No hot water anywhere
Every hot tap runs cold, and the unit may be blank, powered on, or showing a code.
Start here: Start with the unit display, power source, gas valve position, and whether the cold-water supply to the heater is fully open.
Tankless water heater troubleshooting
Rinnai Tankless Water Heater No Hot Water
Direct answer: If a Rinnai tankless water heater gives you no hot water, the most common homeowner-side causes are a tripped power source, closed gas valve, low water flow, or a unit that has locked out and is showing an error code. First figure out whether you have no hot water anywhere or only at one fixture.
Most likely: Most often, this turns out to be a simple supply problem or a flow issue, not a failed internal part.
Start with the easy tells: does the unit power up, does it show a code, do you hear it try to fire, and is the problem at every faucet or just one? Reality check: a tankless heater will not make hot water if the flow through it never reaches the unit's minimum trigger point. Common wrong move: cranking the temperature higher when the real issue is a half-closed valve, dirty inlet screen, or weak fixture flow.
Don’t start with: Do not start by ordering a control board or opening gas components. On tankless units, that is a fast way to waste money and miss the real problem.
Only one sink or shower is cold?Check that fixture first for a clogged aerator, showerhead, or mixing valve problem before blaming the water heater.
No hot water anywhere in the house?Go straight to power, gas supply, unit display, and water flow through the heater.
Last reviewed: 2026-04-17
What no-hot-water looks like on a tankless unit
Only one fixture has the problem
Kitchen sink, one bath sink, or one shower is cold while other fixtures still get hot water.
Start here: Start at that fixture. A clogged aerator, restricted showerhead, or bad mixing valve is more likely than a failed water heater.
Hot water comes for a minute then drops out
The water starts warm, then goes cool or fully cold during the same use.
Start here: Check for low flow, a dirty water inlet screen, partially closed service valves, or a unit that is shutting down and posting a code.
Unit looks normal but never heats
The display is on and water flows, but you do not hear normal ignition or see a temperature rise.
Start here: Check gas supply, recent outages, and any error code history. If gas is present and flow is strong, the unit may be in lockout and need service.
Most likely causes
1. Power loss or a simple lockout
A blank display, recent outage, tripped receptacle, or unit that stopped after a fault points here first.
Quick check: Make sure the unit has power, reset any tripped GFCI or breaker once, and look for an error code on the display.
2. Gas supply is off or restricted
If the unit powers up but never lights, a closed gas valve, empty propane supply, or recent gas work is a strong fit.
Quick check: Confirm the gas shutoff at the heater is fully open and other gas appliances in the home are working normally.
3. Water flow is too low to trigger heating
Tankless heaters need enough flow to fire. A clogged aerator, low-pressure fixture, dirty inlet screen, or half-closed valve can keep it from starting.
Quick check: Open a hot tap fully at a high-flow fixture and compare it to a weak sink or shower. If strong flow works better, chase the restriction.
4. Internal ignition or sensing problem
If power, gas, and flow all check out but the unit still will not fire, the problem is often inside the heater.
Quick check: Look for repeated failed ignition attempts, a fault code, or a unit that clicks then stops. That is usually the point to bring in a qualified tech.
Step-by-step fix
Step 1: Separate a whole-house problem from a single-fixture problem
This saves a lot of wasted time. One cold shower does not mean the tankless unit is bad.
- Test hot water at two or three fixtures, including one high-flow faucet like a tub spout if you have one.
- If only one fixture is affected, remove and rinse that faucet aerator or check the showerhead for mineral buildup.
- If a single shower or sink still has no hot water while others do, suspect that fixture's mixing valve or local blockage instead of the water heater.
Next move: If other fixtures have hot water, the heater is doing its job and you can stay focused on the bad fixture. If no fixture gets hot water, move to the heater itself.
What to conclude: A whole-house loss points to power, gas, flow through the heater, or an internal fault.
Stop if:
- You find leaking water around a fixture wall or under a cabinet.
- Removing an aerator or showerhead reveals damaged threads or seized parts you may worsen by forcing.
Step 2: Check the unit display and basic power supply
A tankless unit that is dead or locked out often tells you right away with a blank screen or code.
- Look at the heater display or controller. Note whether it is blank, normal, or showing an error code.
- If the display is blank, check the receptacle, any nearby GFCI, and the breaker. Reset a tripped breaker or GFCI once only.
- If the unit powers back up, run a hot tap and watch whether the display changes, the burner indicator appears, or a code returns.
- If you recently had a power outage, unplugging the unit for about 30 seconds and restoring power can clear a simple electronic hang-up.
Next move: If the unit powers up and hot water returns, the problem was likely a simple power interruption or temporary lockout. If the display stays blank with confirmed power, or a code returns immediately, the problem is beyond a basic homeowner reset.
What to conclude: Blank with good power suggests an internal electrical problem. A repeating code means the heater is protecting itself and needs the fault addressed, not just reset.
Step 3: Confirm gas and water are both fully available to the heater
A powered tankless heater still will not make hot water if it cannot get fuel or enough water flow.
- Make sure the gas shutoff valve at the heater is fully open. The handle should be in line with the pipe.
- If you use propane, confirm the tank is not empty or recently shut off.
- Check that the cold-water isolation or service valve feeding the heater is fully open.
- Open a hot tap all the way at a strong fixture and listen at the heater for normal ignition sounds.
- If other gas appliances are also acting up, stop chasing the heater and deal with the gas supply issue first.
Next move: If opening a valve or restoring gas supply brings hot water back, you found the cause without replacing anything. If gas and water are clearly available but the unit still does not ignite, keep narrowing it down with flow and filter checks.
Step 4: Rule out low-flow and inlet restriction problems
This is a very common tankless complaint, especially in homes with scale or debris in the water line.
- Run hot water at the highest-flow fixture you have, not a trickling lavatory faucet.
- Compare what happens at a tub spout or laundry sink versus a weak bathroom faucet or low-flow showerhead.
- If the unit works only with stronger flow, clean clogged aerators and showerheads with warm water and mild soap, or descale them if mineral buildup is obvious and the finish allows it.
- If you are comfortable shutting off water and using the service valves, inspect and rinse the water inlet screen on the heater if your setup allows safe access.
- Restore water, purge air at a faucet, and test again.
Next move: If the heater fires once flow improves or the inlet screen is cleaned, the issue was restriction, not a failed major component. If flow is strong and clean but the unit still will not heat, the remaining likely causes are internal and usually not a good DIY gas repair.
Step 5: Use the clues you found to decide between reset, service call, or fixture repair
By now you should know whether the problem is outside the heater, at one fixture, or inside the unit.
- If only one fixture is still cold, repair that fixture's blockage or mixing valve issue.
- If the unit lost power once and now works normally, keep using it but watch for repeat trips or codes.
- If the unit has power, gas, and strong flow but still will not ignite, write down the exact code or behavior and schedule qualified service.
- If the heater clicks, tries to light, then shuts down repeatedly, do not keep cycling it. That usually points to ignition, flame sensing, venting, or other internal safety shutdowns.
- If you have no display and confirmed power at the outlet, stop at diagnosis and arrange professional repair.
A good result: You end with a clear next move instead of guessing at expensive parts.
If not: If you still cannot pin it down, treat it as an internal heater fault and get a service tech involved.
What to conclude: Once power, gas, and flow are ruled out, the remaining fixes are usually inside the heater and often tied to combustion safety or electronic controls.
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FAQ
Why does my Rinnai tankless water heater have no hot water but still has power?
Power only tells you the unit is awake. It still needs gas and enough water flow to fire. A closed gas valve, empty propane supply, dirty inlet screen, weak fixture flow, or a fault lockout can all leave you with a powered unit and cold water.
Can low water pressure cause a tankless water heater to stop heating?
Yes. Tankless heaters need a minimum flow rate before they will ignite. If a faucet aerator, showerhead, inlet screen, or valve restriction cuts flow too much, the unit may never start heating or may drop out during use.
Should I reset my Rinnai tankless water heater?
A simple power reset after an outage or one-time glitch is reasonable. If the same code comes back, or the breaker trips again, stop there. Repeated resets do not fix the cause and can hide a bigger problem.
Why do I have no hot water at one shower but the sinks are fine?
That usually points to the shower, not the heater. The common culprits are a clogged showerhead, a bad shower mixing valve, or debris caught in the fixture after plumbing work.
Do I need to replace a part if my tankless heater starts working again after cleaning a screen or aerator?
Not usually. If the screen or aerator cleaned up well and the heater now runs normally, keep using it. Replace the Rinnai tankless water heater water inlet filter screen only if it is damaged, deformed, or will not clean up enough to reinstall safely.