Combi Flow Rate Calculator — Litres Per Minute

Flow rate (L/min) = (kW × 60) ÷ (4.186 × ΔT), where 4.186 kJ/kg·K is the specific heat of water — this is standard heat-balance physics (Q = mass flow × specific heat × temperature rise), not a single Building Regulation clause. For a 30 kW combi at 35°C rise: (30 × 60) ÷ (4.186 × 35) = 12.3 L/min. A 35°C rise is the most common UK manufacturer quoting basis (10°C cold in → 45°C hot out), but some specs quote at 30°C or 40°C — always check the temperature rise printed on the data badge before comparing models.

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. Check your available pressure with our Water Pressure Calculator.

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. Use our Boiler Size Calculator to compare options, or our Hot Water Cylinder Calculator if switching to stored hot water.

Most combi boilers fire when the DHW flow switch detects roughly 2-3 L/min, though the exact flow-switch threshold is model-specific (typically around 1-2.5 L/min) and is rarely printed on the data badge — check the manufacturer manual if in doubt. If your flow rate is too low (e.g. from partially closed taps or restrictive showerheads), the boiler may not fire. Some models set DHW activation higher, around 3.5 L/min.

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.