When your tankless water heater not heating leaves you shivering under a cold shower, the cause is usually one of five things, and four of them you can check yourself in under 15 minutes before calling a plumber. Tankless units are reliable, but they refuse to fire for logical reasons: no gas, no power, blocked venting, mineral scale, or an ignition fault. Work through them in order and you will usually find the culprit fast.
- Start With the Error Code
- Check Gas Supply First
- Verify Power and Ignition
- Clear the Cold-Water Sandwich Confusion
- Descale the Heat Exchanger
- Inspect Venting and Air Supply
- Low Flow Lockout
- Lukewarm Water: A Different Problem
- Seasonal Failures in Cold Weather
- Preventive Maintenance Schedule
- Gas Pressure Problems in Depth
- Error Codes by Brand: A Quick Reference
- When to Call a Pro
Start With the Error Code
Every modern tankless unit, Rinnai, Navien, Rheem, Takagi, or EcoSmart, shows a digital error code when it faults. Note the code on the display before touching anything, then check the manual or the label inside the front cover. Common codes point you straight at the problem:
- Ignition failure codes (often 11 or 111): the unit is trying to fire but cannot light, usually a gas or igniter issue.
- Flame loss codes (12): it lit but the flame died, often gas pressure.
- Overheat or exhaust codes (14, 10): venting or scale problems.
- No display at all: a power problem.
The code narrows a whole-house guessing game down to a single system. Write it down.
Check Gas Supply First
The most common reason a gas tankless will not heat is no gas reaching the burner. Confirm the gas valve at the unit is fully open, the lever parallel with the pipe. Then check that other gas appliances work; if your stove and furnace are also out, the problem is the meter or supply, not the heater.
Propane users, check the tank level. An empty or nearly empty propane tank starves the burner even though the pilot electronics still light up. And a newly installed unit that never heats often has an undersized gas line that cannot feed the high BTU demand at full fire.
Verify Power and Ignition
Even a gas tankless needs 120-volt electricity for its igniter, fan, and control board. If the display is dark, check the outlet with another device, reset the unit’s breaker, and inspect the power cord. A tripped GFCI outlet is a frequent and easily missed cause.
If the unit clicks repeatedly trying to light but never catches, you likely have an ignition or flame-sensor issue. On some models the igniter or flame rod fouls with carbon and needs cleaning, a job most homeowners hand to a technician.
Clear the Cold-Water Sandwich Confusion
Sometimes the heater works but delivers a slug of cold water mid-shower, the so-called cold-water sandwich. That is not a true failure; it is the brief lag between the burner firing and hot water reaching the tap after a short off-cycle. A recirculation pump or a small buffer tank smooths it out. If you get steady hot water that simply cuts out after a few minutes, though, that points to scale or gas pressure, not a sandwich.
Descale the Heat Exchanger
Mineral scale is the number-one long-term cause of a tankless water heater not heating properly. Scale coats the narrow heat exchanger passages, insulating them from the burner and eventually triggering overheat faults or weak output. If you have hard water and have never flushed the unit, this is likely your problem.
- Shut off the gas and the hot and cold isolation valves.
- Connect a pump and hoses to the service ports.
- Circulate four gallons of white vinegar or descaling solution for 45 to 60 minutes.
- Flush with clean water, reopen the valves, and restore gas.
Do this yearly, or twice a year on very hard water, and you prevent most heating faults outright.
Inspect Venting and Air Supply
A tankless unit needs clear intake and exhaust. A bird nest, wasp nest, or snow drift blocking the exterior vent termination will choke combustion and trip a safety shutdown. Walk outside and confirm the vent caps are clear. Indoors, check that the intake is not smothered by stored boxes or laundry piled against the unit.
Low Flow Lockout
Tankless heaters need a minimum flow rate, often about 0.5 GPM, to activate the burner. If only hot water trickles from a faucet, the unit may never fire because it does not sense enough demand. A clogged aerator or a partially closed valve can drop flow below that threshold. Clean the aerators and confirm the water shutoffs are fully open.
Lukewarm Water: A Different Problem
Not every complaint is a total no-heat situation. Sometimes the unit fires but delivers only lukewarm water, which points to a distinct set of causes. The most common is simply demand exceeding capacity: running two showers and a dishwasher at once forces the burner to spread its output thin, dropping the temperature at every fixture. That is a sizing limit, not a failure.
Other lukewarm causes include a set point that drifted low, a cracked dip tube mixing cold into the output on some hybrid setups, or partial scale buildup insulating the exchanger. If your water is lukewarm only during heavy simultaneous use, your unit is undersized for the demand. If it is lukewarm all the time, check the temperature setting first, then suspect scale.
Seasonal Failures in Cold Weather
Many no-heat calls spike in winter, and for good reason. Incoming groundwater is far colder in January than July, sometimes 40°F versus 70°F, which forces the unit to work far harder for the same output temperature. A tankless heater that comfortably supplied two showers in summer may fall short on the coldest mornings simply because the temperature rise doubled.
Winter also brings frozen condensate lines on condensing units, blocked vent terminations under snow drifts, and, in unheated spaces, the risk of the unit itself freezing. Most tankless heaters include freeze protection that draws power to keep internal components above freezing, but that protection fails during a power outage. If your unit sits in an unconditioned garage or crawlspace, protect the pipes and keep power to the unit through cold snaps.
Preventive Maintenance Schedule
Most no-heat problems are preventable with a simple routine. Keep your tankless unit healthy with this schedule:
- Monthly: glance at the display for any standing error codes and listen for normal ignition.
- Every few months: clean the inlet water filter screen and the faucet aerators.
- Twice a year: check the exterior vent terminations for nests, debris, and, in winter, snow.
- Annually: descale the heat exchanger with vinegar or descaler, and inspect for condensate drainage.
- Every few years: have a technician verify gas pressure and combustion.
Homeowners who keep this schedule rarely face a surprise cold shower, because the two biggest long-term culprits, scale and blocked venting, get caught before they cause a shutdown.
Gas Pressure Problems in Depth
When a tankless water heater is not heating consistently, and you have ruled out the obvious, inadequate gas pressure is a frequent hidden cause, especially on units that light fine but cut out under load. A tankless unit demands a large volume of gas at full fire, far more than the old tank it replaced. If the gas line is undersized, shared with a furnace and range that run simultaneously, or fed by a regulator that cannot keep up, the pressure sags when the burner ramps up and the flame drops out.
Signs point to pressure: the unit heats fine for a low-flow trickle but faults during a strong shower, or it works fine until the furnace kicks on. A technician confirms this with a manometer, measuring both static and dynamic gas pressure at the unit. The fix may be a larger gas line, a dedicated line, or a regulator upgrade. This is not a DIY diagnosis, but recognizing the pattern tells you what to describe when you call, saving time and a return visit.
Error Codes by Brand: A Quick Reference
While every manual is the final word, some patterns hold across brands. Use these as a starting point, not gospel:
- Rinnai: Code 11 is no ignition; Code 12 is flame failure; Code 10 or 14 points to venting or overheat.
- Navien: Codes in the E003 range flag ignition faults; E004 flags false flame; E016 signals overheat, often scale.
- Rheem: Code 11 is ignition failure; Code 13 is a flame-loss condition.
- Takagi: Code 111 is ignition failure; Code 121 is combustion or flame loss.
Across every brand, repeated ignition and flame-loss codes send you back to gas supply and igniter components, while overheat codes send you to scale and venting. Match your code to the manual, then aim your troubleshooting at the right system rather than guessing.
When to Call a Pro
If you have confirmed gas, power, clear venting, adequate flow, and a recent descale, and the unit still throws an ignition or exchanger error, it is time for a licensed technician. Gas pressure testing, control board diagnosis, and flame-rod replacement need a meter and expertise. Expect a service call of roughly $150 to $350. But run through this checklist first; a surprising share of no-heat calls end at a closed gas valve, a tripped GFCI, or a heat exchanger crying out for its first descale.
