Navien E016 Fault Code: Secondary Heat Exchanger Overheat
The E016 code on a Navien boiler signals that the secondary (plate) heat exchanger has reached an unsafe temperature and triggered the built-in safety cutout. To protect the appliance and prevent potential carbon monoxide risks, the boiler locks itself down and stops operating until the underlying cause is resolved. This is not a fault you can ignore or repeatedly reset away — the boiler will keep locking out until the root problem is fixed.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Navien E016 fault code mean?
The E016 code on a Navien boiler signals that the secondary (plate) heat exchanger has reached an unsafe temperature and triggered the built-in safety cutout. To protect the appliance and prevent potential carbon monoxide risks, the boiler locks itself down and stops operating until the underlying cause is resolved. This is not a fault you can ignore or repeatedly reset away — the boiler will keep locking out until the root problem is fixed.
Common causes
- Limescale or sludge blocking the plate heat exchanger Common
In hard-water areas, calcium deposits gradually coat the internal passages of the secondary heat exchanger, cutting heat-transfer efficiency and forcing temperatures to climb until the safety limit trips. Sludge migrating from an unfiltered system has the same effect. This is by far the most frequent trigger for E016 on UK-installed Navien combis.
- Restricted or blocked cold water inlet filter Common
Navien boilers have a strainer filter on the cold water inlet. When this becomes clogged with debris, sediment or pipe scale, the flow rate through the plate heat exchanger drops sharply, causing localised overheating even if the exchanger itself is relatively clean.
- Inadequate water circulation — pump or airlock Sometimes
A circulation pump that is running slowly, seized, or air-locked cannot move water through the heat exchanger quickly enough to carry heat away. The resulting hot-spot is enough to trigger E016. Closed isolation valves elsewhere in the system can produce the same symptom.
- Low system pressure Sometimes
If system pressure has dropped below around 1 bar, there is insufficient water volume circulating. The heat exchanger overheats because the small amount of water present absorbs heat faster than it can be replaced. Pressure loss is often due to a slow leak or failed pressure relief valve.
- Pinhole leak in the heat exchanger or pipework Rare
A small internal or external leak reduces the effective water volume circulating through the exchanger. These leaks can be hard to spot visually but will cause recurring E016 lockouts and may eventually cause water damage if left unaddressed.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. It should read between 1 and 1.5 bar when cold. If it reads below 1 bar, top up the system via the filling loop (usually a braided hose or lever valve beneath the boiler) until the needle sits around 1.2 bar, then close the loop securely. If the pressure drops again within a few days, there is a leak that needs a Gas Safe engineer.
- Allow the boiler to cool, then attempt a single reset DIY safe
Press and hold the reset button (or follow the sequence in your Navien manual) once the boiler has been off for at least 20–30 minutes. If it fires up normally, monitor it closely over the next hour. Do not reset more than two or three times in total — repeated resets without a fix can damage components and may affect your warranty.
- Check and clean the cold water inlet filter DIY safe
Locate the cold water inlet on the boiler (consult your Navien manual for its exact position). Turn off the cold water supply, release pressure carefully, and remove the small strainer mesh. Rinse it under a tap to clear any sediment or debris, then refit it. Restoring proper flow can sometimes resolve E016 if a restricted filter was the sole cause.
- Bleed radiators to clear any airlocks DIY safe
Use a radiator key to open each bleed valve in turn, starting with the ground floor. Release air until water flows steadily, then close the valve. Recheck the system pressure afterwards and top up if it has dropped. Removing airlocks improves overall circulation and may help if sluggish flow contributed to the overheat.
- Have a Gas Safe engineer inspect and clean the secondary heat exchanger Gas Safe engineer
If the fault persists after the checks above, the plate heat exchanger likely needs a professional clean or descale. An engineer can isolate and flush the exchanger using approved equipment and chemicals, restoring proper heat transfer without risking damage to other components. This is the most common professional fix for E016.
- Have a Gas Safe engineer inspect the circulation pump, 3-way valve, and high-limit thermostat Gas Safe engineer
If cleaning the exchanger does not resolve the fault, an engineer should test the pump output, check the 3-way valve for sticking, and verify that the high-limit thermostat and associated sensors are reading accurately. Any faulty components will need replacement by a qualified professional.
- Arrange a full system power flush if sludge contamination is suspected Gas Safe engineer
Where sludge is the likely cause — particularly in older systems without a magnetic filter — a Gas Safe engineer or heating specialist can carry out a power flush to remove debris from throughout the pipework and heat emitters. A magnetic filter should then be fitted to prevent recurrence.
- Call a Gas Safe registered engineer if the fault recurs Gas Safe engineer
If E016 keeps returning after any of the above professional work, further investigation is needed — possibly a pinhole leak, a failing PCB, or a heat exchanger that is beyond cleaning and requires replacement. Only a Gas Safe registered engineer should carry out these diagnostics and repairs.
Parts you may need
- Plate heat exchanger (secondary) · from £180
- Circulation pump · from £95
- Cold water inlet filter/strainer · from £12
- High-limit thermostat/overheat sensor · from £35
- 3-way diverter valve · from £75
- Magnetic system filter (e.g. Magnaclean) · from £60
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear E016?
Resetting once or twice is fine as a first step, but the boiler will lock out again each time if the root cause has not been fixed. Repeated resets without addressing the problem can stress components and, on some Navien models, may affect your warranty. If the fault returns after two resets, stop and call a Gas Safe engineer.
Is E016 more common in hard-water areas?
Yes. In areas with naturally hard water — much of southern and eastern England, for example — limescale builds up inside the plate heat exchanger over time, gradually restricting flow and reducing heat-transfer efficiency until the overheat safety trips. Fitting a scale reducer or inhibitor at installation and having the boiler serviced annually significantly reduces the risk. If you are already seeing E016 and you live in a hard-water area, limescale is the most likely culprit.
How much does it typically cost to fix Navien E016 in the UK?
Most people with this fault pay between £150 and £450 all in. A heat exchanger clean or descale by a Gas Safe engineer usually sits in the £150–£300 range, while a pump replacement typically adds £200–£350 including labour. A full system power flush tends to cost £300–£500 depending on system size. In rare cases where the secondary heat exchanger itself has failed beyond cleaning, replacement parts and labour can push costs higher — if you are facing a quote in that territory on an older boiler, it is worth comparing against the cost of a new unit.
How can I prevent E016 from coming back?
The most effective steps are: have the boiler serviced every year by a Gas Safe engineer; fit a magnetic system filter (if one is not already installed) to catch circulating debris; consider a scale reducer or water softener if you are in a hard-water area; and keep the system topped up to the correct pressure (1–1.5 bar). Annual servicing also gives an engineer the chance to clean the cold water inlet filter before it becomes restricted enough to cause problems.