Radiator Flow Rate Calculator
What is Radiator Flow Rate?
The radiator flow rate is the volume of heated water that passes through a radiator per unit of time to deliver the required thermal energy to a room. Calculating the correct flow rate is essential for "balancing" a heating system. If the flow rate is too high, the water doesn't stay in the radiator long enough to release its heat; if it's too low, the radiator won't get hot enough to meet the room's heat loss demands.
The Science Behind the Calculation
The calculation is based on the specific heat capacity of water. To move a certain amount of heat energy using water as the medium, we use the following physics formula:
Where:
- P: Heat output required (Watts)
- c: Specific heat capacity of water (approx. 4184 Joules/kg°C)
- ΔT: The temperature difference between the water entering the radiator (Flow) and leaving it (Return).
Typical Temperature Drops (ΔT)
In the UK and Europe, a standard gas boiler system is usually designed for a ΔT of 11°C. However, modern condensing boilers are often more efficient at a ΔT of 20°C. Heat pumps typically operate with smaller temperature drops, often around 5°C to 8°C, requiring higher flow rates and larger pipework to move the same amount of heat.
Practical Example
If you have a radiator with an output of 2000 Watts on a standard system with an 11°C temperature drop:
1. Convert Watts to Joules per hour: 2000W × 3600s = 7,200,000 J/h.
2. Calculate heat required per degree: 4184 J/kg°C × 11°C = 46,024 J/kg.
3. Divide: 7,200,000 / 46,024 ≈ 156.44 kg/h (approx 156.44 Liters per hour).