A secondary return is a pipe that carries domestic hot water from the far end of the distribution pipework back to the cylinder, with a small circulation pump keeping the loop warm. Hot water then arrives at distant taps in a few seconds instead of after tens of litres of wasted draw-off. The comfort costs energy: the loop adds continuous pipe losses and disturbs tank stratification, so control strategy matters.
How does a secondary return work?
In a house without recirculation, the hot pipe between cylinder and tap cools between uses, so every draw first pushes out a slug of cooled water. A secondary return closes the circuit: a dedicated return pipe runs from the furthest point of the hot water distribution back to a connection low on the cylinder, and a small pump — typically consuming only 5–25 W — circulates water around the loop so the pipework stays hot. The payoff is immediate hot water at every outlet and far less water poured down the drain; in large buildings with long pipe runs it is the only practical way to deliver acceptable wait times.
How much energy does a recirculation loop cost?
More than most owners expect, through two mechanisms. First, the loop turns the whole distribution network into a radiator: an uninsulated hot water pipe loses heat continuously along its full length whenever the pump runs, and over thousands of pump hours per year those losses routinely exceed the standing loss of the cylinder itself. Second, the returning water — cooled by its trip around the building — re-enters the tank and stirs the middle of the store, degrading the stratification that keeps a hot usable layer at the top. Good insulation on every metre of the loop and a sensible control regime are therefore not optional extras; they decide whether the loop costs a little or a lot. The full energy analysis is in Does a hot water recirculation loop waste energy?
How should a secondary return be controlled?
| Control strategy | How it works | Energy performance |
|---|---|---|
| Continuous circulation | Pump runs 24/7 | Worst — maximum pipe loss |
| Time clock | Pump runs only in occupancy windows | Good, simple |
| Return thermostat | Pump stops once the loop is up to temperature | Good, often combined with timer |
| Demand-triggered | Button, presence or flow sensor starts a short pump burst | Best — loop is hot only when wanted |
A timer plus return-pipe thermostat is the pragmatic standard: the loop is warm during morning and evening peaks and dead at night. Demand-based pumping cuts losses hardest but needs wiring or sensors at the outlets. In hotels and care settings, hygiene rules can override economy — see below.
What does a secondary return mean for legionella control?
A warm, slow-moving loop is exactly the environment legionella bacteria like, so hygiene codes in many countries require circulated systems to keep the whole loop hot — commonly specifying that the return should come back only a few kelvin cooler than the flow, with minimum temperatures set by national regulation. Long switched-off periods in large buildings can create the same risk as a dead leg. Practical consequences: insulate the loop so temperature holds with modest flow, balance parallel risers so no branch stagnates, and follow local rules on temperatures and pasteurisation cycles rather than a generic setting.
When is a secondary return worth installing?
As a rule of thumb, once the pipe run from cylinder to the most-used outlet stores more than a litre or two of water — long single-family homes, multi-storey houses, and virtually all hotels, gyms and apartment buildings. For short runs, thicker insulation and intelligent pipe routing beat a loop. Solimpeks enamel-coated cylinders provide a dedicated recirculation connection at the appropriate height precisely so the returning water re-enters the store without wrecking the temperature layers above it.
Frequently asked questions
Does a secondary return waste energy?
It adds energy cost — continuous pipe losses plus some destratification of the cylinder — but how much depends almost entirely on insulation and control. A well-insulated loop on a timer with a return thermostat costs a fraction of a continuously pumped bare-pipe loop.
Where does the secondary return connect to the cylinder?
At a dedicated connection in the lower-middle region of the cylinder, or via the cold feed where no dedicated boss exists. Returning water is cooler than the top of the store, so it must re-enter below the hot draw-off layer to preserve stratification.
Can I run a secondary return with a heat pump cylinder?
Yes, but control it tightly. Heat pumps regenerate heat more slowly and at lower temperature than boilers, so loop losses eat noticeably into stored capacity. A timer plus return thermostat, generous pipe insulation, and a low-crimp pump schedule are standard practice.
Does a recirculation loop increase legionella risk?
Managed correctly, no — kept hot and moving, a loop is part of legionella control. The risk arises when loops run lukewarm or branches stagnate. Follow national temperature and flushing requirements for circulated hot water systems.