What ΔT should I aim for when balancing radiators?

The target ΔT (difference between flow and return temperature at each radiator) should be 11-12°C for a standard gas boiler system. This ensures each radiator receives the correct proportion of flow for its heat output. For heat pump systems, a ΔT of 5°C is more typical.

How do I balance a heating system?

Turn off the system and open all lockshield valves fully. Turn the system on and allow to reach temperature. Working from the nearest radiator to the furthest from the boiler, close each lockshield until you achieve an 11-12°C ΔT measured with clip-on pipe thermometers on the flow and return pipes.

What is a lockshield valve Kv value?

Kv is the flow coefficient of a valve, representing the flow rate in m³/hr at a pressure drop of 1 bar. A lower Kv means more restriction. Lockshield valves are adjusted (partially closed) to restrict flow to radiators closest to the pump, directing more flow to distant radiators.

Why is heating balancing important for condensing boilers?

A condensing boiler achieves its highest efficiency when the return water temperature is below 55°C (the condensing threshold). Proper system balancing with an 11-12°C ΔT ensures the return temperature stays low enough for condensing operation, saving up to 10-12% on gas bills.

How do I calculate the flow rate for each radiator?

Flow rate (L/min) = BTU/hr × 0.000293071 (conversion to kW) × 1000 ÷ (4.186 × ΔT × 60). For example, a 4,000 BTU radiator with 11°C ΔT needs about 0.42 L/min of flow.

TradeCalculator

Heating Balancing Calculator — Radiator Flow Rates

Calculate the required flow rate per radiator for system balancing. Target ΔT of 11-12°C across each radiator with lock-shield Kv settings.

Total System Output

Total heating system output

Number of Radiators

Total radiators on system

Radiator 1 Output

Nearest to boiler

Radiator 2 Output

Second nearest

Radiator 3 Output

Third in circuit

Radiator 4 Output

Furthest from boiler

How We Calculate This

Heating system balancing ensures each radiator receives the correct flow rate proportional to its heat output. This calculator determines the required flow rate and lock-shield valve setting for each radiator.

Formulas

Flow rate (L/min): (kW × 1000) ÷ (4.186 × ΔT × 60)
BTU to kW: BTU/hr × 0.000293071
Return temp: Flow temp − ΔT
Kv value: Flow ÷ (√ΔP × 1000)

Enter up to four radiator BTU outputs to calculate individual flow rates. Use clip-on pipe thermometers to verify the ΔT at each radiator during the balancing process.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.