How is combi boiler flow rate calculated?

Flow rate (L/min) = (kW × 60) ÷ (4.186 × ΔT). For a 30 kW combi at 35°C rise: (30 × 60) ÷ (4.186 × 35) = 12.3 L/min. The 35°C rise is the UK industry standard test condition (from 10°C to 45°C).

What flow rate do I need for a good shower?

A comfortable shower needs 8-12 litres per minute. A power shower or rainfall head may need 12-15 L/min. A 28-30 kW combi delivers about 11-12 L/min at 35°C rise, which is adequate for one shower. Running a second outlet simultaneously will reduce flow to each.

Can a combi boiler run two showers at once?

Generally no. A standard 30 kW combi delivers about 12 L/min at 35°C rise, which would split to only 6 L/min per shower — below the minimum for a comfortable shower. For two simultaneous showers, consider a 40+ kW combi or a system boiler with stored hot water.

What is the minimum flow rate for a combi boiler?

Most combi boilers need a minimum flow rate of 2-3 L/min to detect flow and ignite the burner. If your flow rate is too low (e.g. from partially closed taps or restrictive showerheads), the boiler may not fire. Some modern combis ignite at as low as 1.5 L/min.

Does water temperature affect combi flow rate?

Yes significantly. In summer, incoming mains water may be 15-18°C, needing only a 25-30°C rise — giving higher flow rates. In winter, mains water drops to 5-8°C, requiring a 40-45°C rise and reducing flow rate. Always size for worst-case winter conditions.

TradeCalculator

Combi Flow Rate Calculator — Litres Per Minute

Calculate combi boiler hot water flow rate at different temperature rises. Check if a combi can meet simultaneous outlet demand.

Boiler Size

DHW output (not heating output)

Temperature Rise

Required temperature rise (35\u00B0C standard)

Inlet Temperature

Cold mains water temperature

Primary Outlet

Main outlet being assessed

Simultaneous Outlets

Number of outlets running at once

How We Calculate This

This calculator determines the hot water flow rate a combi boiler can deliver at various temperature rises, helping assess whether it can meet the property's demand.

Formula

Flow rate (L/min): (kW × 60) ÷ (4.186 × ΔT)
Standard test: 35°C rise (10°C to 45°C)
Summer: ~25°C rise gives higher flow
Winter: ~45°C rise gives lower flow
Effective kW: Rated kW × efficiency factor

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.