For glycol systems, change Cp: 30% EG = 3.60, 50% EG = 3.30 kJ/kg·K. Use the Specific Heat Converter for Cp reference values.
Key Conversion Factors
1 L/min = 0.26417 GPM = 0.01667 L/s = 0.06000 m³/h
1 L/s = 15.8503 GPM = 60 L/min = 3.6 m³/h
1 m³/h = 4.40287 GPM = 16.667 L/min = 0.27778 L/s
Flow (L/s) = Q (kW) / (Cp (kJ/kg·K) x ΔT (°C)) for water: ÷ 4.187 x ΔT
Hydronic Heating Flow Rate Reference (Water, ΔT = 10°C)
| Heat Load (kW) | L/s | L/min | GPM | Typical Application |
|---|---|---|---|---|
| 5 | 0.12 | 7.2 | 1.9 | Single zone / small radiant loop |
| 10 | 0.24 | 14.3 | 3.8 | Small zone / one-pipe baseboard |
| 20 | 0.48 | 28.7 | 7.6 | Residential boiler system (small home) |
| 35 | 0.84 | 50.3 | 13.3 | Residential boiler (typical Canadian home) |
| 50 | 1.19 | 71.7 | 18.9 | Large home or small commercial |
| 100 | 2.39 | 143 | 37.9 | Commercial heating coil or small AHU |
| 200 | 4.78 | 287 | 75.7 | Medium commercial / institutional heating |
| 500 | 11.95 | 717 | 189 | Large commercial heating plant |
| 1000 | 23.9 | 1434 | 379 | District heating main or large plant |
Chilled Water Flow Rate Reference (Water, ΔT = 6°C)
| Cooling Load (kW) | Tons | L/s | L/min | GPM |
|---|---|---|---|---|
| 35 | 10 | 1.39 | 83.6 | 22.1 |
| 70 | 20 | 2.79 | 167 | 44.1 |
| 175 | 50 | 6.97 | 418 | 110 |
| 351 | 100 | 13.9 | 836 | 221 |
| 527 | 150 | 21.0 | 1258 | 332 |
| 703 | 200 | 27.9 | 1675 | 443 |
| 1055 | 300 | 41.9 | 2513 | 664 |
| 1758 | 500 | 69.8 | 4188 | 1107 |
| 3517 | 1000 | 140 | 8376 | 2213 |
Flow rates based on water at 6°C delta-T (typical Canadian chilled water design). Higher delta-T (8-12°C) reduces flow and pump energy. For glycol systems, adjust using the Specific Heat Converter to find the correct Cp.
How to Use the Flow Rate Converter
Type a flow rate in GPM, L/min, L/s, m³/h, m³/s, or US gal/hr. All other units update instantly. Use the quick presets for common HVAC flow rates: residential circulator (7.6 GPM), 100-ton chiller (222 GPM), or cooling tower (1000 m³/h).
Enter heat load in kW, temperature differential in °C, and fluid Cp in kJ/kg·K in the Heat Load to Flow Rate Calculator. For water use Cp = 4.187. For 30% ethylene glycol use 3.60, and for 50% glycol use 3.30. Results show flow in all major units and populate the main converter automatically.
The result panel identifies what HVAC system your flow rate corresponds to: residential circulator, small commercial, chilled water main, or large central plant. This helps confirm a calculated flow is in the right range before selecting pumps or sizing pipes.
Two reference tables cover heating hot water flow at 10°C delta-T (1 kW to 1000 kW) and chilled water flow at 6°C delta-T (10 tons to 1000 tons). Use these to quickly identify the required flow for any load without manual calculation, then feed the result into pump selection and pipe sizing tools.
Fluid Flow Rate in HVAC -- Complete Guide
Fluid flow rate determines pipe sizing, pump selection, heat exchanger performance, and system pressure drop in every hydronic HVAC system. Canadian HVAC engineering uses L/s and m³/h; US pump and equipment ratings use GPM. Converting fluently between these units, and calculating flow from heat load, is a daily requirement in hydronic heating and chilled water design.
GPM to L/s -- The Core Conversion
The most common conversion in Canadian hydronic work is between US GPM and L/s. The factor: 1 GPM = 0.06309 L/s, or 1 L/s = 15.850 GPM. A circulator pump rated at 10 GPM delivers 0.631 L/s. A chiller rated for a 222 GPM flow delivers 14.0 L/s. GPM also converts to L/min by multiplying by 3.785 -- a quick field approximation is 1 GPM ≈ 3.8 L/min. For European equipment ratings in m³/h: 1 GPM = 0.227 m³/h, or 1 m³/h = 4.40 GPM.
Calculating Flow from Heat Load
The fundamental hydronic equation is Q = ṁ x Cp x DeltaT, rearranged for flow: ṁ (kg/s) = Q (kW) / (Cp (kJ/kg·K) x DeltaT (K)). For water (Cp = 4.187 kJ/kg·K, density ≈ 1 kg/L at typical operating temperatures): flow (L/s) = Q (kW) / (4.187 x DeltaT). A 50 kW load at 10°C DeltaT needs 50 / (4.187 x 10) = 1.19 L/s = 71.5 L/min = 18.9 GPM. For glycol systems, use the lower Cp from the Specific Heat Converter -- 30% ethylene glycol has Cp = 3.60 kJ/kg·K, so the same 50 kW load at 10°C DeltaT requires 50 / (3.60 x 10) = 1.39 L/s -- 17% more flow than water.
Delta-T and System Efficiency
A higher temperature differential (delta-T) between supply and return reduces the required flow rate for the same heat transfer. Conventional Canadian hot water heating systems design for 10°C DeltaT (80°C supply / 70°C return). Modern condensing boiler systems use lower water temperatures with higher DeltaT of 15-20°C, reducing flow rate by 33-50% compared to conventional systems. Lower flow means smaller pipes, smaller pumps, lower pump energy, and reduced capital cost. Chilled water systems typically use 6°C DeltaT in Canada (7°C supply / 13°C return), though designers increasingly push to 8-10°C DeltaT to reduce pump energy in variable flow systems.
Pipe Velocity and Flow Rate
Once you have the required flow rate, pipe diameter is sized to keep velocity within acceptable limits. For hydronic systems, typical design velocities are 0.9 to 1.8 m/s for mains (3 to 6 ft/s) and 0.6 to 1.2 m/s for branches (2 to 4 ft/s). Velocity (m/s) = flow (m³/s) / pipe area (m²). For a 2-inch nominal copper pipe (ID approximately 50.8 mm, area = pi x 0.0254² = 0.002027 m²): at 1.0 L/s (0.001 m³/s), velocity = 0.001 / 0.002027 = 0.493 m/s -- well within limits. Use the Air Velocity Converter for velocity conversions and the Area Converter for pipe cross-section areas.
Frequently Asked Questions
Multiply US GPM by 3.78541 to get L/min. So 1 GPM = 3.785 L/min, 10 GPM = 37.85 L/min, and 100 GPM = 378.5 L/min. To go the other way, divide by 3.78541. A quick approximation: 1 GPM ≈ 3.8 L/min. Note that this is US gallons per minute -- if working with Imperial gallons, multiply by 4.546 instead. Most North American pump curves use US GPM.
Flow (L/s) = heat load (kW) / (Cp x DeltaT). For water (Cp = 4.187 kJ/kg·K): flow = kW / (4.187 x DeltaT°C). A 35 kW boiler at 10°C DeltaT needs 35 / (4.187 x 10) = 0.836 L/s = 50.2 L/min = 13.3 GPM. For glycol: use Cp = 3.60 (30% EG) or 3.30 (50% EG). Use the Heat Load to Flow Rate Calculator above -- it handles the math and shows all flow unit equivalents at once.
At a 6°C delta-T (7/13°C supply/return, common in Canadian chilled water design): flow (L/s) = cooling kW / (4.187 x 6) = cooling kW / 25.1. A 100-ton (351.7 kW) chiller needs 351.7 / 25.1 = 14.0 L/s = 221 GPM. A 50-ton chiller needs 7.0 L/s = 111 GPM. Increasing DeltaT to 8°C reduces flow by 25%, lowering pump energy significantly. The chilled water flow reference table above covers 10 to 1000 tons.
A typical Canadian home with 20 kW heating load at 10°C DeltaT needs: flow = 20 / (4.187 x 10) = 0.478 L/s = 28.7 L/min = 7.6 GPM. At 20°C DeltaT (condensing boiler design): 0.239 L/s = 14.3 L/min = 3.8 GPM -- half the flow, smaller pipes, lower pump energy. Enter your home's heat load and DeltaT in the calculator above to get the exact flow rate for your system.