Hydronic System Flow from Load and Delta-T
The most fundamental hydronic number, which every balancing valve, pump, and pipe size starts from.
Example
You enter
- Load (Btu/h, or tons if unit set to 1) 10
- Load unit (0 = Btu/h, 1 = tons) 1
- Design delta-T (degF) 10
- Fluid factor (500 water, about 470 for 30% PG) 500
You get
- Load 120,000 Btu/h
- Design system flow 24.0 gpm
Checked against published worked examples from: Bell & Gossett (Xylem).
Details, formula, and sources
ScopeGPM = Q / (500 dT) from Q = 500 GPM dT for water, or the chilled-water shortcut GPM = 24 tons/dT. A 10-ton coil at a 10 F design delta-T needs 24 GPM; a 100,000 Btu/h boiler at a wide 20 F delta-T only 10 GPM - the delta-T is the lever that shrinks the pump and pipe for the same load. Pure water at the sea-level factor (adjust for glycol); full load on the delta-T. A design aid; the mechanical engineer of record governs.
Formula or tableQ_btuh = (unit == tons) ? load x 12,000 : load; GPM = Q_btuh / (factor x dT); chilled-water shortcut GPM = 24 tons/dT (water).
Source and editionThe water-side sensible-heat transport Q = 500 x GPM x dT and its rearrangement to the design flow, with the chilled-water 24 tons/dT shortcut and the glycol-lowered factor, a standard hydronic relation, by name.
Free accessThe water-side heat-transport equation and its 500 factor are public hydronic-design results.
What governsEstimate. AHJ and licensed professional govern.
API field namesload, unit_tons, dt_f, factor, q_btuh, gpm
- Heat transport GPM = Q / (500 dT) for water; 500 = 8.33 x 60 x 1.0water-side sensible heat
- Chilled-water shortcut GPM = 24 tons/dT (12,000/500 = 24)hydronic design practice
- Fluid factor 468 at 40 F to 476 at 180 F for 30% propylene glycol by volume (until 2026-10-09 this said about 485); glycol-fluid-factor gives it for a concentration and temperatureDow DOWFROST HD typical properties, Form 180-01273-402AMS