Achieving the correct flow rate is critical for the efficiency and longevity of an Air Source Heat Pump (ASHP) or Ground Source Heat Pump (GSHP). Unlike traditional gas boilers that often operate with high temperature differentials (ΔT of 20°C), heat pumps thrive on a narrower ΔT (usually 5°C to 7°C). If the flow rate is too low, the heat pump may cycle frequently, enter "high pressure" error states, or fail to extract enough heat to keep the property warm.
The Flow Rate Formula
The calculation is based on the physics of heat transfer. To find the mass flow rate, we use the following equation:
Flow Rate (L/s) = P / (Cp × ΔT)
Where:
P = Heat Output / Design Load (kW)
Cp = Specific Heat Capacity of the fluid (Water is approx. 4.18)
ΔT = Temperature difference between the flow and return pipes.
Typical ΔT Values for Heat Pumps
Most modern heat pump manufacturers design their systems around a ΔT of 5K. Increasing the ΔT (reducing the flow rate) can lead to a lower COP (Coefficient of Performance) because the compressor must work harder to reach higher temperatures. Conversely, a very low ΔT requires much larger pumps and can lead to excessive noise in the pipework.
Example Calculation
Suppose you have a 7kW heat pump load and are aiming for a ΔT of 5°C using water as the medium:
Load: 7 kW
Specific Heat: 4.18
ΔT: 5
Flow (L/s) = 7 / (4.18 × 5) = 0.335 L/s
Flow (L/min) = 0.335 × 60 = 20.1 L/min
Impact of Glycol
If your system contains anti-freeze (Glycol), the specific heat capacity is lower than pure water. This means you need a higher flow rate to carry the same amount of heat. Using this calculator, you can select glycol mixes to see how it affects your pump requirements.
function calculateFlowRate() {
var power = parseFloat(document.getElementById('hpHeatLoad').value);
var deltaT = parseFloat(document.getElementById('hpDeltaT').value);
var cp = parseFloat(document.getElementById('hpFluid').value);
var resultDiv = document.getElementById('hpResult');
if (isNaN(power) || isNaN(deltaT) || power <= 0 || deltaT <= 0) {
alert("Please enter valid positive numbers for Heat Load and Delta T.");
return;
}
// Formula: Liters per second = Power (kW) / (Cp * DeltaT)
var lps = power / (cp * deltaT);
// Convert to L/min and L/h
var lpm = lps * 60;
var lph = lpm * 60;
var m3h = lph / 1000;
// Display results
document.getElementById('hpDisplayLpm').innerHTML = lpm.toFixed(2) + ' Liters/Min (L/min)';
document.getElementById('hpDisplayLph').innerHTML = lph.toFixed(0) + ' Liters/Hour (L/h)';
document.getElementById('hpDisplayM3h').innerHTML = m3h.toFixed(3) + ' m³/h';
resultDiv.style.display = 'block';
// Smooth scroll to result
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}