Loop Breakdown
Full Calculation Table
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How to Use the Hydronic Pipe Sizing Calculator
Pick Single Loop for a simple system with one pump and one flow rate throughout. Pick Primary or Secondary Loop if the system uses a primary-secondary configuration, since each side of that configuration can run a different flow rate.
Choose the pipe material, then enter the load this loop serves and its design delta T. Use the GPM calculator first if you want to see the flow rate on its own before sizing pipe.
The default of 1.0 m/s suits most branch piping. Increase it toward 2.0 m/s for distribution mains where a smaller pipe is worth slightly more noise risk.
Click Size Supply & Return to get one recommended size for both legs of the loop, since they carry the same flow rate, along with the resulting velocity and pressure drop reference.
Sizing a Hydronic Loop as a Pair, Not a Single Pipe
A hydronic loop is not just one pipe to size, it is a supply and return pair that carries the same flow rate in opposite directions. This calculator starts from the load the loop serves, converts that to a flow rate the same way the GPM calculator does, then sizes one pipe diameter that applies to both the supply and return leg, since there is rarely a reason to size them differently. A secondary loop running on a variable speed pump can see real energy savings versus a constant-speed equivalent, a comparison worth running on the variable speed savings calculator once the loop itself is sized.
Single Loop vs. Primary-Secondary
A single loop system uses one pump and one flow rate for the entire circuit, which is straightforward but ties the heat source's flow rate to whatever the building side needs at any moment. A primary-secondary system decouples these two sides using a closely spaced tee or a hydraulic separator: the primary loop circulates through the boiler or chiller at the flow rate the equipment manufacturer specifies, while the secondary loop serves the building's terminal units at a flow rate that can vary independently, often through a variable speed pump responding to zone demand, sized on the VFD sizing calculator once the motor and flow requirements are known. Pick the loop type that matches your system before sizing, since primary and secondary loops can legitimately end up at different pipe sizes even within the same overall system.
Why Supply and Return Match
In a closed loop, the fluid that leaves through the supply pipe is the same fluid that comes back through the return pipe, so both legs see identical flow rate and should be sized identically. This differs from a one-way distribution system like a domestic water main, where the supply might branch off to multiple uses without an equivalent return path. If your project genuinely needs different supply and return sizes for some unusual reason, run the pipe sizing calculator twice with the appropriate flow rate for each leg instead of using this tool's combined approach.
From This Result to the Rest of the Circuit
This calculator sizes the loop's main supply and return piping. Branch lines feeding individual terminal units typically run at a smaller size and lower flow than the main loop, so size those separately on the pipe sizing calculator. Once every segment is sized, total the friction loss across the full index circuit on the pump head calculator before selecting a pump. The City of Toronto's district energy-ready design guideline shows a real-world example of how a primary loop is sized to interface with a building's secondary distribution system.
A Note on Accuracy
This calculator uses published nominal pipe dimensions and the Hazen-Williams friction method for estimating purposes. For permit-stamped mechanical drawings or large commercial systems, confirm sizing against the current CSA B214 installation code for hydronic heating systems, registered with the Standards Council of Canada, and have a mechanical engineer verify the final design, especially for primary-secondary systems where the hydraulic separation itself needs to be sized correctly.
Frequently Asked Questions
Yes, in almost all cases. Supply and return piping carry the same flow rate in a closed hydronic loop, since the same fluid that goes out comes back, so they should be sized identically. The only common exception is a primary-secondary system where the secondary loop's flow rate differs from the primary loop's flow rate, in which case each loop gets its own appropriately sized supply and return pair.
A primary loop circulates fluid through the heat source or heat sink, such as a boiler or chiller, typically at a constant flow rate set by the equipment manufacturer. A secondary loop branches off the primary loop through a closely spaced tee or hydraulic separator and serves the building's terminal units at a flow rate that can vary independently of the primary loop, often using a variable speed pump. This decoupling lets the primary equipment run at its designed flow rate while the building side responds to actual zone demand.
The pipe sizing calculator sizes a single pipe segment from a flow rate and target velocity you already know. This calculator starts one step earlier, from load and delta T, and applies the result to both the supply and return leg of a loop at once, with primary and secondary loop framing built in. Use this one when sizing a main loop from scratch, and the single-pipe tool for individual branches or segments afterward.
Most small residential systems with a single zone or a small number of zones work fine as a single loop, without the added complexity and cost of a primary-secondary configuration. Primary-secondary setups earn their complexity on systems with multiple boilers, mixed equipment types, or zones with very different flow requirements, where decoupling the primary equipment's flow rate from the building side solves real circulation problems that a single loop would struggle with.
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