UFH Flow Rate Calculator
The total size of the room or heating loop.
Typical new build: 50-70 W/m², Older: 100+ W/m².
Difference between flow and return temp. Standard is 7°C.
Water (100%)
Glycol Mix (30%)
Glycol Mix (50%)
Glycol reduces heat capacity, requiring higher flow.
Recommended Flow Rate
0.0 L/min
Total Circuit Power: 0 Watts
Mass Flow: 0 kg/s
Understanding Underfloor Heating Flow Rates
Correctly setting the flow rate on your underfloor heating (UFH) manifold is critical for system efficiency and comfort. If the flow rate is too low, the water will cool down excessively before reaching the end of the loop, leaving cold spots in the floor. If the flow rate is too high, you waste energy pumping water faster than necessary, and the boiler may not condense efficiently.
Why do we calculate Flow Rate?
Every UFH circuit (loop) has a specific heat requirement based on the floor area it covers and the heat loss of the room. To deliver this energy, a specific volume of warm water must pass through the pipes every minute. This calculator helps you determine the exact setting for the flow meters (toptometers) on your manifold.
The Calculation Formula
The flow rate is calculated using the thermodynamic formula relating Power ($P$), Specific Heat Capacity ($c$), and Temperature Difference ($\Delta T$):
Flow (kg/s) = Power (W) / (Specific Heat (J/kg°C) × ΔT)
To make this practical for manifold settings, we convert the result into Liters per Minute (L/min). The typical variables involved are:
- Zone Area (m²): The square footage of the floor being heated by this specific loop.
- Heat Output (W/m²): The energy density required to heat the room. Modern well-insulated homes typically need 50-70 W/m², while older conservatories or poorly insulated rooms might need 100-120 W/m².
- ΔT (Delta T): The temperature drop between the water entering the floor (Flow) and leaving it (Return). A standard design uses a 7°C drop (e.g., 45°C flow, 38°C return). Heat pumps often run on a tighter Delta T (e.g., 5°C).
- Specific Heat Capacity: Pure water carries heat more efficiently than water mixed with antifreeze (Glycol). If you use Glycol, you need a higher flow rate to move the same amount of heat.
How to Balance Your Manifold
Once you have calculated the required flow rate for each zone using the tool above:
- Turn on the UFH pump and ensure all actuators are open (calling for heat).
- Locate the flow meters on the manifold (usually the clear glass tubes on the top bar).
- Unlock the flow meter (often by lifting a red locking ring or removing a cap).
- Twist the flow meter to adjust the red float until it hovers at the calculated L/min value.
- Lock the meter back in place.
Troubleshooting Tips
- Flow rate won't go high enough: This may indicate air in the system, a kinked pipe, or an undersized pump. Ensure the system is fully bled.
- Room is too cold: If the floor finish is thick (like thick carpet or wood), you may need to increase the flow temperature or the flow rate slightly above the calculation.
- Short loops: Very short loops (e.g., a small bathroom) require very low flow rates (often under 1 L/min). Be careful not to restrict flow completely.