About Chilled & Hot Water Flow Calculator
The chilled & hot water flow calculator returns the water (or glycol) flow rate that a hydronic loop must carry to deliver a given cooling or heating load. It applies the sensible heat-transfer relation Q = m·cp·ΔT, where Q is the load, cp the fluid specific heat, and ΔT the supply-to-return temperature difference, then converts the resulting mass flow to volumetric flow in litres per second, cubic metres per hour, and US gallons per minute.
Enter the load in kW, refrigeration tons, or BTU/h, the design ΔT (commonly 5–8 K for chilled water and 10–20 K for hot water), and the working fluid. The tool reports the pipe velocity for a known internal diameter and suggests a diameter that meets a target velocity, typically held between 1 and 3 m/s to balance pumping energy against erosion and noise. In SI the water relation reduces to L/s = kW / (4.186 · ΔT); in US units it reduces to GPM = BTU/h / (500 · ΔT_F).
How It Works
- Choose a unit system, enter the thermal load (kW, tons, or BTU/h) and the supply-to-return ΔT, and pick the fluid (water, 30% or 50% glycol).
- The calculator normalises the load to kW and ΔT to kelvin, then applies m = Q / (cp·ΔT) to get the mass flow in kg/s.
- Mass flow is divided by the fluid density to give volumetric flow, reported in L/s, m³/h, and US GPM (1 L/s = 15.85 GPM).
- For a known pipe internal diameter it computes velocity v = Q_vol / area; it also suggests a diameter d = √(4·Q_vol / (π·v_target)) to meet your target velocity.
Worked Example
A chiller serves a 100 kW cooling load with chilled water supplied at 6 °C and returned at 12 °C, so ΔT = 6 K. With water (cp = 4.186 kJ/kg·K, ρ ≈ 1000 kg/m³) the mass flow is m = 100 / (4.186 × 6) = 3.98 kg/s, which for water is 3.98 L/s, or 14.33 m³/h, or about 63.1 US GPM. To hold the velocity near 2 m/s the required internal diameter is d = √(4 × 0.003982 / (π × 2)) = 0.0503 m ≈ 50.3 mm, i.e. a DN50 pipe, in which the actual velocity is about 1.83 m/s.
Formulas
- Sensible heat transfer (mass flow)
Q = m_dot * cp * dT => m_dot = Q / (cp * dT)- SI volumetric flow (water)
flow(L/s) = Q(kW) / (rho_kgL * cp * dT) = Q(kW) / (4.186 * dT)- US volumetric flow (water)
flow(GPM) = Q(BTU/h) / (500 * dT_F)- Pipe velocity
v = Q_vol / A , A = (pi/4) * d^2- Suggested diameter for a target velocity
d = sqrt( 4 * Q_vol / (pi * v_target) )
Standards & References
- ASHRAE Handbook — HVAC Systems & Equipment (hydronic heating & cooling)
- ASHRAE Handbook — Fundamentals (fluid flow, secondary-coolant properties)
- Carrier / CIBSE design guidance on pipe velocity limits (1–3 m/s)
- Sensible heat-transfer relation Q = m·cp·ΔT
Frequently Asked Questions
Why does the SI water flow reduce to kW divided by (4.186 × ΔT)?
Mass flow is Q/(cp·ΔT). For water cp = 4.186 kJ/kg·K and density ≈ 1000 kg/m³ = 1 kg/L, so kg/s and L/s are numerically equal. The flow in L/s therefore equals the load in kW divided by 4.186 times the ΔT in kelvin.
Where does the US "500" factor come from?
GPM = BTU/h / (500 · ΔT_F). The 500 is 8.33 lb of water per gallon × 60 minutes per hour × 1.0 BTU/lb·°F specific heat. It is a water-only shortcut; for glycol the specific heat and density change, so the constant differs.
What ΔT and velocity should I design for?
Chilled-water loops commonly use a 5–8 K (≈10–15 °F) ΔT; hot-water loops 10–20 K. A larger ΔT lowers the flow and pump energy. Pipe velocity is usually held between roughly 1 and 3 m/s to limit erosion, noise, and pumping head.
How does glycol change the flow rate?
Glycol mixtures have a lower specific heat than water (about 3.4–3.8 kJ/kg·K for 30–50% mixes) and a higher density, so a glycol loop needs a higher mass and volumetric flow to move the same load at the same ΔT.