Quick Answer: Short cycling is when a tankless water heater fires, shuts off, and re-fires repeatedly instead of running continuously, and it’s almost always caused by flow sitting between two different thresholds most owners never see on a spec sheet: a lower flow rate needed to keep the unit running (Rheem’s RTGH series lists 0.26 GPM) and a higher flow rate needed to fire it in the first place (0.40-0.50 GPM on the same units, per Rheem’s own documentation). A single low-flow fixture, a mismatched recirculation/crossover valve, or a scale-fouled flow sensor misreading actual flow are the three real-world triggers.
Our recirculation loop guide already flags “confirm with your manufacturer that continuous low-flow circulation won’t trip false-firing or short-cycling” as a warning in passing — this guide is the explanation that warning was pointing to: what short cycling actually is, the two-threshold spec-sheet trap that causes most cases, and which fix applies to your situation.
The two-threshold trap
Every tankless unit publishes a minimum flow rate, but that single number on a spec sheet or a retailer listing usually hides a second, higher number that matters more for short cycling.
| Brand / series | Sustain minimum | Activation minimum | Source |
|---|---|---|---|
| Rinnai (most models) | 0.4 GPM | 0.4 GPM | Rinnai installation manuals |
| Rinnai RE180/RE199 (hybrid) | 0.26 GPM | 0.26 GPM | Rinnai installation manuals |
| Rheem RTGH gas series | 0.26 GPM | 0.40 GPM (some models 0.50 GPM) | Rheem RTGH spec/install manuals |
| Rheem RTEX electric (e.g. RTEX-18) | 0.3 GPM | 0.3 GPM | Rheem RTEX spec sheet |
| Navien NPE (standard, non-A2) | ~0.5 GPM | ~0.5 GPM | Navien NPE spec documentation |
| Navien NPE-A2 (ComfortFlow buffer) | No standard minimum | No standard minimum | Navien NPE-A2 FAQ |
Rheem’s own documentation is the clearest example of why this trips people up: the RTGH series lists a 0.26 GPM minimum flow rate prominently, which reads like the number that matters. But that’s the sustain threshold — the flow needed to keep an already-firing unit running. The activation threshold, the flow actually needed to make it fire in the first place, is 0.40 GPM on the same units (0.50 GPM on some models). A fixture flowing at, say, 0.32 GPM sits in the gap between those two numbers: high enough that the unit thinks it should be able to sustain a fire, too low to reliably trigger one — so it hunts, firing and dropping out repeatedly instead of settling into one state.
Navien’s NPE-A2 series (the ComfortFlow line, covered in our buffer tank guide) sidesteps the whole mechanism differently than that guide’s cold-water-sandwich framing suggests. The internal 0.5-gallon buffer tank and pump don’t just pre-heat water to shorten wait time — because the buffer tank and heat exchanger are already hot, the unit can supply low-flow demand straight from the buffer without needing to trigger the burner’s normal minimum-flow activation logic at all, which is a second, distinct benefit from the buffer tank most coverage (including our own) frames purely as a cold-water-sandwich fix.
The three real-world causes
| Cause | How it shows up | Fix |
|---|---|---|
| Single low-flow fixture | Cycling only on one specific faucet/showerhead, fine everywhere else | Run a second fixture simultaneously, or swap to a higher-flow aerator/showerhead |
| Mismatched crossover valve or recirc pump | Cycling starts after installing a recirculation system, worse right after a pump cycle | Confirm the pump/valve's trickle rate against your unit's activation minimum — see our recirculation loop guide |
| Scale-fouled flow sensor | Inconsistent cycling even on normal-flow fixtures, often paired with a worsened cold-water sandwich | Clean or replace the sensor — see our flow sensor guide |
The first two causes are both, functionally, the same two-threshold problem from a different angle: not enough real flow reaching the unit. The third is different — it’s not that flow is actually too low, it’s that the sensor reporting flow to the control board is fouled with mineral scale (the same mechanism our descaling guide covers for the heat exchanger) and misreports it, so the board’s on/off logic reacts to bad data instead of real conditions. Ruling out which of the three applies starts with a simple test: does it happen on every fixture, or just one? One fixture points to a flow-rate problem at that tap; every fixture points to either the sensor or a whole-house recirculation mismatch.
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Why it’s worth fixing, not living with
A single short-cycling episode won’t damage anything, but repeated ignition cycling puts more wear on the igniter, gas valve, and heat exchanger than one continuous burn does — and Leridian Dynamics’ own documentation for its Maxx-Flow crossover valve kit warns installers directly that an incorrectly-matched flow rate causing short-cycling can void the tankless unit’s manufacturer warranty. That’s a good reason to diagnose the actual cause using the table above rather than just tolerating pulsing temperature as “how tankless works.”
The bottom line
Short cycling almost always comes down to flow sitting in the gap between a tankless unit’s sustain minimum and its higher activation minimum — a distinction most spec sheets, including Rheem’s own RTGH documentation, don’t make obvious at a glance. Test whether it happens on one fixture or every fixture: one fixture means a flow-rate fix (a higher-flow aerator, or running two fixtures together); every fixture means checking a mismatched recirculation/crossover valve or a scale-fouled flow sensor. Navien’s NPE-A2 buffer-tank design is the one mainstream unit that avoids the standard activation threshold altogether, worth knowing if you’re shopping new rather than fixing an existing install.