Forest Hill Water Hardness & Quality Report (2026)

Thames Water supplies Forest Hill in the London Borough of Lewisham in south-east London, via the Thames Water south London ring main drawing from Bough Beech Reservoir in the Eden Valley of west Kent and from the River Thames at Walton. At 327 mg/L (22.9°Clark) and a TDS of 967.3 mg/L — approaching 1000 mg/L TDS — Forest Hill's water is extremely hard, placing it among the very hardest tap water supplies anywhere in Greater London and rivalling the most extreme confined chalk aquifer zones in the entire UK.

The Forest Hill and Honor Oak supply zone within the Thames Water south London distribution network appears to receive an exceptional chalk mineral fraction — possibly reflecting the routing of a high proportion of the most mineralised North Downs Chalk source water from Bough Beech through this specific south-east London sub-zone. The TDS of 967.3 mg/L places Forest Hill in the same extreme category as the deeply confined chalk aquifer supplies of Swanscombe (344.5 mg/L, TDS 1031.7 mg/L) and Royston (316.5 mg/L, TDS 948.3 mg/L) — a quite extraordinary degree of mineralisation for an inner London supply.

At 327 mg/L, limescale is an extreme and relentless household challenge in Forest Hill. Kettles must be descaled weekly to prevent rapid element failure. The combi-boiler faces very high risk of premature breakdown without a properly fitted, annually replaced scale inhibitor and frequent professional servicing of the heat exchanger. Washing-up liquid barely lathers without generous quantities of product. Taps, shower heads, and all basin fittings develop heavy chalk encrustation within days; at minimum a weekly descaling with white vinegar or a proprietary product is essential. A whole-house water softener is very strongly recommended for every Forest Hill household to protect all appliances and plumbing.