Price water-only heat-network DHW without a plant-efficiency divisor 🟩

SAP 10.2 §4.3 (p.24) puts the water-heating efficiency in the Table 12
heat price, and C4 (p.53) scopes the plant efficiency to CO2/PE only, so
worksheet (310) = (64) × (305a) × (306) carries no efficiency term. The
dwelling-side divisor was applying it a second time.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -1187,6 +1187,13 @@ _HEAT_NETWORK_DHW_CHARGING_FACTOR_BY_CODE: Final[dict[int, float]] = {
     2306: 1.00, 2308: 1.00, 2309: 1.00, 2310: 1.00, 2312: 1.00, 2314: 1.00,
 }
 
+# SAP 10.2 Table 4c(3) (PDF p.169) worksheet (305a), "DHW-only system,
+# flat rate charging" row — the RdSAP 10 (spec p.59) default for a
+# water-ONLY heat network (WHC 950/951/952), whose own charging method is
+# never lodged: the dwelling's heating-control code describes the SPACE
+# system's charging, not this network's.
+_TABLE_4C3_FLAT_RATE_CHARGING_FACTOR: Final[float] = 1.05
+
 
 def _heat_network_space_charging_factor(main: Optional[MainHeatingDetail]) -> float:
     """SAP 10.2 Table 4c(3) (PDF p.169) worksheet (305) — heat-network
@@ -8001,13 +8008,28 @@ def cert_to_inputs(
             * _heat_network_dhw_charging_factor(main)
         )
     elif epc.sap_heating.water_heating_code in _WATER_HEAT_NETWORK_ONLY_CODES:
-        # HW-only heat network (whc 950/951/952): the Table 4a plant
-        # efficiency is already in `water_eff`; apply the Table 12c
-        # distribution loss on top per RdSAP 10 §10 (spec p.36 "water
-        # heating only ... distribution loss"). q_generated = q_useful ×
-        # DLF, so delivered-per-fuel efficiency = plant_eff / DLF. Fires
-        # on the WHC alone — the HW network is independent of the main.
-        water_eff = water_eff / _heat_network_dlf(primary_age)
+        # HW-only heat network (whc 950/951/952). SAP 10.2 §4.3 (p.24):
+        # "The efficiency for water heating is incorporated in the price of
+        # heat from a heat network in Table 12" — the dwelling buys
+        # DELIVERED heat (fuel 51), so the Table 4a plant efficiency must
+        # NOT divide here as well. C4 (p.53) scopes it: "For heat networks
+        # not listed in the PCDB, for the calculation of CO2 emissions and
+        # Primary Energy, the efficiency to be used is that of the heat
+        # generator" — CO2/PE only, never delivered energy.
+        #
+        # C6 "Heat network providing DHW only" (p.54): "Allow for the
+        # control factor from Table 4c(3), for the distribution loss factor
+        # as in C3.1" — i.e. worksheet (310) = (64) × (305a) × (306), with
+        # no efficiency term. Same shape as the WHC-901/902/914 branch
+        # above, differing only in that (305a) takes the Table 4c(3)
+        # DHW-only flat-rate default rather than the space system's lodged
+        # charging code. Fires on the WHC alone — §4.3 keys on the DHW
+        # source ("Where hot water is provided by a heat network"), not on
+        # the space main.
+        water_eff = 1.0 / (
+            _heat_network_dlf(primary_age)
+            * _TABLE_4C3_FLAT_RATE_CHARGING_FACTOR
+        )
     is_instantaneous = epc.sap_heating.water_heating_code in _INSTANTANEOUS_WATER_CODES
     # §9a Table 11 secondary fraction — pulled forward of §4 so the
     # post-§8 Equation D1 cascade can derive Q_space = (98c)m × (204)

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -1399,17 +1399,22 @@ def test_heat_network_main_with_hw_from_main_dlf_scales_hot_water_kwh() -> None:
     assert abs(hn_hw_fuel / wh_result.output_kwh_per_yr - 1.41) <= 1e-6
 
 
-def test_hot_water_only_heat_network_whc_950_applies_table12c_dlf() -> None:
+def test_hot_water_only_heat_network_whc_950_costs_more_fuel_than_boiler_dhw() -> None:
     # Arrange — water_heating_code 950 = "hot-water-only heat network
-    # (boilers)" (SAP 10.2 Table 4a HW section, plant eff 0.80). RdSAP 10
-    # §10 (spec p.36) requires the Table 12c distribution loss applied on
-    # top of the plant efficiency for water-heating-only heat networks, so
-    # the delivered HW fuel = q_useful × DLF / 0.80. The DLF must fire on
-    # the WHC ALONE — independent of the main, which here is an ordinary
-    # gas boiler (NOT a heat network), mirroring cert 9093 (whc 950 + a
-    # warm-air main). Compare against a non-heat-network baseline at the
-    # same 0.80 water efficiency (whc 901 from the same gas main): the 950
-    # HW fuel must exceed it by exactly the DLF (age E → 1.41).
+    # (boilers)". The Table 12c distribution loss must fire on the WHC
+    # ALONE — independent of the main, which here is an ordinary gas boiler
+    # (NOT a heat network), mirroring cert 9093 (whc 950 + a warm-air main).
+    # Compared against the SAME dwelling taking DHW from that gas boiler
+    # (whc 901, Table 4a eff 0.80), the water-only network bills:
+    #
+    #   950: q_useful × (306) DLF 1.41 × (305a) 1.05  = q × 1.4805
+    #   901: q_useful ÷ 0.80                          = q × 1.25
+    #
+    # → the network's delivered fuel is 1.4805/1.25 ≈ 1.18× the boiler's.
+    # The network pays a distribution loss and a flat-rate charging penalty
+    # but NO plant-efficiency divisor (SAP 10.2 §4.3 p.24: that efficiency
+    # is in the Table 12 heat price) — see the absolute-efficiency pin in
+    # `test_water_only_heat_network_dhw_prices_heat_without_plant_efficiency`.
     part = make_building_part(construction_age_band="E")
     gas_main = MainHeatingDetail(
         has_fghrs=False,
@@ -1445,8 +1450,8 @@ def test_hot_water_only_heat_network_whc_950_applies_table12c_dlf() -> None:
     hw_network: float = cert_to_inputs(hw_network_epc).hot_water_kwh_per_yr
     baseline: float = cert_to_inputs(baseline_epc).hot_water_kwh_per_yr
 
-    # Assert — the HW-only-heat-network fuel is scaled up by the age-E DLF.
-    assert abs(hw_network / baseline - 1.41) <= 0.02
+    # Assert — DLF × flat-rate charging, against the boiler's 1/0.80.
+    assert abs(hw_network / baseline - (1.41 * 1.05) * 0.80) <= 0.02
 
 
 def test_water_only_heat_network_dhw_prices_heat_without_plant_efficiency() -> None: