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Glycol Loop Heat Transfer (Capacity Derate)

Also known as glycol derate · propylene glycol heat transfer

Q˙=ρcV˙ ΔT\dot{Q} = \rho c \dot{V} \, \Delta T

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Worked example: 30% PG: 1024 kg/m3, 3850 J/(kg.K), 2 L/s, 12 K → 94.6 kW. Press Try an example to run it live, then adjust anything.

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See also

Hydronic Heat Transfer (Water): Both use heat transfer rate and supply-to-return ΔT.Condenser Water Flow Rate: Both use supply-to-return ΔT.Conduction Through a Pipe Wall: Both use heat transfer rate.Fin Heat Transfer Rate: Both use heat transfer rate.Heat Flow from Thermal Resistance: Both use heat transfer rate.Heat Loss Through an Assembly (Q = A·ΔT/R): Both use heat transfer rate.
2 more relatedNet Radiation Exchange Between Surfaces: Both use heat transfer rate.Newton's Law of Cooling (Q = hAΔT): Both use heat transfer rate.
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Glycol Loop Heat Transfer (Capacity Derate) explained

V̇ρcpΔTQ

Antifreeze buys you freeze protection and charges you capacity. Propylene glycol is denser than water but has a markedly lower specific heat: a 30 % mix at 20 °C runs about 1025 kg/m³ and 3850 J/(kg·K), so the product ρc falls from water's 4.18 MJ/(m³·K) to about 3.94, roughly a 6 % derate. Push to 50 % PG and you lose about 13 %, and the fluid gets thick enough that pump head rises too. That is why the same emitter that made design on water goes cold on the day the glycol truck leaves.

Field practice at HYDRONIC Water Treatment: check the mix with a refractometer, not with the invoice. We have opened plenty of "40 % systems" that measured 22 % because someone topped off a leaking loop with a garden hose for two winters: freeze protection gone, inhibitor package diluted below its threshold, and steel corroding underneath. Worked example: 25 gpm of 64 lb/ft³, 0.92 BTU/(lb·°F) glycol carrying 150,000 BTU/hr needs ΔT = 150,000 ÷ (500 × 0.92 × 0.96 × 25) ≈ 12.7 °F, where the same load on plain water would show 12 °F, a small number that tells you the pump has to move more fluid for the same job.

Glycol Loop Heat Transfer (Capacity Derate) formula

Q˙=ρcV˙ ΔT\dot{Q} = \rho c \dot{V} \, \Delta T
Where
  • Q˙\dot{Q}= Heat transfer rate (W)
  • ρ\rho= Fluid density (kg/m³)
  • cpc_p= Fluid specific heat (J/(kg·K))
  • V˙\dot{V}= Flow rate (L/min)
  • ΔT\Delta T= Supply-to-return ΔT (C°)
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The HVAC heat transfer constants

6 formulasThe 500, 1.08, 0.68 and 4.5 rules for water and air heat transfer — where each constant comes from and when it stops being true.

Revised 1 November 2026
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formula.expert. (2026, November 1). Glycol Loop Heat Transfer (Capacity Derate) Calculator. https://formula.expert/solvers/glycol-hydronic-heat-rate

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