No display and no hot water
The unit looks dead, no lights or screen, and every hot tap stays cold.
Start here: Start with power to the unit, then confirm it is actually a tankless heater and not a separate recirculation or mixing issue.
No hot water from a tankless unit
Tankless Water Heater Not Heating
Direct answer: When a tankless water heater is not heating, the usual causes are no power to the unit, no gas supply on gas models, too little water flow to trigger heating, a clogged inlet screen, or a lockout from scale, venting, or an internal fault.
Most likely: Start with the easy tells: does the unit power up, does any display show an error, do other gas appliances work, and does hot water improve when you open one fixture fully instead of barely cracking it.
Tankless heaters act different from tank-style heaters. They need enough water moving through them before they fire, and they will shut themselves down fast when they see a venting, flame, freeze, or overheat problem. Reality check: a lot of no-heat calls end up being low flow, a closed valve, or a tripped power source. Common wrong move: replacing parts before checking whether the unit is even being told to fire.
Don’t start with: Do not start by ordering a control board or taking apart the burner section. On tankless units, simple flow and supply problems fool people all the time.
If the display is darkCheck the disconnect, breaker, outlet, and any reset on the unit before anything else.
If the unit has power but no hot waterRun one hot fixture at full flow and watch for an error code, ignition attempt, or temperature change.
Last reviewed: 2026-04-04
What the no-heat problem looks like
Display is on but burner never fires
You open hot water, the screen wakes up or changes, but you do not hear ignition or feel any temperature rise.
Start here: Check water flow first, then inlet screen blockage, then gas supply on gas models.
Brief warm water then cold
You get a short burst of heat and then it drops off, or it heats only at one faucet.
Start here: Look for low flow, crossed hot-cold mixing, scale buildup, or a different symptom page if the unit repeatedly goes cold mid-use.
Error code with no heat
The display shows a fault or lockout and the heater will not recover on its own.
Start here: Use the code as a clue, but first rule out simple supply issues, freeze damage signs, and blocked intake or vent openings.
Most likely causes
1. Power loss or reset issue
A dark display, no fan noise, and no response at any fixture usually means the unit is not getting power or has tripped a reset.
Quick check: Check the breaker, service switch or disconnect, outlet if it plugs in, and any reset button or obvious power indicator on the heater.
2. Not enough water flow to trigger heating
Tankless heaters need a minimum flow rate. A partly closed stop, clogged aerator, low-pressure fixture, or small trickle can leave the unit awake but not firing.
Quick check: Open one hot faucet fully, preferably a tub spout or laundry sink, and see whether the unit starts heating with stronger flow.
3. Clogged tankless water heater inlet screen or scale restriction
Debris at the cold-water inlet or mineral buildup inside the heat exchanger can cut flow enough that the heater will not stay lit or cannot transfer heat well.
Quick check: If flow is weak only on hot and the unit has not been serviced in a long time, inspect the inlet screen and look for scale-service reminders or related fault history.
4. Gas supply, ignition, venting, or internal fault
On gas tankless units, no gas, failed ignition, blocked venting, or a safety lockout can stop heating even when the display is on.
Quick check: See whether other gas appliances are working, whether the gas shutoff is parallel with the pipe, and whether the unit shows a fault after trying to fire.
Step-by-step fix
Step 1: Confirm the exact failure pattern first
You want to separate a dead unit from a unit that has power but is not being triggered. That saves a lot of wasted time.
- Open a hot tap at one fixture and leave it fully open for at least 30 to 60 seconds.
- Watch the tankless water heater display or indicator lights while the water is running.
- Listen for any fan, clicking, or ignition sound.
- Check whether cold-water flow at that same fixture is normal compared with hot-water flow.
- If the unit is actually a tank-style heater, stop here and use the matching electric or gas water heater no-hot-water guide instead.
Next move: If the heater starts and hot water arrives with one fixture fully open, the problem is likely low flow at certain fixtures, clogged aerators, or a mixing issue rather than a dead heater. If nothing changes, keep going. You still need to decide whether this is power, flow restriction, gas supply, or a fault lockout.
What to conclude: A tankless heater has to see enough flow and have all its safeties satisfied before it will heat.
Stop if:
- You smell gas anywhere near the unit.
- You see water dripping inside the cabinet or from a split pipe.
- The vent pipe is loose, scorched, or disconnected.
Step 2: Check power and basic reset conditions
A tankless unit that has lost power cannot open gas valves, run fans, or energize heating components. This is the fastest safe check.
- Make sure the breaker is fully on. If it looks tripped, switch it fully off and then back on once.
- Check any nearby service switch or disconnect for the heater.
- If the unit plugs into an outlet, confirm the outlet has power with another device.
- Look for a reset or power button on the front panel and restart the unit once if the manual controls allow it.
- After restoring power, run hot water again and watch for the display to wake up or for a fault code to appear.
Next move: If the display comes back and the heater runs normally, you likely had a simple power interruption or reset issue. If the display stays dark or the breaker trips again, stop DIY and have the electrical supply and heater checked professionally.
What to conclude: A dark or repeatedly tripping unit points to a supply problem or an internal electrical fault, not a routine cleaning issue.
Step 3: Rule out low flow and a clogged inlet screen
This is one of the most common tankless no-heat causes, especially when the unit powers up but never really fires or only works at one fixture.
- Open a high-flow hot outlet like a tub spout or laundry sink instead of a low-flow bathroom faucet.
- If hot water works there but not at smaller fixtures, clean the faucet aerators and showerheads and retest.
- Check that the cold-water isolation valve and hot-water outlet valve at the heater are fully open.
- If you know how to isolate the heater and relieve pressure safely, inspect the tankless water heater inlet screen for debris and rinse it with clean water.
- Reassemble, reopen valves fully, and test again with one fixture running hot at full flow.
Next move: If the heater now fires and stays hot, the trigger problem was low flow or inlet restriction. If flow is strong and the unit still does not heat, move on to supply and fault checks.
Step 4: Check gas supply and visible venting conditions on gas models
Gas tankless heaters need fuel, combustion air, and a clear vent path. If any of those are missing, the unit may lock out with no heat.
- Confirm other gas appliances in the home are working if you have them.
- Make sure the gas shutoff at the heater is fully open, with the handle parallel to the pipe.
- Look outside at the intake and exhaust terminations if they are accessible. Clear leaves, lint, snow, or nests from around the openings without taking the vent apart.
- Run hot water and watch whether the unit tries to ignite, then faults out.
- If the display shows a code after an ignition attempt, write it down for service reference.
Next move: If restoring gas supply or clearing an obvious outside blockage brings heat back, monitor the unit through a full hot-water run. If the unit still will not ignite, or it faults on combustion or venting, stop here and call a qualified tankless technician.
Step 5: Decide between maintenance, a pro service call, or a confirmed electric-part repair
By now you should know whether the problem is simple restriction, overdue descaling, or a deeper internal failure. This is where you make the right next move instead of throwing parts at it.
- If the unit has hard-water history, weak hot-side flow, or scale-related fault behavior, schedule or perform a proper tankless descaling service if your setup and skill level support it.
- If you have an electric tankless model with power present but no heat on one or more stages, have the heating circuit diagnosed before replacing parts.
- If testing confirms a failed tankless water heater heating element or tankless water heater thermostat on an electric model, replace only the failed component with an exact fit part.
- If the unit is gas-fired and still not heating after the checks above, book professional service for ignition, combustion, venting, or internal control diagnosis.
- After any repair or maintenance, run hot water at two fixtures one at a time and confirm stable outlet temperature.
A good result: If the heater now delivers steady hot water without faulting, the repair path was correct.
If not: If the unit still gives no heat, repeated lockouts, or signs of internal leakage, stop and have it professionally diagnosed.
What to conclude: Electric tankless units can have replaceable heating parts, but gas tankless no-heat problems usually need proper testing instead of parts guessing.
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FAQ
Why does my tankless water heater have power but no hot water?
Usually because it is not seeing enough water flow, the inlet screen is restricted, the gas supply is off on a gas model, or the unit is in fault lockout. Power alone does not mean it is being told to fire.
Can low water pressure keep a tankless heater from heating?
Yes. Tankless heaters need a minimum flow rate before they start heating. A clogged aerator, partly closed valve, dirty inlet screen, or weak fixture flow can keep the burner or heating elements from coming on.
Should I reset my tankless water heater?
A basic reset is fine once after checking power and obvious supply issues. If it trips or faults again right away, do not keep resetting it. Repeated lockouts usually mean the heater is protecting itself from a real problem.
Does a tankless water heater need to be descaled when it stops heating?
Often, yes, especially in hard-water areas. Scale can choke flow and reduce heat transfer enough that the unit will not heat properly or will shut down on temperature-related faults. Descaling makes sense when the symptoms and maintenance history support it.
Can I replace parts myself on a tankless water heater?
On an electric tankless model, a confirmed heating element or thermostat failure may be a reasonable DIY repair if you can safely isolate power and verify fit. On gas tankless units, no-heat problems usually move into pro territory once basic power, flow, and visible vent checks are done.