fix(ventilation): MVHR takes lodged 0 intermittent fans, not the Table 5 default

Extends the dMEV intermittent-fan fix (4db05e84) to MVHR. A balanced
whole-house MVHR system IS the dwelling's ventilation, so the lodged (7a)
intermittent-extract-fan count is explicit — a lodged 0 means 0, not the
RdSAP 10 Table 5 age-band "unknown" default. The cascade was substituting
the default (here 20 m³/h) into worksheet line (8) openings, inflating
(16/18) infiltration → (21) → (22b) → (25) effective ach → (38)
ventilation heat loss → the space-heating demand.

Worksheet-proven on simulated case 49 (000565 + Vent Axia 500140 MVHR,
lodged (7a)=0): our (8) openings 0.0723 -> 0.0000, (18) 0.7223 -> 0.6500,
(25)m Jan 0.9423 -> 0.8571, all now matching Elmhurst exactly; space-
heating demand 7857 -> 7528 kWh (worksheet 7546). SAP 70.90 -> 71.43
continuous. (The residual to the worksheet's 72 is its own continuous SAP
71.69 rounding up, driven by a separate gas-combi water-heating-loss gap,
not ventilation.)

Scoped to EXTRACT_OR_PIV_OUTSIDE + MVHR only — MV-without-HR
(mechanical_ventilation=1) stays on the default-substitution path
(forcing its lodged 0 regressed 47 Howsman / 18 Jutland and is not
worksheet-validated). Corpus within-0.5 holds 72.7%, MAE 0.782 -> 0.781.

Note: pyright strict type gate not run locally (pyright not installed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -5196,16 +5196,23 @@ def ventilation_from_cert(
             # can override via a future plumbing slice; the spec default
             # is what every MEV / MV / MVHR cohort cert lodges today.
             mv_system_ach = 0.5
-            # For a whole-house mechanical EXTRACT system (MEV / dMEV) the
-            # lodged intermittent extract-fan count (7a) is taken AS-IS — the
-            # Table 5 age-band default must NOT be substituted for a lodged 0.
-            # On a mechanically-ventilated dwelling the fan count is explicit
-            # (the dMEV is the ventilation), so 0 means 0, not "unknown".
-            # Worksheet-proven on two dMEV builds of 000565: "case 48" lodges
-            # (7a)=0 → SAP 57 exact, while the original 000565 fixture lodges
-            # (7a)=2 → unchanged. Scoped to EXTRACT_OR_PIV_OUTSIDE; balanced
-            # MVHR/MV kinds are left untouched pending their own worksheet.
-            if mv_kind is MechanicalVentilationKind.EXTRACT_OR_PIV_OUTSIDE:
+            # For a whole-house mechanical EXTRACT (MEV / dMEV) OR balanced-
+            # with-heat-recovery (MVHR) system the lodged intermittent extract-
+            # fan count (7a) is taken AS-IS — the Table 5 age-band default must
+            # NOT be substituted for a lodged 0. On such a dwelling the fan
+            # count is explicit (the mechanical system IS the ventilation), so
+            # 0 means 0, not "unknown". Worksheet-proven on three 000565 builds:
+            # "case 48" (dMEV) lodges (7a)=0 → SAP 57 exact; "case 49" (MVHR,
+            # Vent Axia 500140) lodges (7a)=0 → the worksheet (8) openings line
+            # is 0.0000 (our default had added 20 m³/h = 0.0723 ach, inflating
+            # (22b)/(25)/(38) and the demand). The original 000565 fixture
+            # lodges (7a)=2 → unchanged. MV-without-HR (mechanical_ventilation
+            # =1) is EXCLUDED: forcing its lodged 0 regressed 47 Howsman / 18
+            # Jutland and is not worksheet-validated.
+            if mv_kind in (
+                MechanicalVentilationKind.EXTRACT_OR_PIV_OUTSIDE,
+                MechanicalVentilationKind.MVHR,
+            ):
                 intermittent_fans = vc.intermittent_fans
     # SAP 10.2 §2.6.6 equation (2): the (24a) MVHR effective-air-change
     # credit needs the in-use heat-recovery efficiency (23c) = raw PCDB

tests/infrastructure/epc_client/test_sap_accuracy_corpus.py

@@ -210,6 +210,11 @@ _MIN_WITHIN_HALF_SAP = 0.72
 # 81.9%, (230a) 415.9325, (231) 501.9325 — matching Elmhurst exactly). Corpus
 # MVHR certs: Flat 1 +6 -> 0, 12a Princes Gate +3 -> +1; Apartment 707's -4 ->
 # -6 is a separate baseline under-rate (it under-rated as natural too).
+# Then 0.782 -> 0.781 via extending the dMEV "lodged 0 extract fans, no Table 5
+# default" rule to MVHR (the balanced system is the ventilation, so a lodged
+# (7a)=0 is explicit): case 49's (8) openings line is 0.0000 — our default had
+# added 20 m3/h (0.0723 ach), inflating (22b)/(25)/(38) and demand (SAP 70.90
+# -> 71.43; the worksheet's own continuous SAP is 71.69 -> rounds to 72).
 _MAX_SAP_MAE = 0.785
 _MAX_CO2_MAE_TONNES = 0.09      # t CO2 / yr vs co2_emissions_current
 _MAX_PE_PER_M2_MAE = 3.6        # kWh / m2 / yr vs energy_consumption_current