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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.
Whole-house boiler duty — sets the Total System Flow. The 4 radiators below are a sample circuit, so their branch flows only sum to this if every emitter is entered.
Nearest to boiler
Second nearest
Third in circuit
Furthest from boiler
Flow−return difference across each radiator. 11-12°C is the common UK field-balancing target for gas systems; ~5°C for heat pumps.
Boiler flow temperature
Pressure drop across LSV/TRV — typically 0.05-0.15 bar (Grundfos UPS2 / CIBSE Guide B1)
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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 × 60) ÷ (4.186 × ΔT)
- BTU to kW: BTU/hr × 0.000293071
- Return temp: Flow temp − ΔT
- Kv value (DIN EN 60534): Kv = Q(m³/h) ÷ √ΔP(bar), where Q(m³/h) = flow(L/min) × 0.06
Kv is the valve flow coefficient — the flow in m³/h that passes through the fully-open valve at a 1 bar pressure drop. The lock-shield is closed down toward this Kv to throttle each radiator's flow.
The headline Total System Flowis the boiler's whole-house duty (from the Total System Output you enter). The four radiators are a worked sample circuit, so their branch flows will only sum to the total flow if every emitter on the system has been entered — expect the sample branch flows to be smaller than the headline total.
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. Commissioning and balancing of water-based heating systems is covered by BS EN 14336.
Frequently Asked Questions
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. Use our Radiator Sizing Calculator to check each radiator's output matches the room's heat loss.
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.
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.
A condensing boiler only condenses (and reaches its highest efficiency) when the return water temperature is below about 55°C — the natural-gas flue-gas dew point. What matters is the actual return temperature, not the ΔT alone: return = flow − ΔT, so you also need a low enough flow temperature. At 70°C flow with an 11°C ΔT the return is 59°C, which is too hot to condense; dropping the flow to around 60-65°C (giving a 49-54°C return at the same ΔT) keeps the boiler condensing. Good balancing then ensures every radiator achieves that ΔT so the boiler sees a consistently low return.
Flow rate (L/min) = (BTU/hr × 0.000293071 × 60) ÷ (4.186 × ΔT). For example, a 4,000 BTU radiator is 1.172 kW; at 11 °C ΔT this gives (1.172 × 60) ÷ (4.186 × 11) ≈ 1.53 L/min (heat equation per the CIBSE Domestic Heating Design Guide; balancing and commissioning to BS EN 14336). Use our Radiator Sizing Calculator to find each radiator's BTU output, and our TRV Sizing Calculator for valve settings.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.