Pairing recirculation with a tankless water heater requires more careful matching than with a storage tank, since a tankless unit has no standing reserve to draw from.
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Quick answer
A tankless water heater has no stored volume of hot water, so recirculation with a tankless unit must actively trigger the unit to fire in response to each recirculation cycle rather than simply drawing down a standing reserve the way a storage tank does. Done without the right controls, this can lead to a phenomenon sometimes called the cold water sandwich, where a brief slug of lukewarm water reaches the tap between recirculation-heated water and freshly triggered hot water. Many tankless units support recirculation through dedicated ports or compatible controls designed to minimize this, but not every unit and every recirculation setup pair together cleanly, so this should be assessed and configured by a licensed technician familiar with your specific equipment.
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A storage tank keeps a volume of water hot and ready, so a recirculation pump drawing from it simply moves already-heated water through the loop with the tank's thermostat topping it up as needed, much like normal household use. A tankless unit has no such reserve; it heats water only as it flows through, triggered by flow itself. When a recirculation pump calls for water without a real fixture being opened, the tankless unit needs to be told to fire specifically for that recirculation demand, which requires either a dedicated recirculation port built into the unit's design or an external control system designed to communicate that demand correctly.
If a tankless unit's minimum flow rate for ignition is not properly matched to the recirculation pump's flow, or if the timing between the unit shutting off after a recirculation cycle and a fixture opening is not well managed, a slug of water that never got fully heated can pass through before the unit re-ignites for the actual fixture draw. This shows up as a brief cold or lukewarm patch in what should be a steady stream of hot water, sometimes right after recirculation has just run. Proper configuration, and in many cases using a unit's dedicated recirculation-compatible plumbing and controls, is the main way to avoid or minimize this.
| Factor | Why it matters |
|---|---|
| Built-in recirculation port or compatible control | Lets the unit distinguish a recirculation call from a normal fixture draw and respond correctly |
| Minimum flow rate for ignition | Must be matched to the recirculation pump's flow to avoid partial or failed ignition |
| Control strategy (timer, on-demand, crossover valve) | Affects how often the unit is asked to fire for recirculation alone |
| Installation and setup quality | Even compatible equipment can misbehave if not configured correctly during installation |
If an existing tankless unit has no practical way to support recirculation without significant additional plumbing or controls, and the cost or complexity outweighs the convenience gained, a technician may reasonably recommend against forcing the pairing. This is a legitimate outcome of the assessment, not a failure to find a solution, and it is better determined honestly upfront than discovered after installation through recurring lukewarm water complaints.
Safety note
If you ever smell gas near a boiler, water heater, or vent, do not investigate further: leave the home first, then call FortisBC at 1-800-663-9911. Combustion appliances should only be serviced by a Technical Safety BC licensed gas fitter.
Not automatically; it depends on whether the specific unit has documented recirculation support or compatible controls, which should be verified against the manual rather than assumed.
It is a brief slug of lukewarm water that can pass through a recirculating tankless system between a recirculation cycle and a fresh fixture draw if flow and timing are not properly matched.
Yes, since a tankless unit has no standing reserve, it must actively fire in response to recirculation calls, which adds cycling compared to a household without recirculation.
It can be, but it needs to be matched to the specific unit's minimum flow requirements and control options, which a technician should confirm.
Yes, an on-demand or thermostatic control strategy generally reduces unnecessary firing compared to continuous recirculation with a tankless unit.
A technician may recommend against adding it if the cost or complexity outweighs the benefit, which is a legitimate and honest outcome of an assessment.
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