diff --git a/.claude/skills/expand-sap-accuracy-corpus/worklist.md b/.claude/skills/expand-sap-accuracy-corpus/worklist.md
index 3838dbc8e..b34a142da 100644
--- a/.claude/skills/expand-sap-accuracy-corpus/worklist.md
+++ b/.claude/skills/expand-sap-accuracy-corpus/worklist.md
@@ -405,7 +405,7 @@ methodology) is still open — see comment in test_real_cert_sap_accuracy.py.
 All 20 pinned as `RealCertExpectation`s to the engine's OWN observed output.
 - [x] 🔧 100010359769 — SAP-15.0 · eng 68 (2011 lodged 68, not a target)
 - [x] 🔧 100010359788 — SAP-15.0 · eng 72 (2011 lodged 73, not a target)
-- [x] 🔍 100010086084 — SAP-15.0 · eng 53, Elmhurst worksheet 51 (2011 lodged 45, not a target) · FULL Elmhurst RdSAP10 rebuild REDONE 2026-07-10 — the prior 46/48 readings were CONTAMINATED (stale open-chimneys=2 and wall-thickness=280mm carried over from a PRIOR cert build in the shared Elmhurst session; the build script never explicitly zeroed `TextBoxOpenChimneys`). Fixed the build script (explicit chimney zeroing + wall thickness set to 220mm, since `CheckBoxWallThicknessUnknown` doesn't persist via Playwright and leaving it blank makes the Walls section invalid) and a real build-script bug (`ref.input_value()` truthiness check treated the string `"0"` as truthy, causing spurious navigation to Home.aspx that broke the SpaceHeating nav). Clean rebuild: eng 53 vs Elmhurst 51 — only a ~2pt gap now (was ~5-7pt). No mapper.py change was needed/made — the gov-API mapper's open_chimneys_count=0 was correct all along; the register itself lodges open_fireplaces_count=0. Remaining ~2pt gap not chased further: single-window-orientation approximation (South, max solar gain) plus untriaged minor divergences. See test_real_cert_sap_accuracy.py comment for full detail, incl. a possible Elmhurst-PDF-parser mismatch (cylinder size / boiler flue type) noted but out of scope.
+- [x] 🔍 100010086084 — SAP-15.0 · eng 53, Elmhurst worksheet 51 (2011 lodged 45, not a target) · FULL Elmhurst RdSAP10 rebuild REDONE 2026-07-10 — the prior 46/48 readings were CONTAMINATED (stale open-chimneys=2 and wall-thickness=280mm carried over from a PRIOR cert build in the shared Elmhurst session; the build script never explicitly zeroed `TextBoxOpenChimneys`). Fixed the build script (explicit chimney zeroing + wall thickness set to 220mm, since `CheckBoxWallThicknessUnknown` doesn't persist via Playwright and leaving it blank makes the Walls section invalid) and a real build-script bug (`ref.input_value()` truthiness check treated the string `"0"` as truthy, causing spurious navigation to Home.aspx that broke the SpaceHeating nav). Clean rebuild: eng 53 vs Elmhurst 51 — only a ~2pt gap now (was ~5-7pt). No mapper.py change was needed/made — the gov-API mapper's open_chimneys_count=0 was correct all along; the register itself lodges open_fireplaces_count=0. Remaining ~2pt gap not chased further: single-window-orientation approximation (South, max solar gain) plus untriaged minor divergences. FOLLOW-UP (2026-07-10): root-caused + fixed the cylinder_size / boiler_flue_type mismatches, both (a) Elmhurst input errors (same silent-stale-value class as chimneys/wall-thickness) — build script's cylinder-size select assumed option VALUE==numeric code, but this DOM's option values are the visible litre-band text, so it silently no-opped and left a prior cert's "Medium" selected (fixed: match by text, "Normal"); the boiler-code dialog's combined "Balanced/Open Flue" category doesn't drive the separate FlueType field, which was left at a stale "Balanced" (fixed: explicit select to "Open"). Added mapper dict entries for the litre-suffixed cylinder labels (mapper.py `_ELMHURST_CYLINDER_SIZE_LABEL_TO_SAP10`). boiler_flue_type isn't consumed by the calculator at all (ML/generator-only), so zero SAP-score effect; cylinder fix moved Elmhurst-PDF-inputs SAP 46→47 only. gov-API SAP (53) and Elmhurst worksheet (51) unchanged. Full suite re-run clean (67 passed/67 skipped/1 xfailed). See test_real_cert_sap_accuracy.py comment for full detail.
 - [x] 🔧 100010090369 — SAP-15.0 · eng 53 (2011 lodged 47, not a target) · ditto
 - [x] 🔧 100010074470 — SAP-15.0 · eng 46 (2011 lodged 39, not a target) · ditto
 - [x] 🔧 100010100813 — SAP-15.0 · eng 59 (2011 lodged 47, not a target) · ditto
diff --git a/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_summary.pdf b/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_summary.pdf
index b67bc1da4..6ce8e8ebb 100644
Binary files a/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_summary.pdf and b/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_summary.pdf differ
diff --git a/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_worksheet.pdf b/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_worksheet.pdf
index 887cf0bc0..00c6fb183 100644
Binary files a/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_worksheet.pdf and b/backend/epc_api/json_samples/real_life_examples/SAP-Schema-15.0/uprn_100010086084/elmhurst_worksheet.pdf differ
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index da1753453..20ef69613 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -7555,7 +7555,9 @@ _ELMHURST_CYLINDER_SIZE_LABEL_TO_SAP10: Dict[str, int] = {
     # The live RdSAP10-Online tool renders the size band WITH its litre range
     # suffix (observed on the SAP-Schema-15.0 UPRN 100010086084 build) rather
     # than the bare label the cohort fixtures used above.
+    "Normal (Less than 130L)": 2,
     "Medium (131-170L)": 3,
+    "Large (Greater than 171L)": 4,
 }
 
 
diff --git a/scripts/hyde/build_100010086084.py b/scripts/hyde/build_100010086084.py
index fb67bfa65..a700725da 100644
--- a/scripts/hyde/build_100010086084.py
+++ b/scripts/hyde/build_100010086084.py
@@ -5,7 +5,7 @@ UNKNOWN, not a specific mm value), pitched 300mm loft insulation, SOLID
 ground floor uninsulated, mains-gas generic (no-PCDB) REGULAR boiler — SAP
 Table 4b code 115 (pre-1998, balanced/open flue, "BGL Pre 98 Wall mount
 BF/OF") + programmer/room-thermostat control (2104/CBC) + radiators +
-cylinder (size 2 = "Medium (131-170L)" on the live tool, foam, 25mm), gas
+cylinder (size 2 = "Normal (Less than 130L)" on the live tool, foam, 25mm), gas
 room-heater SECONDARY (603, approximated as RGB "Modern fire with open flue
 post 1980" -- the cert gives no finer detail), TFA 68, single meter,
 2-storey / 2 floor-dims (no extensions). Partial double glazing
@@ -244,6 +244,15 @@ def space_heating(page):
     E.set_heating_dialog(page, f"{MH1B}ButtonMainHeatingControls",
                          "^Boilers", "^Standard", "CBC Programmer and room thermostat")
     print("control:", page.locator(f"#{E.MH1}TextBoxMainHeatingControls").input_value())
+    # Explicit flue type (BUG fixed): the "Balanced/Open Flue" boiler-code
+    # CATEGORY picked above is a combined Table 4b group name (BF/OF) that
+    # does NOT set the separate `RadioButtonListFlueType` select on this
+    # page -- that field kept its inherited/default "Balanced" from a
+    # prior cert (silent-stale-value class of bug, see docstring), which
+    # mismatches the lodged gov-API `boiler_flue_type=1` ("Open"). Set it
+    # explicitly to match.
+    E.set_select(page, f"{MH1B}RadioButtonListFlueType", "Open")
+    print("flue type:", page.locator(f"#{E.FP}{MH1B}RadioButtonListFlueType").input_value())
     E.save_close(page)
 
 
@@ -269,12 +278,18 @@ def water_heating(page):
     E.set_heating_dialog(page, f"{WH}ButtonWaterHeatingCode",
                          "From Space Heating", "From the primary heating system")
     print("water code:", page.locator(f"#{E.FP}{WH}TextBoxWaterHeatingCode").input_value())
-    # cylinder: size 2 (lodged cylinder_size code -- set_select matches by
-    # <option VALUE>, not visible text; value "2" renders as "Medium
-    # (131-170L)" on the live tool -- see the mapper fix for that label).
-    # foam insulation (type 1), 25mm (value happens to equal visible text
-    # here: "25 mm").
-    E.set_select(page, f"{WH}DropDownListCylinderSize", "2")
+    # cylinder: lodged cylinder_size code 2 = SAP10 "Normal" (110 L; see
+    # cert_to_inputs.py _CYLINDER_SIZE_CODE_TO_LITRES). BUG (fixed): this
+    # dropdown's <option VALUE> IS its visible text on this build ("No
+    # Access" / "Normal (Less than 130L)" / "Medium (131-170L)" / "Large
+    # (Greater than 171L)" / "Value known") -- passing the bare numeric
+    # code "2" to `set_select` silently matched nothing (Playwright
+    # select_option no-ops on a non-matching value) and left the field at
+    # whatever a PRIOR cert's build had set ("Medium (131-170L)"), the
+    # same silent-failure/stale-value contamination class as the wall-
+    # thickness gotcha documented in this file's docstring. Match by the
+    # real option text instead.
+    E.select_by_contains(page, f"{WH}DropDownListCylinderSize", "Normal")
     E.set_select(page, f"{WH}DropDownListInsulated", "Foam")
     E.set_select(page, f"{WH}DropDownListInsulationThickness", "25 mm")
     print("cylinder size:", page.locator(f"#{E.FP}{WH}DropDownListCylinderSize").input_value())
diff --git a/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py b/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
index 18987de3a..841eb753d 100644
--- a/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
+++ b/tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
@@ -1182,14 +1182,53 @@ _EXPECTATIONS: Final[tuple[RealCertExpectation, ...]] = (
     # — true multi-orientation entry does not reliably stick, see
     # elmhurst_lib.py). South-facing (max solar gain) was used here as the
     # fairer single-orientation bound.
-    # Also noted but NOT chased further (secondary to the primary gov-API vs
-    # Elmhurst-worksheet reconciliation, and outside this mapper): the
-    # Elmhurst-PDF ingestion path (`_map_elmhurst_*` in mapper.py) parses this
-    # rebuild's cylinder size as code 3 ("Large") though "Medium (131-170L)"
-    # (code 2) was selected in the tool, and boiler_flue_type as 2 though
-    # "Balanced/Open Flue" (code 1) was selected — possible PDF-parser
-    # mismatches worth a follow-up if the Elmhurst-PDF-inputs path is ever
-    # made a reconciliation target for this cert.
+    # UPDATE (2026-07-10, follow-up): the two flagged cylinder_size /
+    # boiler_flue_type mismatches were root-caused and fixed — both were (a)
+    # Elmhurst input errors, the same silent-stale-value contamination class
+    # as the chimneys/wall-thickness bugs above, NOT parser or mapper bugs:
+    #   * cylinder_size: `water_heating()`'s `E.set_select(...,
+    #     "DropDownListCylinderSize", "2")` assumed the <option> VALUE
+    #     attribute was the bare numeric SAP code, but on this build's DOM
+    #     the option VALUE *is* its visible text (`select_options()` dump:
+    #     "No Access" / "Normal (Less than 130L)" / "Medium (131-170L)" /
+    #     "Large (Greater than 171L)" / "Value known" — no "2" among them).
+    #     Playwright's `select_option("2")` silently no-ops on a non-matching
+    #     value, leaving the field at "Medium (131-170L)" inherited from a
+    #     prior cert. Confirmed live: `.input_value()` read back "Medium
+    #     (131-170L)" before the fix. Fixed the build script to match by
+    #     visible text (`E.select_by_contains(..., "Normal")`) — verified
+    #     `.input_value()` now reads "Normal (Less than 130L)", matching the
+    #     lodged gov-API `cylinder_size=2` (Normal/110L per
+    #     `cert_to_inputs.py` `_CYLINDER_SIZE_CODE_TO_LITRES`). Added the
+    #     matching mapper dict entries for the litre-range-suffixed labels
+    #     ("Normal (Less than 130L)": 2, "Large (Greater than 171L)": 4) next
+    #     to the existing "Medium (131-170L)": 3 entry in
+    #     `_ELMHURST_CYLINDER_SIZE_LABEL_TO_SAP10` (mapper.py) — a real
+    #     mapper-coverage gap, since the calculator raises
+    #     `UnmappedElmhurstLabel` rather than silently mis-mapping.
+    #   * boiler_flue_type: the boiler-code SEARCH DIALOG's "Balanced/Open
+    #     Flue" (matched via `"Balanced.*Open"`) is a combined Table 4b
+    #     BOILER CATEGORY name (covers both flue types under one SAP code,
+    #     115) — it does NOT set the separate, independently-selectable
+    #     `RadioButtonListFlueType` field on the SpaceHeating page (options:
+    #     "Balanced" / "Open"). That field was left at its inherited/default
+    #     "Balanced" from a prior cert (confirmed live via DOM dump: selected
+    #     option was "Balanced" pre-fix), which the PDF/parser correctly read
+    #     as code 2 — mismatching the lodged gov-API `boiler_flue_type=1`
+    #     ("Open"). This field is NOT consumed by `Sap10Calculator`/
+    #     `cert_to_inputs.py` (only by ML/recommendation-generator code), so
+    #     it had zero effect on the SAP score either way — but fixed for data
+    #     fidelity: `space_heating()` now explicitly sets
+    #     `RadioButtonListFlueType` to "Open" after picking the boiler code.
+    # Both fixes applied, PDFs re-downloaded (Recommendations page confirmed
+    # clean beforehand). Elmhurst-PDF-inputs path moved 46 → 47 SAP (cylinder
+    # volume correction, `HW` 3436 kWh vs the prior smaller "Large" reading);
+    # this engine's gov-API SAP (53) and Elmhurst's own worksheet (51) are
+    # unaffected/unchanged by this follow-up — no calculator-input field for
+    # THIS reconciliation target actually depended on either mismatch.
+    # Full `test_real_cert_sap_accuracy.py` + `test_sap_accuracy_corpus.py`
+    # suites re-run clean after the mapper dict addition (67 passed, 67
+    # skipped, 1 xfailed — same as before the change).
     RealCertExpectation(
         schema="SAP-Schema-15.0",
         sample="uprn_100010086084",