Determine the required mass flow rate for your hydronic heating system.
Enter the total power output of your boiler or the heat loss calculation for the zone.
Standard radiators typically use 20°C (older) or 11°C (modern/heat pumps). Underfloor usually 5-7°C.
Required Flow Rates
Flow Rate (Litres per Minute):–
Flow Rate (Cubic Metres per Hour):–
Flow Rate (Litres per Second):–
function calculatePumpFlow() {
// Get Input Values
var loadKW = document.getElementById('heatLoadKW').value;
var deltaT = document.getElementById('deltaT').value;
// Validation
if (loadKW === "" || deltaT === "") {
alert("Please fill in both the Heating Load and Delta T fields.");
return;
}
var kw = parseFloat(loadKW);
var dt = parseFloat(deltaT);
if (isNaN(kw) || kw <= 0) {
alert("Please enter a valid positive number for the Heating Load.");
return;
}
if (isNaN(dt) || dt <= 0) {
alert("Please enter a valid positive number for Delta T.");
return;
}
// Calculation Logic
// Specific Heat Capacity of Water (approx) = 4.186 kJ/(kg*K)
// Formula: Flow (l/s) = kW / (Specific Heat * Delta T)
var specificHeat = 4.186;
// 1. Calculate Litres per Second (l/s)
// Note: Density of water changes with temp, but 1kg approx 1 litre is standard for pump sizing.
var flowLPS = kw / (specificHeat * dt);
// 2. Calculate Litres per Minute (l/m)
var flowLPM = flowLPS * 60;
// 3. Calculate Cubic Meters per Hour (m3/h)
var flowM3H = flowLPM * 0.06; // (LPM * 60) / 1000
// Display Results
document.getElementById('resLPS').innerHTML = flowLPS.toFixed(3) + " l/s";
document.getElementById('resLPM').innerHTML = flowLPM.toFixed(2) + " l/min";
document.getElementById('resM3H').innerHTML = flowM3H.toFixed(2) + " m³/h";
// Show result div
document.getElementById('resultsArea').style.display = "block";
}
Understanding Central Heating Pump Flow Rates
Correctly sizing a central heating pump (often called a circulator pump) is critical for the efficiency and longevity of your heating system. If the flow rate is too low, the water travels too slowly, losing too much heat before it reaches the last radiator, resulting in cold spots. If the flow rate is too high, it can cause system noise, increased electricity usage, and accelerated wear on the pipework (due to velocity erosion).
The Golden Formula:
Flow Rate (l/s) = Power (kW) ÷ (Specific Heat Capacity × Temperature Difference)
What is Delta T (ΔT)?
The "Temperature Differential," or Delta T, is the designed temperature drop between the water leaving the boiler (flow) and the water returning to it (return).
20°C (Standard Boiler): Historically, gas and oil boiler systems with radiators were designed with a flow of 80°C and return of 60°C.
11°C (Condensing Boiler): Modern condensing boilers often aim for a smaller differential to ensure the return temperature is low enough for condensation to occur (improving efficiency).
5°C – 7°C (Underfloor Heating): Heat pumps and underfloor systems operate with much higher flow rates and smaller temperature drops.
How to Use This Calculator
Enter Heat Load (kW): Input the total output of your boiler or the calculated heat loss of the zone the pump is serving. If you have a 30kW boiler, enter 30.
Enter Delta T: Select your system's design temperature drop. If you are unsure, 11°C is a safe assumption for modern radiator systems, while 20°C is common for older gravity conversions or high-temp systems.
Calculate: Click the button to see the required flow rate in three common units used by pump manufacturers (Grundfos, Wilo, etc.).
Selecting the Right Pump
Once you have the flow rate (e.g., 1.5 m³/h), you also need to calculate the Head Loss (pressure drop) of the system, which depends on the pipe diameter, length, and number of fittings. Pump selection charts map "Flow Rate" on the horizontal axis and "Head (meters)" on the vertical axis. Your calculated flow rate tells you how much water needs to move; the head tells you how hard the pump needs to push to move that amount.
Why Specific Heat Matters
This calculator uses the specific heat capacity of water (approx. 4.186 kJ/kgK). If you are adding Glycol (antifreeze) to your system, the specific heat capacity drops, meaning you will need a higher flow rate to transfer the same amount of heat.