Pipe sizing in hydronic systems balances first cost against operating cost and reliability. Smaller pipes cost less to buy and install, but they increase friction losses, pump energy and the risk of noise and erosion. Larger pipes cost more but run quieter and cheaper. Most designers size pipe using a combination of a target friction rate and maximum velocity limits.
This guide explains the methods, typical values from industry guidance and the factors that should push a designer toward larger or smaller pipe.
Why velocity and friction matter
Friction and pump energy
Friction loss in pipe rises roughly with the square of velocity (more precisely, around the 1.8 to 2 power, depending on flow regime). Pump power depends on flow and head. Doubling velocity in a pipe roughly quadruples its friction loss. Since pumps often run thousands of hours per year, friction rate is a direct driver of lifetime energy cost.
Noise
High velocities produce flow noise and can transmit noise through piping, especially at fittings, valves and where pipes pass through occupied spaces. Entrained air makes noise worse.
Erosion
High velocities, particularly in copper pipe and at fittings, can erode pipe walls over time. Erosion corrosion in copper is a known issue at high velocities and high temperatures, often seen at elbows and tees downstream of pumps.
Air removal
Velocities that are too low can allow air to collect at high points and dirt to settle. A minimum velocity, often around 0.6 m/s (2 fps), helps carry entrained air to separators in horizontal piping, though dedicated air separation and vents remain essential.
Typical friction rate targets
A commonly used design range for hydronic piping is a friction rate of about 100 to 400 Pa/m (roughly 1 to 4 ft of head per 100 ft of pipe). Many designers target the middle of that range, around 250 Pa/m (about 2.5 ft per 100 ft), and lower in long runs or where pump energy matters most.
The ASHRAE Handbook, Fundamentals (pipe sizing chapter) provides friction charts and discusses typical ranges. Designers increasingly use lower friction rates than in the past to meet energy code pump power limits and reduce operating cost.
Typical velocity limits
Velocity limits vary by pipe size, material, service and noise sensitivity. The following are typical guidance values drawn from ASHRAE and common industry practice. Project specifications or owner standards may differ.
| Pipe size | Typical maximum velocity |
|---|---|
| Up to 50 mm (2 in.) | About 1.2 m/s (4 fps) |
| 65 to 150 mm (2.5 to 6 in.) | About 1.2 to 2.4 m/s (4 to 8 fps), often limited by friction rate |
| 200 mm (8 in.) and larger | Up to about 3 m/s (10 fps), often limited by friction rate |
For small pipes, the friction rate usually limits sizing before velocity does. For large pipes, velocity becomes the limit. In noise-sensitive locations, such as piping near bedrooms, patient rooms or recording studios, lower velocities are prudent.
Copper pipe
Copper is more susceptible to erosion at high velocities, particularly with hot water. The Copper Development Association recommends lower maximum velocities for hot water than for cold water. Many designers limit copper heating water piping to about 1.2 to 1.5 m/s (4 to 5 fps) or lower.
Plastic pipe
PEX and other plastic piping used in radiant and small distribution systems have their own manufacturer limits. Plastic pipe is generally less prone to erosion but may have higher noise transmission in some installations.
Sizing procedure
- Determine the design flow in each section from loads and design delta T.
- Select a pipe size that keeps friction within the target range and velocity below the limit for that size and service.
- Check whether the next smaller size would still meet both limits, and whether the next larger size meaningfully reduces pump head on the critical path.
- Calculate total head on the critical path, including fittings and equipment, to select the pump. See pump selection and system curves.
- Review sections near the pump, where flow and velocity are highest, for noise and erosion risk.
- Confirm minimum velocities in horizontal mains and provide air and dirt separation.
Effect of glycol
Glycol solutions have higher viscosity and lower specific heat than water, especially at low temperatures. Effects on piping include:
- More flow for the same load, because of lower specific heat.
- Higher friction loss at the same velocity, because of higher viscosity.
- Larger pumps or pipes may be needed to stay within design limits.
Correction factors for flow and pressure drop are available from glycol suppliers and in the ASHRAE Handbook. Corrections are largest at low temperatures and high glycol concentrations. See glycol systems, concentration and freeze protection.
Variable flow considerations
In variable flow systems with two-way valves, flow in most pipes is well below design most of the time. That reduces friction and pump energy at part load, and velocities fall below design. Two implications follow:
- Size for design flow, but recognize that part-load conditions dominate operating hours.
- Be cautious about upsizing purely for part-load efficiency: the savings at low flow are small because friction is already low.
Pipe sizing also affects control. Large pressure drops in mains relative to coils reduce valve authority at remote coils. A well-sized distribution system with modest main losses helps coils near the end of the system control well.
Special cases
Condenser water
Open condenser water systems have additional considerations: static lift to the tower, pump suction from the tower basin, and air entrainment at the basin. Lower suction velocities and adequate submergence prevent vortexing and cavitation.
Steam and condensate
Steam piping follows very different sizing rules based on pressure drop and velocity of steam, with separate rules for condensate return. This guide does not cover steam.
Data centres and critical systems
Critical systems sometimes use lower velocities to limit noise, reduce erosion and allow future capacity increases. Ring mains and redundant paths are sized so that any single path can carry the required flow.
Frequently asked questions
Should I always size for the lowest friction rate?
No. Very low friction rates mean large, costly pipes with little additional energy benefit, and can make air removal harder. A life-cycle cost view, considering pump energy and first cost, usually points to a moderate friction rate.
Why does my existing system make noise at valves?
Common causes are excessive differential pressure across control valves, entrained air, or high velocity at specific fittings. Reducing pump differential pressure setpoints, improving air removal and balancing the system often helps. Measurements during water balancing identify the cause.
Key takeaways
- Size hydronic piping using both a target friction rate and maximum velocity limits.
- Typical friction rates are about 100 to 400 Pa/m (1 to 4 ft per 100 ft), with many designers targeting the middle of the range.
- Velocity limits rise with pipe size; copper hot water piping needs more conservative limits.
- Glycol increases required flow and friction, so apply correction factors.
- Good pipe sizing supports pump efficiency, valve authority and quiet operation.
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