diff --git a/scripts/hyde/elmhurst_lib.py b/scripts/hyde/elmhurst_lib.py
index fb99966fb..72f2fb9bc 100644
--- a/scripts/hyde/elmhurst_lib.py
+++ b/scripts/hyde/elmhurst_lib.py
@@ -467,6 +467,63 @@ def select_boiler(page: Page, query: str, ref: str) -> str:
     return "ok"
 
 
+def clear_main_heating_code(page: Page) -> None:
+    """Empty Main Heating 1's SAP code via a real server postback (set the
+    cascade's top-level dropdown to '' + OK). REQUIRED before `set_pcdb_boiler`:
+    while any code is set, the PCDF reference textbox and the boiler-search
+    magnifying glass both render `aspNetDisabled`. They only enable when the
+    code is empty server-side (a JS-clear alone flips the class cosmetically but
+    leaves the postback unwired)."""
+    page.evaluate(
+        "(id)=>{const e=document.getElementById(id); if(e)e.click();}",
+        f"{MH1}ButtonMainHeatingCode",
+    )
+    page.wait_for_timeout(SETTLE_MS * 3)
+    page.evaluate(
+        """(id)=>{const s=document.getElementById(id);if(s){s.value='';
+        s.dispatchEvent(new Event('change',{bubbles:true}));}}""",
+        f"{HEATING_DIALOG}DropDownList1",
+    )
+    page.wait_for_timeout(SETTLE_MS * 3)
+    dialog_commit(page, "OK")
+    page.wait_for_timeout(SETTLE_MS * 2)
+
+
+def set_pcdb_boiler(page: Page, pcdb_index: "int | str") -> str:
+    """Set Main Heating 1 to the EXACT PCDB boiler by its EPC
+    `main_heating_index_number` (e.g. 18908 = Worcester Greenstar 4000,
+    GR4700iW 25 C NG, 88.70%). Elmhurst resolves it to the product's PCDB
+    winter efficiency — the same value the SAP engine uses — so the worksheet
+    and our calculator compare like-for-like (not against the generic BGW
+    SAP-Table 84%). Returns the resolved boiler description.
+
+    Hard-won mechanism (supersedes the old "PCDB disabled" note):
+      1. The PCDF reference textbox is only writable while the main heating CODE
+         is empty — call `clear_main_heating_code(page)` first.
+      2. The index must be entered via REAL keyboard + Tab. A JS-set `.value`
+         does NOT survive the field's AutoPostBack, so the boiler never
+         resolves (the original failure mode). Ctrl-A + type + Tab works.
+      3. The `TextBoxPCDFBoilerReference` direct-entry path is far simpler than
+         the magnifying-glass grid (whose modal has no layout box, so
+         mouse/Playwright clicks can't target rows). The grid search DOES work
+         (JS-focus the FullSearch box + `keyboard.type` + fire
+         HyperLinkActionBoilerSearch → "Ref No" column == the PCDB index) if a
+         name lookup is ever needed, but direct entry is preferred."""
+    fld = f"{MH1}TextBoxPCDFBoilerReference"
+    page.evaluate(
+        "(id)=>{const e=document.getElementById(id);if(e){e.focus();e.select();}}", fld
+    )
+    page.wait_for_timeout(SETTLE_MS)
+    page.keyboard.press("Control+a")
+    page.keyboard.type(str(pcdb_index), delay=40)
+    page.keyboard.press("Tab")
+    page.wait_for_timeout(SETTLE_MS * 5)  # AutoPostBack resolves the PCDB record
+    return page.evaluate(
+        """()=>{const e=document.querySelector('[id*=LabelMHSSapCodeDescription]');
+        return e?e.innerText.trim():'';}"""
+    )
+
+
 def read_default_uvalue(
     page: Page, suffix: Optional[str] = None, label_regex: str = r"Default U-value"
 ) -> Optional[float]: