Navien E352 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E352 fault code mean?
The E352 fault code on Navien boilers signals that the system has detected excessively high water pressure. Specifically, the boiler's water pressure sensor has registered a reading of approximately 3.9 bar (around 56.6 PSI) or above for at least three consecutive seconds, prompting an automatic shutdown to protect the system. The fault will clear itself automatically once pressure falls back to around 3.0 bar (43.5 PSI) or below. This is a safety-driven lockout — the boiler is doing its job by refusing to operate under potentially damaging conditions.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
Common causes
- Failing or waterlogged expansion vessel Common
The expansion vessel absorbs pressure fluctuations as the system heats and cools. If its internal diaphragm has failed or it has lost its nitrogen charge, it can no longer buffer pressure rises — causing readings to spike rapidly, especially when the boiler fires up. Navien UK has noted that a significant proportion of their warranty callouts relate to expansion vessel issues that were missed during routine servicing. A waterlogged vessel may also cause the pressure relief valve to weep or discharge repeatedly.
- System overpressurised via filling loop Common
If the filling loop has been left partially open, or was used to top up pressure and not fully closed afterwards, water can continue slowly entering the system. This pushes pressure well above the safe operating range of 1.0–2.0 bar, easily triggering the E352 threshold. This is one of the simplest causes to identify — check the pressure gauge and the filling loop connection immediately.
- Faulty or stuck auto-fill valve Sometimes
Some Navien installations include an automatic filling valve that maintains system pressure without manual intervention. If this valve sticks open or fails in the open position, it continuously admits mains water into the system, causing pressure to climb steadily until it exceeds safe limits and triggers the E352 fault.
- Faulty water pressure sensor Sometimes
The E352 error is triggered by a signal from the water pressure sensor, so a defective or moisture-damaged sensor can generate false high-pressure readings even when actual system pressure is normal. Engineers can verify sensor output voltage — the normal range is approximately 0.3–2.8V — to determine whether the sensor is reading accurately or needs replacement. Sensor wiring should also be checked for corrosion or loose connections.
- Pressure relief valve (PRV) not functioning correctly Sometimes
The PRV is designed to open and release water if pressure climbs dangerously high, acting as a last line of defence. If it has stuck closed, it cannot perform this protective role and system pressure will continue to build unchecked. Conversely, a PRV that is weeping or discharging regularly is often a symptom of an underlying pressure problem rather than a cause — but a stuck or incorrectly rated PRV should be inspected as part of diagnosis.
- Internal heat exchanger cross-contamination Rare
In rare cases, a crack or failure in the domestic hot water heat exchanger can allow mains-pressure cold water (which is typically at 2–6 bar) to bleed into the lower-pressure central heating circuit. Because mains pressure is higher than normal system pressure, this can push the heating circuit above 3.9 bar and trigger E352. This is a more serious fault and will usually require heat exchanger replacement.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler or on the pipework nearby. Normal operating pressure for a Navien boiler is between 1.0 and 2.0 bar when the system is cold. If the gauge reads significantly above 2.0 bar — and especially if it is approaching or above 3.0 bar — you have confirmed the system is overpressurised. Note the reading before doing anything else.
- Check whether the filling loop is open or leaking DIY safe
Locate the filling loop — usually a flexible braided hose with one or two valves connecting the cold mains supply to the heating pipework. Both valve handles should be perpendicular to the pipe (closed position). If either handle is in line with the pipe, it is open and allowing water in. Close both valves fully and monitor whether pressure continues to rise. If the loop has been disconnected after use, confirm it is fully removed and capped.
- Attempt a boiler reset DIY safe
On most Navien boilers, you can reset the fault by pressing the Back button on the front panel. If the boiler restarts and runs normally without the fault reappearing, monitor the pressure gauge over the next 30–60 minutes. If E352 returns quickly or repeatedly, do not continue resetting — repeated lockouts indicate an underlying issue that requires an engineer.
- Do not attempt to bleed pressure by opening valves or components Gas Safe engineer
Releasing pressure from a heating system by opening drain cocks, bleed valves on the boiler body, or any internal component is not a safe DIY task on a system that may be at very high pressure. Scalding water or steam can be released suddenly. Leave any pressure reduction to a Gas Safe registered engineer who has the correct tools and training.
- Do not attempt to inspect or replace the expansion vessel, PRV, auto-fill valve, or pressure sensor Gas Safe engineer
These components sit within the sealed heating system or involve pressurised connections. Testing the expansion vessel pre-charge, replacing a PRV, or verifying sensor output voltage all require specialist equipment and Gas Safe competency. Attempting these without the correct qualifications may also invalidate your Navien warranty.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler has locked out on E352, the most common repairs — expansion vessel recharge or replacement, PRV replacement, auto-fill valve repair, or pressure sensor replacement — all require a qualified engineer. When you call, mention the E352 code and describe what the pressure gauge shows. This will help the engineer arrive prepared with appropriate parts. You can verify an engineer's credentials at gassaferegister.co.uk.
Parts you may need
- Expansion vessel (Navien-compatible, typically 8–12 litre) · from £55
- Water pressure sensor · from £22
- Pressure relief valve (3 bar PRV) · from £18
- Auto-fill / pressure reducing valve · from £35
- Filling loop assembly · from £14
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 £100–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Navien E352 fault myself without calling an engineer?
There are a couple of safe checks you can do yourself: confirm the filling loop is fully closed, read the pressure gauge, and attempt a single reset using the Back button. However, if the boiler locks out again or the pressure remains above 2 bar, the underlying fix — whether that is recharging the expansion vessel, replacing the PRV, or swapping the pressure sensor — must be carried out by a Gas Safe registered engineer. Attempting internal system work yourself risks scalding injuries, voiding your Navien warranty, and potentially invalidating your home insurance.
Why does my Navien boiler keep showing E352 even after I reset it?
A fault that keeps returning after resetting almost always points to an ongoing condition rather than a one-off glitch. The most common culprits are a waterlogged expansion vessel that can no longer absorb pressure changes, a filling loop or auto-fill valve that is continuously admitting water, or a faulty pressure sensor generating false readings. Each of these requires professional diagnosis and repair — repeated resetting without addressing the root cause can mask a worsening problem and may eventually cause the PRV to discharge, which wastes water and can damage flooring or ceilings.
What pressure should my Navien boiler run at, and what triggers E352?
Navien boilers operate correctly at between 1.0 and 2.0 bar when the system is cold (cold fill pressure). As the system warms up, pressure naturally rises slightly — this is normal. The E352 fault is triggered when pressure reaches approximately 3.9 bar and holds there for three or more seconds. The fault clears automatically once pressure drops back to around 3.0 bar or below. If your gauge regularly climbs above 2.5 bar, you should have the expansion vessel inspected at your next service even if E352 has not yet appeared.
How much does it cost to fix a Navien E352 fault in the UK?
Most people with an E352 fault pay somewhere between £100 and £380 all-in. A pressure sensor swap or PRV replacement at the lower end of this range (£100–£200) is straightforward for an experienced engineer. Expansion vessel replacement — the most common root cause — typically costs £180–£350 depending on access and the vessel size required. In more complex cases where an internal heat exchanger has failed and is allowing mains water into the heating circuit, costs can rise considerably higher, but this is a rare outcome. Getting two or three quotes from Gas Safe registered engineers is always worthwhile.