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The emitter's output at the design water temperature, or the whole circuit's load.
For one radiator, its output at the mean water temperature the system will actually run — not the catalogue figure at a higher one. For a circuit or a boiler, the load it serves.
The difference between flow and return water temperatures the system is designed for.
Boiler systems have long been designed on 20 K (36 °F) or 11 K (20 °F); heat pumps on 5 K (9 °F) or so. The flow is inversely proportional to it, so a quarter of the drop is four times the water for the same heat.
The inside diameter of the pipe carrying this flow, not its nominal size.
15 mm copper tube to BS EN 1057 at 0.7 mm wall has a 13.6 mm bore (about 0.54 in) and 22 mm has 20.2 mm (about 0.8 in); US copper and PEX tables give the inside diameter for each nominal size and type.
Design flow rate
1.893 gal/min
Flow = heat ÷ (the specific heat of water × the temperature drop). Halve the drop and the flow doubles; a quarter of it, as a heat pump runs, is four times the water through the same emitter and the same pipe.
- Water velocity in the pipe
- 3.09 ft/s
- Mass of water per second
- 0.26 lb/s
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Heating Circuit Flow Rate Calculator (Heat Output, ΔT and Pipe Velocity): 1.89 gal/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Heat carried by water: Q = ṁ × c × ΔT, so the flow is the heat output ÷ (4.186 kJ/kg·K × the design temperature drop), with a litre of water taken as a kilogram
- CIBSE Guide C, Reference data — the properties of water; BS EN 12828, Heating systems in buildings — Design for water-based heating systems
- Velocity in the pipe = the volume flow ÷ the bore's cross-sectional area, π × d² ÷ 4
Inputs used
- Heat Output to Carry
- 34121.42 BTU/hr
- Design Temperature Drop (Flow to Return)
- 36 °F
- Internal Bore of the Pipe
- 0.5 in
Intermediate steps
- Water velocity in the pipe
- 3.09 ft/s
- Mass of water per second
- 0.26 lb/s
Confidence note: Flow = heat ÷ (the specific heat of water × the temperature drop). Halve the drop and the flow doubles; a quarter of it, as a heat pump runs, is four times the water through the same emitter and the same pipe.
What this calculation does not cover
- Water is taken at a kilogram a litre — 8.34 lb a US gallon. Hot water is slightly lighter — about 2 per cent at 70 °C (158 °F) — so the volume flow at working temperature is that much higher than shown.
- Glycol mixtures carry less heat per degree than water and are more viscous; a system with antifreeze needs the fluid maker's specific heat and a larger flow.
- The velocity is reported, not judged. Noise, erosion and the pump's duty set the limits for a given pipe, and the pressure loss that comes with the velocity is a separate calculation.
Add the equipment this sizes
This result is a specification — 1.893 gal/min — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-22 · v1.0.0
Regulatory standards & verification citations3
- Heat carried by water: Q = ṁ × c × ΔT, so the flow is the heat output ÷ (4.186 kJ/kg·K × the design temperature drop), with a litre of water taken as a kilogram
- CIBSE Guide C, Reference data — the properties of water; BS EN 12828, Heating systems in buildings — Design for water-based heating systems
- Velocity in the pipe = the volume flow ÷ the bore's cross-sectional area, π × d² ÷ 4
Which documents these citations point at
Standards referenced: BS EN 12828 (European Committee for Standardization, as published in the UK by BSI, European (EN)).
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