How to Size Hot Water Systems Correctly – Avoid Cold Showers in Large Homes
SNZ Plumbing Estimating – Accurate, Compliant Quotes Every Time
Cold showers during peak morning or evening use are one of the most common complaints from clients in large homes. The key to preventing this is correct sizing per AS/NZS 3500.4:2025 (Heated water services), the national standard referenced in the Plumbing Code of Australia (PCA).
Step 1: Understand Fixture Units & Peak Demand
AS/NZS 3500.4 uses fixture unit ratings (similar to cold water in 3500.1) to estimate simultaneous demand. Common values:
- Shower: 4 units
- Bath: 10 units
- Kitchen sink: 4 units
- Laundry tub: 3 units
For a 4–5 bathroom house (typical large family home):
- Peak use: 3 showers (12 units) + bath (10 units) + kitchen/laundry (7 units) = ~29 units
- Convert to flow: Approximately 30–45 L/min at peak (real-world simultaneous use).
Step 2: Storage vs Continuous Flow – The Comparison
Table 3.1 (Minimum storage capacities for single-family dwellings) gives guidance:
- 4–5 bedrooms + 4–5 bathrooms → 315–400 L storage recommended for electric/gas storage systems.
- But storage has limits: A 400 L tank depletes fast during peak (e.g., multiple showers + bath fill), with recovery taking 1–4 hours depending on type.
Why 26–32 L/min continuous flow gas beats 400 L storage:
- Unlimited supply — Heats water on demand; no tank to run out.
- Handles peak — 32 L/min supports 3–4 showers (9–10 L/min each) + sink/laundry simultaneously without temperature drop (Sydney inlet ~15–20°C in winter gives real flow ~35–40 L/min).
- No standby losses — Only uses gas when hot water is needed (saves $200–$600/year vs storage for high-demand homes).
- Compact — Wall-mounted, saves space in large or multi-level homes.
- Manufacturer rating — Rinnai Infinity 32 and Rheem 32 are explicitly rated for large homes with 4–6 bathrooms in temperate climates.
Step 3: When to Choose Each
Choose 26–32 L/min continuous flow gas for:
- 4+ bathrooms or high simultaneous use
- Long pipe runs (instant hot at distant taps)
- Energy & water efficiency (no waste waiting for hot)
Choose 400 L storage if:
- Client prefers electric off-peak (cheaper running with solar)
- Budget constraint (lower upfront cost)
- Staggered usage (not everyone showers at once)
Pro Tip for Estimators
Always calculate fixture units from AS/NZS 3500.1 Table 3.1 (cold demand) and cross-check with 3500.4 Table 3.1. Add recirculation (ring main + pump) for long layouts — it eliminates waiting and waste.
At SNZ Plumbing Estimating, we size systems accurately using the latest 2025 standards — no undersupply, no overspending.
What’s your biggest hot water sizing headache? Comment below or DM us for a quick check on your next job!
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