Summary
- Part G covers cold and hot water supply, hot water safety (the G3 unvented cylinder rules), sanitary facilities and water efficiency — new dwellings must demonstrate 125 litres/person/day (or 110 where a planning condition applies) via the water efficiency calculator.
- Unvented hot water cylinders must be installed by a competent (registered) installer with the full set of safety devices and a compliant discharge pipe — the D1/D2 discharge route is a design matter, not a plumber's improvisation.
- Bath outlet temperature in new dwellings is capped at 48°C (TMV), and as PD you should hold the water calc, G3 certificate and commissioning records in the handover pack.
What the Building Safety Act requires for Part G on non-HRB domestic jobs
Requirements G1–G6 run from wholesome water supply through to food preparation areas. For small practice work the two recurring items are G2 water efficiency — every new dwelling (including conversions creating dwellings) needs a water efficiency calculation showing ≤125 l/person/day, tightened to 110 where the local plan requires it — and G3 hot water supply and systems, which governs unvented cylinders.
G3 is safety-critical: an unvented cylinder stores water under pressure, and the stacked safety devices (thermostat, thermal cut-out, temperature/pressure relief valve) plus a correctly detailed discharge pipe are what stand between a fault and a steam explosion. Installation is notifiable and must be by an operative holding a recognised G3 qualification, with the certificate retained.
As Principal Designer (BSA), your coordination evidence is the water calculation at design stage, the cylinder location and discharge route resolved on drawings (not left to site), and the commissioning certificates in the handover pack.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: confirm whether the 110 l/p/d planning condition applies (common across water-stressed authorities); flag cylinder location early — lofts complicate discharge routes.
- Stage 3: coordinate the D2 discharge pipe route to a safe, visible termination; check sanitary layouts against Part M/T requirements; confirm fittings schedule feeds the water calculator.
- Stage 4: complete the water efficiency calculation from the final fittings schedule; specify TMVs to baths (48°C max in new dwellings); require the G3 installation/commissioning certificate as a handover deliverable.
Standards and references you should be citing
- The Water Efficiency Calculator for New Dwellings (the methodology behind the 125/110 l/p/d figures).
- BS EN 12897 — specification for unvented storage water heaters.
- BS EN 806 series and BS 8558 — design and installation of water supply systems inside buildings.
- BS EN 1111 / BS EN 1287 (TMV2/TMV3 schemes) — thermostatic mixing valves for the 48°C bath outlet limit.
Vented vs unvented hot water: the compliance difference
| Aspect | Vented cylinder | Unvented cylinder (G3) |
|---|---|---|
| Safety devices | Open vent to atmosphere | Thermostat + thermal cut-out + T&P relief valve, all factory-fitted |
| Installer | Competent plumber | G3-qualified registered installer; work is notifiable |
| Discharge pipework | Not applicable | D1/D2 pipe sized and routed to safe visible termination, shown on drawings |
| Evidence to log | Commissioning record | G3 certificate, discharge route drawing, commissioning record |
Common pitfalls that catch out sole practitioners
- Water efficiency calc done from the sales brochure, then failed when the client upgrades to a rainfall shower — recalculate at final fittings selection.
- Discharge pipes terminating invisibly into a boxed-in soil stack — the termination must be safe and apparent so a discharging valve is noticed.
- Assuming the 110 l/p/d standard only applies to volume housebuilders; it binds any new dwelling where the planning condition is imposed, including one-off houses and conversions.
- No TMV on the bath in a new dwelling — a cheap omission that fails final inspection.
Frequently asked questions
Does a domestic extension need a water efficiency calculation?
No — G2 water efficiency applies to new dwellings, including those formed by material change of use. An extension to an existing house does not trigger the calculation, though any new sanitary installations must still meet G1, G3 and G4 as applicable.
Can any plumber install an unvented cylinder?
No. G3 installation is notifiable building work and should be carried out by an installer holding a recognised unvented hot water qualification, typically registered with a competent person scheme. Keep the certificate — it is the compliance evidence for one of the few genuinely dangerous systems in a house.
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