No display and no hot water
The front panel is dark, the unit seems dead, and every hot tap runs cold.
Start here: Start with house power, the water heater breaker, any nearby service switch, and the receptacle if the unit plugs in.
Tankless Water Heater Troubleshooting
Navien Tankless Water Heater No Hot Water
Direct answer: When a Navien tankless water heater gives no hot water, the most common homeowner-side causes are lost power, gas shutoff, low water flow, or the unit locking out with an error on the display. Start there before blaming an internal part.
Most likely: A simple supply issue or lockout is more common than a failed internal component, especially if the unit was working normally and then quit all at once.
First figure out which version of 'no hot water' you have: no display at all, display on but water stays cold, hot water starts then drops out, or one fixture works while another does not. That split tells you whether you’re chasing power, fuel, flow, or an internal fault. Reality check: a tankless heater will not fire if it does not see enough water moving through it. Common wrong move: turning the temperature higher when the real problem is low flow or a lockout code.
Don’t start with: Don’t start by opening the cabinet, replacing electronics, or guessing at gas-side parts. On tankless units, that gets expensive fast and can turn a simple reset or flow issue into a service call.
No display on the unit?Check the breaker, service switch, and outlet power before anything else.
Display is on but water is cold?Run one hot fixture fully open and look for an error code, ignition attempt, or burner icon.
Last reviewed: 2026-04-17
What no-hot-water looks like on a Navien tankless unit
Display is on but burner never lights
You open a hot tap, the unit wakes up or shows temperature, but you do not get heat.
Start here: Check that the gas shutoff is open, another gas appliance works, and the hot tap is flowing strong enough to trigger the heater.
Hot water comes on briefly then goes cold
You get a short burst of warm water, then it fades or cycles hot and cold.
Start here: Look for restricted flow, a dirty inlet screen, scale buildup, or a fixture mixing issue before assuming a bad heater part.
One fixture has no hot water but others do
The heater can make hot water somewhere in the house, but one sink or shower stays cold.
Start here: Focus on that fixture first. The water heater is probably not the main problem if other taps still get hot.
Most likely causes
1. Power loss or unit lockout
If the display is blank or the unit stopped suddenly after working fine, lost power or a fault lockout is the first thing to check.
Quick check: Look for a dark display, tripped breaker, switched-off service disconnect, or an error code on the front panel.
2. Gas supply problem
A tankless heater can have power and water flow but still make no heat if the gas valve is closed or gas service is interrupted.
Quick check: Make sure the gas shutoff at the heater is parallel with the pipe and confirm another gas appliance is operating normally if you have one.
3. Water flow too low to trigger heating
Tankless units need enough flow to prove demand. A partly closed valve, clogged aerator, dirty inlet screen, or very low-flow fixture can keep the burner off.
Quick check: Open one hot faucet fully and see whether the unit responds differently than it does with a weak trickle.
4. Internal heating or sensing fault
If power, gas, and flow are all present but the unit still will not heat, the problem may be inside the water heater, often with ignition, flame sensing, or a temperature-related component.
Quick check: Watch for repeated ignition attempts, a fault code, or a unit that starts and shuts down right away.
Step-by-step fix
Step 1: Check whether the unit has power and is actually awake
A blank display points you toward supply power first. That is safer and more common than an internal failure.
- Look at the front display before opening any hot tap. Note whether it is blank, lit, or showing an error.
- Check the water heater breaker in the main panel. Reset it once only if it is tripped.
- Check for a nearby service switch or disconnect and make sure it is on.
- If the unit uses a plug, make sure the plug is fully seated and the outlet has power.
- After restoring power, open one hot faucet and watch the display for any change.
Next move: If the display comes back and the unit heats normally, the problem was a power interruption or a simple reset condition. If the display stays blank or the breaker trips again, stop chasing hot-water settings and treat it as a power or internal electrical problem.
What to conclude: No display usually means the heater cannot even begin its normal ignition sequence. A lit display with no heat means move on to gas and flow checks.
Stop if:
- The breaker trips again right away.
- You smell burning, see scorch marks, or hear arcing.
- There is water inside the cabinet or dripping onto wiring.
Step 2: Make sure the heater has gas and a real hot-water call
A tankless unit needs both fuel and enough water movement. If either one is missing, it will sit there looking normal but never heat.
- Open one hot fixture fully, not halfway. A bathroom faucet on a low-flow setting can be misleading.
- Check that the cold-water isolation valve to the heater is fully open and the hot outlet valve is fully open if accessible.
- Check the gas shutoff at the heater. The handle should be in line with the pipe when open.
- If your home has another gas appliance, confirm it is working. If nothing gas-fired works, the issue may be upstream of the heater.
- Watch the display while water is running. Look for any sign that the unit recognizes flow or tries to fire.
Next move: If a stronger hot-water flow suddenly brings back heat, you were likely below the heater’s activation threshold or dealing with a restriction. If you have good water flow, the gas valve is open, and the unit still does not try to heat, the next clue is the display behavior or fault code.
What to conclude: This separates a simple demand problem from a true heater fault. One weak fixture can fool you into thinking the whole unit is bad.
Step 3: Look for an error code or a pattern on the display
On these units, the display often tells you whether the heater is seeing flow, failing ignition, overheating, or shutting down for protection.
- With one hot tap running, read the display carefully and write down any code exactly as shown.
- If there is no code, note whether the unit shows temperature only, whether a burner icon appears, or whether it starts then drops out.
- Turn the hot water off, wait a minute, then run it again to see if the same pattern repeats.
- If the manual is available on-site, use it only to identify the general fault category, not to jump straight to parts replacement.
- If the unit has a simple reset function available from the user controls, perform it once and test again.
Next move: If a single reset clears the issue and hot water returns, keep using the heater but watch for the same code to come back. If the same code or same shutdown pattern returns immediately, the heater is protecting itself and needs a more specific repair path.
Step 4: Rule out low-flow and scale-related problems before blaming electronics
Tankless heaters often lose hot water because water cannot move through them properly. That can look like a bad control problem when it is really a flow problem.
- Check whether the problem happens at every fixture or only at one shower or faucet.
- Remove and rinse a clogged faucet aerator or showerhead if that fixture has weak hot flow.
- If your unit has a serviceable cold-water inlet screen and you can access it without disturbing gas or wiring, shut off water, relieve pressure, and inspect it for debris.
- Think about recent symptoms: reduced hot-water performance, temperature swings, or a unit that has not been descaled in a long time all point toward restriction or scale.
- If multiple fixtures are affected and flow through the heater seems weak, schedule a proper tankless flush or service rather than guessing at parts.
Next move: If cleaning a restricted fixture or inlet screen restores steady hot water, the heater was likely not seeing enough flow or was being starved by debris. If flow is strong and clean but the unit still will not heat or keeps dropping out, the remaining likely causes are inside the heater.
Step 5: Decide between a homeowner reset-and-monitor fix and a service call
By this point you should know whether the problem was power, gas, flow, or a repeat internal fault. The last step is choosing the right next action without wasting money on the wrong part.
- If the heater now works after restoring power, opening a shutoff, or cleaning a restriction, run hot water at two fixtures one at a time and confirm the heat stays stable.
- If only one fixture still has no hot water, repair that faucet, shower valve, or local plumbing issue instead of the water heater.
- If the unit repeatedly shows the same ignition, flame, or overheat behavior, book service with the exact code and your observations ready.
- If a technician confirms a failed user-serviceable water-heater component on the water side, buy only the exact fit for your unit.
- If the unit is older, heavily scaled, or has repeat combustion-related faults, ask for a full condition check before approving parts.
A good result: If hot water is back and stays steady, you have likely solved the immediate problem or at least narrowed it to a fixture-side issue.
If not: If there is still no hot water after these checks, the safe next move is professional diagnosis of the heater itself rather than trial-and-error parts buying.
What to conclude: A tankless heater with confirmed power, gas, and flow but no heat usually needs targeted service, not guessing. The useful win here is arriving at that conclusion cleanly and safely.
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FAQ
Why does my Navien tankless water heater have power but no hot water?
That usually points to one of three things: no gas, not enough water flow to trigger heating, or the unit is locked out on a fault. A lit display does not mean the burner is actually firing.
Can low water pressure cause a tankless water heater to stop heating?
Yes. Tankless units need a minimum flow rate before they will fire. A clogged aerator, restricted inlet screen, partly closed valve, or very low-flow fixture can leave you with cold water even though the heater has power.
Should I reset the unit if there is no hot water?
A single reset is reasonable after you check power, gas, and flow. If the same code or same no-heat pattern comes right back, stop resetting it and get the fault diagnosed.
If one shower has no hot water, is the water heater bad?
Usually not. If other fixtures still get hot water, the problem is more likely in that shower valve, faucet cartridge, anti-scald setting, or a local restriction at the fixture.
What parts should I not guess and replace on a tankless water heater?
Do not guess on gas valves, ignition parts, flame-sensing parts, or control boards. Those are high-fitment, higher-risk repairs, and the wrong call is expensive. Confirm the fault first.