Code appears as soon as hot water starts
The unit fires, then shuts down quickly with little or no sustained hot water.
Start here: Look for a serious flow restriction first, especially a clogged water inlet filter or a valve not fully open.
Tankless water heater overheating code
Navien Tankless Water Heater Code E016
Direct answer: A Navien tankless water heater code E016 usually means the unit is seeing overheating or a heat-exchanger temperature that is climbing too fast. The most common homeowner-level causes are restricted water flow, a dirty water inlet filter, scale inside the heat exchanger, or a circulation problem around the unit.
Most likely: Start with the simple stuff: make sure the service valves are fully open, clean the water inlet filter, and see whether hot water flow at faucets is weak or surging. If flow is poor, the heater can overheat even though the burner and controls are doing what they should.
This code is one of those faults that can look dramatic but often starts with a plain mechanical problem. Reality check: a tankless unit that cannot move enough water through the heat exchanger will trip on temperature fast. Common wrong move: resetting the heater over and over without fixing the restriction that caused the code in the first place.
Don’t start with: Do not start by ordering a control board or opening gas-side components. On this code, flow and scale problems are more common than an electronic failure.
If hot water flow is weak at several fixtures,check the water inlet filter and any partially closed service valves first.
If flow seems normal but the code returns during longer hot water use,suspect scale buildup or an internal circulation issue and plan on a pro visit.
Last reviewed: 2026-04-17
What E016 usually looks like in the house
Code shows up during longer showers or tub fills
Short draws may work, but longer hot water use trips the code.
Start here: Scale buildup in the heat exchanger becomes more likely when the problem shows up under longer run time.
Hot water flow is weak or pulses
Fixtures may surge from hot to cooler water, or overall hot-side flow feels reduced.
Start here: Check the water inlet filter, service valves, and any obvious plumbing restriction before assuming an internal part failed.
Code returns right after a reset
The heater clears briefly, then faults again on the next call for hot water.
Start here: Treat that as an active problem, not a glitch. Verify flow and stop using repeated resets as the fix.
Most likely causes
1. Dirty water inlet filter
A clogged inlet screen cuts water flow through the tankless heat exchanger, and that can drive temperature up fast enough to trigger E016.
Quick check: Shut off power and water isolation as needed, remove the water inlet filter, and look for grit, scale flakes, or debris packed into the screen.
2. Scale buildup in the tankless heat exchanger
Mineral scale narrows the water path and insulates the heat-transfer surface, so the unit runs hotter than it should, especially on long hot water draws.
Quick check: Think about water hardness and service history. If the unit has not been descaled in a long time and the code shows up during longer use, scale is high on the list.
3. Restricted or unstable water flow through the unit
Partly closed service valves, clogged fixture aerators after plumbing work, or a broader supply restriction can all reduce flow enough to trip an overheat code.
Quick check: Compare hot water flow at more than one fixture. If several hot taps are weak, the problem is likely upstream of the faucet.
4. Internal circulation or sensor-related fault
If flow is good, the filter is clean, and the unit still overheats, the issue may be inside the heater, such as a circulation problem or a temperature-sensing problem.
Quick check: If the code returns with normal house flow and no obvious restriction, stop at basic checks and schedule service rather than guessing at internal parts.
Step-by-step fix
Step 1: Confirm this is a real overheating pattern, not a one-off glitch
You want to separate a brief nuisance trip from a repeatable problem before taking anything apart.
- Run one hot water fixture that normally gives a steady draw, like a shower or tub spout.
- Watch whether the heater starts normally, then faults within seconds, or only after a minute or two.
- Notice whether hot water flow at the fixture is weak, pulsing, or normal before the code appears.
- Reset the unit once if needed, but only once for testing.
Next move: If the code does not return and hot water stays steady, keep an eye on it, but still move to the next simple checks because intermittent restrictions often come back. If E016 returns right away or during the same test draw, move on to flow and filter checks.
What to conclude: A repeatable return points to a real overheating condition, and poor or unstable flow is the first thing to rule out.
Stop if:
- You smell gas anywhere near the unit.
- The cabinet is unusually hot, scorched, or making sharp banging noises.
- Water is leaking from the unit or nearby piping.
Step 2: Check for obvious flow restrictions around the water heater
Partially closed valves and simple flow restrictions are common, safe to inspect, and easy to miss.
- Make sure the cold-water supply valve to the tankless water heater is fully open.
- If the unit has service isolation valves, confirm they are in the normal operating position, not partly closed after past maintenance.
- Open hot water at two different fixtures and compare flow strength.
- If only one fixture is weak, clean that fixture aerator or showerhead before blaming the heater.
Next move: If flow improves and the code stops returning, the heater was likely overheating because water could not move through it properly. If several fixtures still have weak hot flow or the code returns with normal-looking flow, continue to the inlet filter check.
What to conclude: House-side flow clues help you separate a local faucet issue from a restriction at or inside the tankless unit.
Step 3: Clean the tankless water heater inlet filter
A dirty inlet screen is one of the most common homeowner-fix causes of overheating codes on tankless units.
- Turn off power to the tankless water heater.
- Shut off the water supply to the unit using the proper isolation valve if present.
- Relieve pressure by opening a nearby hot water tap.
- Remove the water inlet filter or screen cap carefully and pull out the filter.
- Rinse the screen under clean water and gently remove grit or scale flakes. Use a soft brush only if needed.
- Reinstall the filter, reopen the water valve fully, restore power, and test hot water again.
Next move: If the code clears and hot water stays steady, the clogged inlet filter was likely the cause. If the filter was clean or the code still returns, scale inside the heat exchanger becomes more likely.
Step 4: Decide whether scale buildup is the likely next problem
If the inlet filter is clean and the code shows up under longer hot water use, internal scale is the next most common cause.
- Think about your water hardness and whether the unit has been flushed or descaled on a regular schedule.
- Notice whether hot water starts fine but drops out during longer draws like showers or tub fills.
- Look for a history of reduced hot water performance, temperature swings, or repeated overheat-type faults.
- If you are already set up and experienced with tankless descaling using the unit's service valves, follow the normal maintenance procedure for your unit. If not, schedule service.
Next move: If a proper descaling restores steady operation, scale restriction was likely the root problem. If descaling does not help, or you are not equipped to do it safely, the problem is likely inside the unit and needs service diagnosis.
Step 5: Stop at the unit and book service if flow checks did not solve it
Once basic flow restrictions and maintenance are ruled out, the remaining causes are usually internal and not good guess-and-buy territory for homeowners.
- Tell the service company the exact code, when it appears, and whether hot water flow is weak or normal.
- Mention whether you cleaned the tankless water heater inlet filter and whether the unit has been descaled recently.
- If the heater is needed urgently, use only as much as necessary until it is inspected, and avoid repeated resets if the code keeps returning.
- If the unit leaks, smells like gas, or overheats immediately, shut it down and leave it off until serviced.
A good result: If the technician confirms a circulation or sensing issue and repairs it, verify the unit can run a full shower or tub fill without tripping the code.
If not: If the code persists even after service, ask for a full flow, scale, and sensor diagnosis rather than another blind reset.
What to conclude: At this point the likely causes are internal to the tankless water heater, and the safe next move is targeted service, not random parts replacement.
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FAQ
What does Navien code E016 mean?
It generally points to an overheating condition. In plain terms, the unit is seeing temperature rise too high or too fast, often because water is not moving through the heat exchanger the way it should.
Can a dirty filter really cause E016?
Yes. A clogged tankless water heater inlet filter can cut flow enough to make the heat exchanger run too hot, and that is one of the first things worth checking.
Will resetting the unit fix E016?
Usually only temporarily, if at all. If the code comes back during the next hot water call, the restriction or overheating cause is still there.
Is E016 usually a bad control board?
No, not as a first guess. Flow restriction and scale are more common than a board failure, and control parts are not a smart first purchase on this symptom.
Can I descale the tankless water heater myself?
Some homeowners do, but only if the unit has proper service valves and they already know the maintenance procedure. If you are not set up for that job, it is better to schedule service than improvise.
When should I call a pro right away for E016?
Call right away if you smell gas, see leaking from the unit, the cabinet gets unusually hot, or the code returns after you confirmed good valve position and cleaned the inlet filter.