When a tankless water heater is not producing hot water, the cause is usually one of five things: too little water flowing to fire the burner, more hot water demand than the unit can heat at once, mineral scale or a clogged inlet filter, a gas, venting or power problem, or a brief cold burst that is normal behaviour rather than a fault. The first three are often simple to confirm at home, and the last two are where a licensed technician takes over.
No Hot Water at All, or Hot Then Cold?
Start by pinning down the symptom, because it points to different causes.
- No hot water at any tap. Think power, gas supply, closed valves, or a unit that has locked out on an error code.
- No hot water at one tap only, usually a low-flow bathroom faucet. Think minimum flow.
- Hot, then cold, then hot again. Think too much demand at once, or the "cold water sandwich" covered below.
- Hot water that slowly got weaker over months. Think scale and a clogged inlet filter.
If the unit's display shows an error code, write it down before resetting anything. It is the fastest route to a diagnosis, and a code that keeps coming back after a reset is telling you something.
Too Little Flow to Fire the Burner
A tankless heater does not keep water hot. It fires only when it senses enough water moving through it. Rinnai spells out the numbers for its gas units: they "need a demand of at least 0.4 to 0.6 gallons per minute of water flow through the hot side to ignite and 0.26 gallons per minute to stay in operation." Below that, Rinnai says, the unit "will turn off because it senses that hot water isn't needed."
That explains a common complaint: the shower is fine, but the bathroom sink never gets hot. Barely cracking a hot tap, or a faucet aerator clogged with grit, can keep flow under the ignition threshold. Open the tap fully for a minute, and clean the aerator if flow looks weak. Also check that the water valves under the unit are fully open. A valve left partly closed after a flush is an easy one to miss.
Too Much Demand at Once, Especially in Winter
A tankless unit heats water as it passes through, so it has a limit on how much hot water it can deliver at any one moment. A.O. Smith lists an undersized unit among the common tankless problems: it "struggles with high demand," and the fix is to "limit simultaneous hot water use or upgrade to a higher-capacity model."
Here, that limit shows up most in winter. The colder the incoming water, the more heat the unit has to add to reach the set temperature, so a heater that keeps up with two showers in September can fall short on a January morning. If the hot water only fails when the washer, dishwasher and a shower overlap, the unit may simply be too small for the house. Our tankless vs tank comparison explains why sizing matters so much more for tankless.
Scale, a Clogged Inlet Filter and Error Codes
Hard water leaves mineral scale wherever it is heated, and a tankless heat exchanger has narrow passages. A.O. Smith's list puts it plainly: "Mineral Buildup: Clogs and damages heating elements. Fix: Descale the heater annually, especially in hard water areas." Rinnai also says "it's important to flush those mineral deposits at least once a year."
The inlet water filter is the other half of it. It sits at the cold water connection and catches grit before it reaches the unit, and when it clogs, hot water flow drops. On private wells across Chester County and much of Cecil County, where water is often harder, this happens faster. If your unit has never been flushed, start with our guide on how often to flush a tankless water heater.
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Gas, Venting and Power
A gas tankless unit needs three things to fire: gas, a clear vent and electricity. A.O. Smith lists "Ignition Failures: Stops the unit from starting. Fix: Check gas supply, venting, and ignition pack; may need professional help."
What you can safely check yourself:
- Power. The unit plugs in or is hardwired. Rinnai's FAQ is direct: "If you experience a power outage, your Rinnai Tankless Water Heater will not operate." Check the outlet and the breaker.
- Gas. Are other gas appliances working? Is the gas valve to the unit open?
- Vent. Look at the outside vent terminations. Leaves, nests or snow drifts can block them.
What you should not do is open the unit or work on the gas controls. And if you ever smell gas, A.O. Smith's advice applies to every gas appliance: "turn off the gas supply valve immediately and call a professional—don't continue to troubleshoot."
The Cold Water Sandwich Is Not Always a Fault
Some cold water is normal. Rinnai notes that "without a Rinnai recirculation system, it's normal to have a slight 1 - 2 second temperature drop as the water in the pipes clears the line." A.O. Smith describes the related "cold water sandwich" as an "unexpected cold water burst." In practice it usually happens when a hot tap is turned off and back on within a minute or so. A.O. Smith suggests you "minimize demand fluctuation or consider a model with a built-in buffer tank."
A short burst of cool water between two hot runs is how tankless heaters behave. Long stretches of cold water are not, and those point back to the causes above.
When to Call a Technician
Call if the unit shows an error code that returns after a reset, if gas and power are fine but it still will not fire, if hot water has faded even after a flush, or if you smell gas.
Water Heater Now services and repairs tankless water heaters in Wilmington and across New Castle County, Chester County and Cecil County. For a tank-style heater, see our water heater repair service instead. Call (302) 240-2818. We answer 24 hours a day.
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Sources, verified September 30, 2026: Rinnai, Residential FAQ; A.O. Smith, Is Your Hot Water Not Getting Hot? Common Causes & What To Do About Them.
