Hot Water Recirculation: Losses, Legionella, Balancing

How long does hot water take to reach the farthest tap in your building? If nobody on the team knows, that answer is itself informative: the hot water recirculation loop in that building has probably not been checked since commissioning. It runs, it consumes, and its settings rest on whatever an installer judged reasonable on start-up day. In a Greater Montreal commercial or institutional building, that loop is one of the few circuits running 8,760 hours a year — and one of the few whose misadjustment produces an unnecessary bill and a health risk at the same time.

What does a hot water recirculation loop actually do?

A hot water recirculation loop is a return pipe that continuously carries hot water from the farthest point back to the water heater, pushed by a small circulator. It removes the wait at the tap and it stops water from cooling as it sits in the risers. Two distinct jobs, then: one about comfort, the other about water safety.

That dual nature is what makes the subject awkward. A manager looking at the utility bill sees a pump that never stops and a warm pipe heating a crawl space; anyone responsible for water safety looks at the same network and sees the only barrier keeping lukewarm water from lingering in the branches. Both readings are correct, and the reconciliation is not a temperature compromise — it is balancing.

What the loop really costs, 8,760 hours a year

A recirculation loop sheds heat everywhere its piping crosses a space colder than itself: parking garage, service chase, shaft, roof. Unlike a boiler, it never shuts down for the summer. Those losses therefore accumulate year-round, while building heating only recovers them during the cold season — in July they are a net load on the cooling system.

Three factors dominate, and only one is genuinely expensive to fix:

• Missing or degraded insulation. The most common and the cheapest to correct. Bare return sections cluster in the same predictable places: downstream of a repair, in garages, and near connection points.
• Excessive circulation flow. An oversized circulator, or balancing valves left wide open, moves far more water than needed. The return comes back at nearly supply temperature and the energy disappears into the structure.
• Supply temperature raised “for safety”. It never compensates for a poorly balanced loop; it mostly overheats the branches that were already hot enough.

The circulator deserves particular attention because it is the only moving part in the circuit. Checking it follows the same logic as circulator pump maintenance on a heating network: noise, motor temperature, and the actual position of the isolation valve.

The temperatures that are not negotiable in Quebec

Here the room to manoeuvre is narrow and well documented. Legionella multiplies in lukewarm water, roughly across the band between room temperature and 45 °C; it stops multiplying above that and dies faster the higher the temperature climbs. That is why Quebec public health and standards bodies converge on storage held at 60 °C, with the whole volume reaching that temperature at least daily.

The practical consequence is blunt: you do not turn the tank down to save energy. Scald protection happens elsewhere, at the fixtures. Chapter III, Plumbing, of the Construction Code caps the temperature at the bathtub tap or showerhead at 43 °C in private seniors’ residences and care facilities, and at 49 °C in other regulated buildings, using thermostatic type T or combined type TP mixing valves — pressure-balancing-only devices are excluded in those facilities.

On the loop itself, the accepted practice is to keep circulation hot enough along the entire path, return included, to stay out of the growth band. Chapter III also requires a temperature maintenance system once a hot water distribution network reaches a certain length (article 2.6.1.1.): the loop is not a comfort refinement, it is a regulated component.

Why is an unbalanced loop a health problem before it is a comfort problem?

Because water, like electricity, takes the easiest path. With no adjustment, nearly all the flow runs through the shortest, least restrictive branch — the one hugging the mechanical room — while the riser at the far end of the building gets a trickle. The result is a two-speed network: branches at 55 °C and others drifting slowly into the lukewarm band. The top-floor tenant complains about the wait; what they are actually reporting is a section of network no longer protected by temperature.

Two balancing approaches coexist on the Quebec market:

In an older downtown Montreal building whose risers have been modified through successive floor renovations, the second approach avoids rebalancing the whole network after every job.

Field case: a Laval seniors’ residence and five degrees of spread

The call sounded ordinary: top-floor residents waiting for hot water, ground-floor residents finding it scalding. Readings taken on the return of each branch, before any intervention, produced the full picture in an hour. The branch near the mechanical room came back at 57 °C; the top-floor branch at 41 °C — squarely in the lukewarm band, in the exact part of the building where the occupants are most vulnerable.

Three findings on inspection, none exotic. The balancing valves on all four branches were wide open, never adjusted since construction. Roughly fifteen metres of return crossing the garage had lost its insulation after a repair. And the tank, correctly set at 60 °C, was feeding showers whose mixing valves had never been verified — which explained the scalding at the bottom of the building.

The fixes ran in the reverse order of the complaint: insulation restored, balancing valves adjusted branch by branch until the return temperatures tightened, then mixing valves verified at the fixtures to bring the showers back under the regulated ceiling. Supply temperature was left alone. The wait on the top floor disappeared, and the report handed to the manager contained what the file had been missing from the start: four dated return temperatures, measured at the same points, which now serve as the baseline for later visits across Laval.

Three adjustments that cut the bill without touching safety

Restore the insulation before anything else. It is the only measure that reduces losses without changing a single network temperature. Schedule it alongside mechanical plumbing work already planned rather than as a standalone project.

Bring the flow back to what is needed. A properly balanced loop runs with a measurable difference between supply and return. A return arriving at supply temperature signals excess flow — a circulator working for nothing.

Control scaling instead of chasing temperature. In a hot water network, scale narrows the usable bore of the branches and invalidates any balancing done afterwards. The logic mirrors water treatment on a heating circuit, with one added parameter: here the water is consumed, so it is continuously renewed.

Where to start on an existing loop

One round of measurements is enough to stop guessing. Have the return temperature read on every branch, at the same time of day and under comparable conditions, and log the values in the maintenance file with the date. If the spread between the hottest and coldest branch exceeds a few degrees, balancing is the first job — before any discussion about replacing the water heater or scheduling the circulator.

Fall is the right moment for that round: the building is normally occupied, hot water demand is representative, and insulation repairs get done before unheated spaces drop in temperature. At Montréal Combustion we take those readings during an energy optimization visit, because the same numbers serve both files: the utility bill and the safety of the water network.