Boiler Distribution Pipe Sizing
Hydronic GPM = Q / (500 × delta-T), the smallest copper / steel / PEX size at or below the velocity ceiling, velocity.
Example
You enter
- Boiler output (BTU/hr) 200000
- Supply-return delta-T (°F) 20
- Pipe material copper
- Max velocity (ft/s, blank = material default) 4
- Run length one-way (ft) 100
You get
- Required flow 20 GPM
- Velocity 3.607
- Friction / pump head 3.416
- Head (ft) 3.416
Checked against published worked examples from: ASHRAE / Bell & Gossett.
Details, formula, and sources
ScopeHazen-Williams head loss per 100 ft, and pump head at the run length. Per ASHRAE Systems and Equipment 2020 Ch. 13.
Formula or tableGPM = Q / (500 × delta-T) (hydronic water energy balance). Velocity (ft/s) = GPM / (2.448 × d_in²). The smallest standard inner diameter whose velocity is at or below the ceiling is recommended. Head loss = Hazen-Williams (C = 150 copper/PEX, 130 steel); pump head = head per 100 ft × run length / 100.
Source and editionASHRAE Systems and Equipment 2020 Chapter 13 (hydronic heating) by name; Bell & Gossett / Taco system-design velocity limits by name. Hazen-Williams (1905) public domain.
Free accessASHRAE Handbook TOC free at ashrae.org; B&G / Taco design guides free at bellgossett.com / tacocomfort.com.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesboiler_btu_hr, delta_T_F, material, max_velocity_fps, length_ft, gpm, velocity_fps, friction_ft_per_100ft, head_ft
- Water factor 500 BTU/(hr · gpm · °F)60 min/hr × 8.33 lb/gal × 1 BTU/lb-°F (physical fact for water)
- Velocity constant v = GPM / (2.448 × d²)pipe-area / 448.83 gal·ft⁻³·min⁻¹ identity
- Hazen-Williams C 150 copper / PEX, 130 black steelAWWA water-flow C-values
- Default velocity ceiling 4 / 6 / 3 ft/s (copper / steel / PEX)Bell & Gossett / Taco quiet-operation limits
- Water near room temperature Hazen-Williams is an empirical fit for cool water; hot hydronic water is less viscous, so the head loss shown runs high (conservative), and it does not apply to glycol solutions, whose heat capacity also changes the 500 factorASHRAE Fundamentals pipe-sizing chapter