Heating and Chilled Water Flow Rate Calculator

The formula

Every heating or cooling circuit follows the same energy balance: the heat delivered equals the mass of water moved, times its specific heat, times the temperature drop across the load.

Q is the heat load in kW, ṁ the mass flow in kg/s, c_p the specific heat in kJ/(kg·K) and ΔT the difference between supply and return in kelvin (the same number as in °C). To get volume flow, divide the mass flow by the density. For water, c_p is about 4.19 kJ/(kg·K) and barely changes between 10 and 90 °C; density falls from about 1000 to 965 kg/m³ over the same range, which is why the calculator corrects for temperature.

A handy shortcut for water: flow in m³/h ≈ 0.86 × kW ÷ ΔT. For 500 kW at 80/60 that gives 0.86 × 500 ÷ 20 ≈ 21.5 m³/h; the exact value with hot-water density is about 22 m³/h.

Why ΔT is the number to watch

The temperature difference does more to the size and running cost of a system than almost anything else. Take that same 500 kW load. At 80/60 it needs about 22 m³/h. Run it at 80/70 and the flow doubles to around 44 m³/h. Pressure drop in the pipes rises roughly with the square of flow, so pumping power climbs about eightfold in theory, and in practice the pump simply cannot deliver and the far end of the network goes cold.

Once you know the flow, the pipe size calculator picks a standard diameter for it. This is the classic symptom of a poorly balanced heating network: return temperatures creep up, ΔT collapses, pumps run flat out and consumers at the end of the line still complain. If your measured ΔT is well below the design value, the fix is usually balancing valves and controls, not a bigger pump.

Typical temperature schedules

Notice how low-temperature systems and chilled water need several times more flow per kW than a classic radiator circuit. That is the hidden cost of efficient heat pumps and comfortable underfloor heating: bigger pipes and more pumping, which is why ΔT has to be designed in from the start.

Two other numbers usually come up at the same stage of a design. How much water the system holds decides the size of the expansion vessel, which the expansion tank calculator works out. And how much of the heat you produce never reaches a radiator is shown by the pipe heat loss calculator.

Glycol mixtures

Antifreeze has lower specific heat than water, so the same load needs more flow. A 30% propylene glycol mix needs roughly 8% more mass flow than water, and its higher viscosity adds pressure drop on top. If a system is switched from water to glycol without rechecking the pump, it will usually fall short of its design capacity.

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Frequently asked questions

Which temperature should I use for the water properties?

The calculator uses the mean of supply and return. Density and specific heat change only slightly across a typical heating range, so the difference between using supply, return or mean temperature is around 1 to 2 percent. If you need the volume flow exactly where the pump sits, which is often the return, that small difference is the reason.

Why does a smaller temperature difference need so much more flow?

Because the heat carried is flow times temperature difference. Halve the ΔT and you must double the flow to deliver the same power. That means larger pipes, bigger pumps and much more pumping energy, since pressure drop rises roughly with the square of flow. A system designed for 80/60 that actually runs at 80/70 is moving twice the water it should.

Does this work for chilled water and cooling?

Yes. The equation is the same; only the direction of heat flow changes. Enter the chilled water supply and return temperatures, for example 7/12 °C, and the cooling load. The calculator uses the absolute temperature difference.

How accurate are the glycol values?

They are typical values for the mixture at around 20 °C and are fine for preliminary sizing. Glycol properties vary with concentration, temperature and the exact product, so for final design use the data sheet from your glycol supplier.