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PM Assist

Water heat load and ΔT calculator

Duty from flow and ΔT on a chilled water or LTHW circuit, using the water properties at your actual temperature rather than a flat 4.18 and 1000 — which is right on CHW and nearly 3% out on LTHW.

Duty

125.88 kW

Mass flow

4.999 kg/s

Properties used

ρ 999.76 kg/m³

cp 4.1971 kJ/kg·K

Interpolated from the NIST table

The common shorthand — flow × 4.18 × ΔT, which assumes ρ = 1000 kg/m³ — gives 125.40 kW here, understating the duty by 0.4%.

The water properties this uses

NIST Chemistry WebBook (IAPWS-95), liquid water at 101.325 kPa. Published here so the assumption behind every answer is visible rather than buried.

TemperatureDensity (kg/m³)cp (kJ/kg·K)Error if you assume 1000
6 °C999.944.2028+0.01%
10 °C999.704.1952+0.03%
20 °C998.214.1841+0.18%
40 °C992.224.1794+0.78%
60 °C983.204.1850+1.71%
80 °C971.794.1968+2.90%

Method and sources

Equations used

Q (kW) = ṁ (kg/s) × c_p (kJ/kg·K) × ΔT (K)

Sensible heat transfer to a liquid stream.

ṁ (kg/s) = flow (l/s) ÷ 1000 × ρ (kg/m³)

Volumetric flow converted to mass flow at the water's actual density.

Q (kW) ≈ flow (l/s) × 4.18 × ΔT (K)

The common shorthand, shown alongside every result for comparison. Assumes ρ = 1000 kg/m³, which is a CHW assumption.

Constants assumed

SymbolValueSource
ρ, c_pInterpolated from the table aboveNIST Chemistry WebBook (IAPWS-95), 101.325 kPa
ρ at 6 °C999.94 kg/m³NIST — why 1000 is a safe CHW assumption
ρ at 80 °C971.79 kg/m³NIST — why 1000 is not a safe LTHW assumption

Sources

  • •NIST Chemistry WebBook, thermophysical properties of water (IAPWS-95 formulation), at 101.325 kPa.
  • •Properties are interpolated linearly between tabulated points and clamped outside 6–80 °C rather than extrapolated.

What this calculator deliberately does not do

Glycol mixtures are not supported.

Glycol changes both specific heat and density materially — a 30% ethylene glycol mix has roughly 10% lower cp — so a glycol circuit calculated on water properties is significantly out. Publishing figures would require sourcing cp and ρ per concentration from a manufacturer or published engineering data table; that was not obtained, and estimating them would give a confidently wrong answer.

This is the sensible heat carried by a water stream. It does not account for latent load, pipe heat gain or loss, pump heat, or the difference between the duty a coil is asked for and the duty it actually achieves at the air conditions on the day.

Frequently asked questions

What is the formula for water heat load?
Q = mass flow × specific heat capacity × ΔT. With flow in litres per second that becomes Q (kW) = flow (l/s) × density (kg/m³) ÷ 1000 × cp (kJ/kg·K) × ΔT (K). The familiar shorthand, Q ≈ flow × 4.18 × ΔT, is the same equation with cp fixed at 4.18 and density assumed to be exactly 1000 kg/m³.
Why does this ask for a water temperature?
Because the shorthand's density assumption is only right at chilled water temperatures. Water is 999.94 kg/m³ at 6 °C, so assuming 1000 is fine on a CHW circuit — but it is 971.79 kg/m³ at 80 °C, so on an LTHW circuit the shorthand overstates the duty by nearly 3%. That is the wrong direction to be wrong in when you are checking whether a heat exchanger is underperforming. Specific heat varies far less, between about 4.179 and 4.203 across the same range, so 4.18 is sound throughout.
Where do the water properties come from?
The NIST Chemistry WebBook, which implements the IAPWS-95 formulation, at 101.325 kPa. The tabulated values are published in the method section below so they can be checked, and the calculator interpolates linearly between them. Outside 6–80 °C it clamps to the nearest tabulated point rather than extrapolating, because past 80 °C the density curve steepens and a straight-line extension would start inventing values.
Can I use this for a glycol circuit?
No, and it deliberately does not offer the option. Glycol changes both properties materially — a 30% ethylene glycol mixture has roughly 10% lower specific heat than water — so a glycol circuit calculated on water properties will be significantly out. Publishing glycol figures would have meant sourcing specific heat and density for each concentration from a manufacturer or a published engineering data table, which was not obtained, and estimating them would produce exactly the kind of confident wrong number this tool exists to avoid.
What ΔT should a chilled water circuit run at?
That is a design decision for the system, not something a calculator can tell you, and it is set by the coil selections and the distribution design. What this calculator is useful for is the reverse question: if a circuit was designed for a given ΔT and is running at a much smaller one, the flow is too high for the load it is meeting — the classic low-ΔT syndrome — and the duty being delivered is lower than the flow rate suggests.