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
Details, formula, and sources
Hazen-Williams head loss per 100 ft, and pump head at the run length. Per ASHRAE Systems and Equipment 2020 Ch. 13.
GPM = 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.
ASHRAE Systems and Equipment 2020 Chapter 13 (hydronic heating) by name; Bell & Gossett / Taco system-design velocity limits by name. Hazen-Williams (1905) public domain.
ASHRAE Handbook TOC free at ashrae.org; B&G / Taco design guides free at bellgossett.com / tacocomfort.com.
Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
Field names used by the API: boiler_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