How to Size Hot Water Systems Correctly – Avoid Cold Showers in Large Homes

01How to Size Hot Water Systems Correctly – Avoid Cold Showers in Large Homes

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).

02Step 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).

03Step 2: Storage vs Continuous Flow – The Comparison

Typical storage sizes for single-family dwellings (a rule of thumb, not a table in AS/NZS 3500.4 - check the heater maker's sizing guide):

• 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.

04Why 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.

05Step 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)

• 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 work out loading units and probable simultaneous flow from AS/NZS 3500.1:2025 Tables 3.2.1, 3.2.3 and 3.2.4, then check the heater against the maker's sizing guide. 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.

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06Related reading

• Grease Traps for Commercial Kitchens – Sizing & Compliance
• NDIS SDA Plumbing Requirements – Class 3 Dwellings Guide
• Top 5 Estimating Mistakes Plumbers Make (And How to Avoid Them)
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