A recirculation loop keeps hot water moving through a return line so it is already hot near every fixture. The plumbing code limits hot water pipe from its source to 50 feet developed length and counts a recirculating loop as a source. The CDC also advises circulating hot water stay at or above 120°F.
What does a hot water recirculation loop do?
It keeps hot water circulating in a circuit: out from the heater to the fixtures' branch connections, then back through a return line, driven by a small pump. The branches that feed each fixture stay short, so the wait for hot water stays short too. Our page on how an apartment building's loop is supposed to work shows the loop from the tenant's side, and what a dead leg is covers the branches that fall outside it.
Does the plumbing code require one?
Not by name, but the length limit effectively forces the issue in a larger building. IPC Section 607.2 says: "The developed length of hot or tempered water piping, from the source of hot water to the fixtures that require hot or tempered water, shall not exceed 50 feet." The next sentence is the key one: "Recirculating system piping and heat-traced piping shall be considered to be sources of hot or tempered water." A designer who cannot put a heater within 50 feet of every fixture can meet the rule by running a loop past them, so each branch stays within that distance of the loop.
Phoenix's published amendments to the 2024 IPC do not touch Section 607, so the base text applies to IPC-based plans there. In Maricopa County, food businesses have a separate rule about how fast hot water must arrive, covered in whether hot water has to reach a restaurant sink in 45 seconds.
What else does the code say about loops?
Several things, all in Section 607:
- Energy code link. For buildings under the commercial energy provisions, 607.2.1 says the developed length "shall be limited in accordance with Sections C404.5.1 through C404.5.2.1" of the International Energy Conservation Code. Section 607.2.2 sends the installation of circulation and heat trace systems to Section C404.6 of that code for most commercial occupancies.
- Pump controls for storage loops. The controls on pumps that circulate water between a heater and a storage tank must limit operation "from heating cycle startup to not greater than 5 minutes after the end of the cycle."
- Demand recirculation. A system with pumps that return water "through a cold water supply pipe" must be a demand recirculation system. The control starts the pump when a user acts on a fixture or is sensed, and "shall limit the temperature of the water entering the cold water piping to 104°F."
- Mixing valves. Where a temperature-actuated mixing valve is used with a recirculating pump, the return line must be routed to the water heater's cold inlet and to the valve's cold inlet or hot return connection.
- Insulation. Section 607.5 requires piping that conveys heated water to be insulated under the energy code's rules.
Why does temperature matter for health?
Because lukewarm water in a pipe is where Legionella grows. The CDC's potable water guidance says to "Store hot water at temperatures above 140°F (60°C)," "Ensure hot water in circulation does not fall below 120°F (49°C)," and "Recirculate hot water continuously, if possible." It lists the favorable range for Legionella as 77 to 113°F and notes the bacteria "may grow at temperatures as low as 68°F." To prevent scalding at the fixtures while the loop runs hot, the CDC says to "Install thermostatic mixing valves as close as possible to fixtures." Our page on why commercial buildings use a thermostatic mixing valve covers that piece, and the Legionella water plan page covers who needs a program.
Note the tension. The CDC says "continuously, if possible," while the IPC's demand-control section limits pump operation for certain return arrangements. The right design for a given building comes from the engineer and the water management plan, not from a rule of thumb.
What does a failing loop look like?
Hot water that takes a long time to arrive at far fixtures or upper floors, return lines that are cold when they should be warm, or a pump that hums but does not move water. Those symptoms are covered in why an apartment building loses hot water on upper floors and what to do when hot water works only while the recirculating pump runs. Check the pump, controls and balancing before replacing a water heater.
HQ is licensed and insured, replaces commercial water heaters, and gives free estimates with upfront pricing, so a building with slow hot water can be checked and quoted before work starts.