system but
+    count in `Noutlets` for SAP10.2 Appendix J (p.81) step 1a and route
+    Nbath through the "shower also present" branch in step 2a (0.13N +
+    0.19 instead of 0.35N + 0.50). Defaults False when unlodged."""
+    n = epc.sap_heating.electric_shower_count if epc.sap_heating is not None else None
+    return (n or 0) >= 1
 
 
 def _has_bath_from_cert(epc: EpcPropertyData) -> bool:
@@ -1052,12 +1069,14 @@ def _water_heating_worksheet_and_gains(
     zero_monthly = (0.0,) * 12
     if epc.total_floor_area_m2 is None:
         return None, zero_monthly
+    has_electric_shower = _has_electric_shower_from_cert(epc)
     bootstrap = water_heating_from_cert(
         epc=epc,
         mixer_shower_flow_rates_l_per_min=_mixer_shower_flow_rates_from_cert(epc),
         has_bath=_has_bath_from_cert(epc),
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
+        has_electric_shower=has_electric_shower,
     )
     combi_loss_override = pcdb_combi_loss_override(
         pcdb_record,
@@ -1071,6 +1090,7 @@ def _water_heating_worksheet_and_gains(
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
         combi_loss_monthly_kwh_override=combi_loss_override,
+        has_electric_shower=has_electric_shower,
     )
     return wh_result, wh_result.heat_gains_monthly_kwh
 

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py

@@ -172,6 +172,12 @@ def build_epc() -> EpcPropertyData:
             # radiant heaters" → SAP code 691. Table 11 fraction 0.10.
             secondary_heating_type=691,
             number_baths=1,  # 000487 line 123: Total number of baths = 1
+            # 000487 cert: 1 instantaneous electric shower, no mixer
+            # outlet (worksheet text shows only Doors/Windows openings,
+            # no shower outlet under Heat Loss). Drives Appendix J step 2a
+            # to use Nbath = 0.13N + 0.19 ("shower also present").
+            electric_shower_count=1,
+            mixer_shower_count=0,
         ),
     )
 

packages/domain/src/domain/sap/worksheet/water_heating.py

@@ -595,6 +595,7 @@ def water_heating_from_cert(
     low_water_use: bool,
     combi_loss_monthly_kwh_override: Optional[tuple[float, ...]] = None,
     electric_shower_monthly_kwh_override: Optional[tuple[float, ...]] = None,
+    has_electric_shower: bool = False,
 ) -> WaterHeatingResult:
     """SAP 10.2 §4 orchestrator — chain every line ref from (42) through
     (65) for a combi-gas dwelling with optional PCDB-backed combi loss.
@@ -635,6 +636,7 @@ def water_heating_from_cert(
     has_shower = (
         len(mixer_shower_flow_rates_l_per_min) > 0
         or electric_shower_monthly_kwh_override is not None
+        or has_electric_shower
     )
     baths = hot_water_baths_monthly_l_per_day(
         n_occupants=n,