{
 "id": "hp_limit_change_reason",
 "name": "Why the limit moved from 42 to 37 dB(A)",
 "category": "heat_pump_noise",
 "value_num": null,
 "display": "MCS dropped the old method's assumed 40 dB(A) background noise step and set a corresponding limit of 37 dB(A)",
 "unit": "reason for the change, from MCS's consultation response",
 "period_start": "2026-09-30",
 "period_end": null,
 "period_kind": "page_undated",
 "deadline": null,
 "source_url": "https://mcscertified.com/wp-content/uploads/2024/11/MCS-020-Consultation-Response-Final.pdf",
 "source_org": "MCS, consultation response on MCS 020",
 "checked_at": "2026-09-30",
 "recheck_by": "2026-12-29",
 "history": [],
 "public_note": "The old MCS 020 sum, checked against 42 dB(A), had a step that assumed 40 dB(A) of background noise. Where a heat pump came in quieter than that, the installer had to use a figure higher than the heat pump's own. MCS removed that step and set what it calls a corresponding limit of 37 dB(A). MCS tested 48 real assessments both ways. On average they cleared the old 42 by 1.08 dB and the new 37 by 7.3 dB, and every one that passed the old sum also passed the new one. So the lower number is not simply a tougher rule. It is a different sum.",
 "next": null,
 "url": "https://hearthsum.co.uk/api/hp_limit_change_reason.json"
}