Recirculation Pump and Timer Commissioning

Purpose

A recirculation loop is the only thing a plumber installs whose whole job is to run when nobody is using water. Set up on the customer's stated goal, instant hot at every tap at all times, it does three things they did not ask for: it runs the hot line fast enough to erode copper from the inside, it holds the loop hot around the clock, and on a crossover arrangement it warms the cold line. This procedure commissions on a calculated velocity and a schedule matched to when the household actually draws water, and treats time-to-hot as the last check rather than the first.

Scope

Covers commissioning a domestic hot water recirculation pump on a residential or small-commercial system, on either a dedicated return line or a crossover arrangement at the far fixture, including the control strategy and its schedule.

It does not cover sizing the hot distribution itself, pinhole leak investigation on an already-eroded system, which the pinhole leak assessment SOP owns, or mixed-temperature delivery, which the thermostatic mixing valve SOP owns.

Roles and the handoff

The handoff that breaks: the customer says "just leave it running", the tech obliges, and four years later the same shop is repairing pinholes in a system it commissioned. The velocity calculation is what turns that conversation from opinion into arithmetic.

Procedure

1. Identify which architecture you have before anything else, because everything downstream forks here. A dedicated return runs a separate line from the far end back to the heater; a crossover arrangement uses a thermal bypass valve at the far fixture that pushes hot into the cold line. Acceptance: architecture named, and on a crossover system the fixtures downstream of the bypass listed. Wrong looks like assuming a dedicated return because there is a pump at the heater: a crossover system with no return line behaves differently and warms the cold side. Hazard: tracing a line into a crawl or attic puts the access rules for that space in charge before this procedure resumes.

2. Verify where the return lands and that it is checked and isolated. Acceptance: return connected at a tapping the water heater manufacturer permits, a check valve fitted in the correct direction, and isolation each side of the pump. Wrong looks like a return teed into the cold inlet upstream of the heater's own connection, or no check valve at all: without a check the loop thermosiphons whenever the pump is off, so the loop runs all night on a timer that says it is off. Hazard: opening a hot line means it is drained and cooled first, and if a torch comes out, ventilate, keep flux fume out of your breathing zone, shield framing and hold a 30 minute fire watch with an extinguisher in reach.

3. Calculate the velocity the selected pump will produce in the return pipe, before it is energized. Velocity in feet per second equals gallons per minute times 0.4085 divided by the square of the internal diameter in inches, so read the actual internal diameter for the tube type rather than the nominal size. Acceptance: calculated velocity inside the commonly applied limit for continuously recirculated hot water in copper, which is on the order of 2 to 3 feet per second at temperatures up to about 140 F and lower above that, per the Copper Development Association's Copper Tube Handbook. Wrong looks like a standard 6 gallon per minute pump on a 1/2 in return: the arithmetic below shows what that produces, and the failure mode is erosion-corrosion pinholes years later, downstream of every fitting. Stop rule: do not commission an over-velocity loop, fit a balancing valve or a lower-flow pump setting first. Hazard: none, this is arithmetic done at the truck.

4. Set the control strategy against the hours the household actually draws water. A timer alone runs blind; an aquastat alone holds the loop hot whenever it cools; the pairing runs the pump only inside the scheduled windows and only when the return has dropped. Acceptance: schedule windows written down from the customer's own stated hours, aquastat setpoint recorded, and an on-demand button noted where fitted. Wrong looks like 24 hour operation set as a default: state the runtime ratio to the customer instead and let them choose with a number in front of them. Hazard: wiring a timer or aquastat is electrical work, so open the pump's circuit and prove it dead with a meter proved on a known live source before and after, per NFPA 70E-2021, 120.5, with practices at 29 CFR 1910.333(b)(2) and the qualified-person requirement at 1910.332 and 1910.399.

5. Commission the thermal performance with the pump running and the loop stabilized. Acceptance: supply measured at the loop's start and return measured at the heater inlet, with the return within about 10 F of the supply once the loop has settled, and both recorded. Wrong looks like a return far colder: the loop is uninsulated, oversized or losing heat somewhere findable, and it costs the customer that heat every hour the pump runs. Hazard: readings at pipe surfaces mean reaching around hot lines, so use the instrument rather than a hand to judge temperature.

6. On a crossover system, measure the cold line at the far fixture after the pump has run. Acceptance: cold water at that fixture back to its normal supply temperature within a reasonable draw, and no fixture that needs genuinely cold water sitting downstream of the bypass. Wrong looks like a cold tap running warm for a long draw: that is warmed stagnant water in the cold line, which is both a comfort complaint and a growth-friendly temperature band, so the bypass is relocated or the loop is converted to a dedicated return. Hazard: run this test at a tap into the basin with hands clear, because the first water out can be near loop temperature.

7. Prove the loop does not run itself with the pump off. Shut the pump down and leave it off for at least half an hour, then read the return line at the heater inlet. Acceptance: return line at or close to room temperature, which shows the check valve is holding. Wrong looks like a return still hot: the check valve is missing, backwards or passing, and the loop is thermosiphoning, so every hour of the schedule you just set is fiction. Hazard: none at this step, it is a wait and a reading.

8. Restore, label and hand over with the schedule where the customer can read it. Acceptance: pump energized on its intended schedule and observed starting and stopping at a window boundary, check valve confirmed holding from step 7, a durable label at the pump giving the windows, the aquastat setpoint and the date, and the balancing valve position marked if one was fitted. Wrong looks like a commissioned loop with the schedule only in the tech's head: the first person to lose power resets it to continuous. Hazard: this step puts electrical energy and a running pump back into a system you opened, so confirm the pump's cover and any junction are closed before energizing, stand clear of the pump on first start, and re-check every joint you disturbed for weep at 20 minutes under full static.

When the loop should not be commissioned as designed

Velocity over the gate and no way to throttle: the pump is wrong for that return size, so change the pump or the setting. Copper already showing pinholes: this becomes a pinhole leak assessment, not a commissioning, because adding velocity to an eroding system accelerates it. Customer insists on continuous operation: put the velocity figure, the erosion mechanism and the standby-loss ratio in writing, get it acknowledged, then set what they asked for. Crossover bypass upstream of a drinking water point or an ice maker: relocate it, because those fixtures are why cold has to stay cold.

The record this produces

• Architecture as found or as built, and on a crossover system the fixtures downstream of the bypass
• Return connection point, check valve present and its direction, isolation fitted
• Return pipe material, tube type and the internal diameter used in the calculation
• Pump flow used, calculated velocity, and the gate it was compared against
• Balancing valve fitted, and its marked position
• Supply and return temperatures with the loop stabilized, and the drop between them
• Schedule windows and aquastat setpoint as set, and the runtime ratio against continuous
• Thermosiphon check result after the pump was off, and time-to-hot at the farthest fixture with the pump on and off

The velocity line is the one that protects the shop. It is the difference between a loop the customer chose and a loop the shop built too fast.

Worked pass: dedicated 1/2 in return, standard pump, four bathroom house

Existing dedicated return in type L copper, internal diameter 0.545 in for that size and type. Heater setpoint 130 F, no mixing valve on this system. Customer wants "instant hot".

• Step 1: dedicated return confirmed by following the line from the far bathroom back to the heater. No crossover valve anywhere.
• Step 2: return lands at the manufacturer-permitted tapping, isolation both sides, but no check valve found. One fitted in the correct direction as part of the work.
• Step 3: FAILED as originally specified. The pump on the truck moves about 6 gallons per minute at this loop's head. Velocity is 6 times 0.4085 divided by 0.545 squared, and 0.545 squared is 0.297, so 2.451 divided by 0.297 gives 8.25 feet per second, which is far above the 2 to 3 feet per second gate. Stop rule taken: not commissioned as specified. A balancing valve was fitted and throttled to 1.5 gallons per minute, which gives 1.5 times 0.4085 divided by 0.297, or 0.613 divided by 0.297, which is 2.06 feet per second and inside the gate.
• Step 4: customer's actual hours are 6:00 to 8:30 in the morning and 5:30 to 9:30 in the evening, so 2.5 plus 4.0 is 6.5 hours a day out of 24, which is about 27 percent of continuous. Timer set to those two windows with an aquastat cutting the pump when the return is satisfied, so the pump runs less than that ratio again inside the windows. The loop's added standby loss falls roughly in proportion to the hours it is held hot, allowing for the line staying warm a while after each window closes.
• Step 5: supply measured 129 F at the loop start, return 121 F at the heater inlet with the loop settled, so a drop of 8 F, inside the 10 F gate.
• Step 6: not applicable, dedicated return.
• Step 7: pump off 35 minutes, return line at the heater inlet read 74 F against a 72 F room. The new check valve is holding.
• Step 8: pump energized, observed starting at the 5:30 window boundary and stopping at 9:30. Label fitted at the pump with both windows, the aquastat setting, the balancing valve position and the date. Time-to-hot at the farthest fixture read 6 seconds inside a window against 95 seconds with the pump off, which is the number the customer actually wanted and the last one taken.

The customer got what they asked for at roughly a quarter of the running hours and at a velocity their copper will survive, and the ticket carries the arithmetic that decided it.

References

• Copper Development Association, Copper Tube Handbook, current edition, for erosion-corrosion velocity limits in copper tube and the internal diameters of each tube type
• Water heater manufacturer installation manual, for permitted return connection points on that model
• NFPA 70E-2021, 120.5 with 29 CFR 1910.333(b)(2), 1910.332 and 1910.399 for proving the pump circuit dead and who may do the electrical work
• See related: the pinhole leak assessment and repipe recommendation SOP, the thermostatic mixing valve setup and verification SOP, and the water heater annual service SOP