Ravenheat E29 Fault Code: Hot Water System Overheating
The E29 code on a Ravenheat boiler signals that the domestic hot water or central heating circuit has reached a dangerously high temperature. The boiler's built-in safety system detects that water temperatures have climbed beyond their safe operating limit and triggers a lockout to protect the appliance and your home. The boiler will not restart until the underlying cause is identified and resolved — resetting without fixing the root problem will simply bring the fault back.
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 Ravenheat E29 fault code mean?
The E29 code on a Ravenheat boiler signals that the domestic hot water or central heating circuit has reached a dangerously high temperature. The boiler's built-in safety system detects that water temperatures have climbed beyond their safe operating limit and triggers a lockout to protect the appliance and your home. The boiler will not restart until the underlying cause is identified and resolved — resetting without fixing the root problem will simply bring the fault back.
Common causes
- Limescale build-up in the heat exchanger Common
In hard-water areas of the UK, limescale gradually coats the inside of the heat exchanger, forming hotspots where water is superheated before the NTC thermistor can respond. A characteristic kettling or rumbling noise while the boiler runs is a strong indicator. If left untreated, the restricted heat exchange eventually pushes outlet water temperatures high enough to trigger the E29 lockout.
- Heating sludge blocking water flow Common
Rust and debris shed by ageing radiators, pipes, and fittings mix with system water to create a thick black sludge. This material accumulates inside the heat exchanger and around the pump, slowing circulation so that heated water lingers in the boiler rather than being carried away around the system. The resulting temperature spike causes the safety cut-out to fire.
- Faulty NTC thermistor (temperature sensor) Common
The NTC thermistor continuously feeds water-temperature readings to the PCB so the boiler can regulate its output. A failing thermistor — a well-documented weak point on models such as the CSI 85T — can send inaccurate readings that either allow the boiler to overheat before a shutdown is triggered, or cause a spurious E29 lockout when temperatures are actually within range.
- Circulation pump failure Sometimes
If the pump stops moving water around the system — whether through mechanical failure, a seized impeller, or sludge ingress — heat builds up inside the boiler body with nowhere to go. The safety thermostat detects the rapid temperature rise and shuts the boiler down. A pump that is running but producing a grinding or humming noise may be on the verge of total failure.
- Faulty pressure relief valve (PRV) Sometimes
The PRV is designed to vent excess pressure before it becomes dangerous. When this valve sticks or deteriorates, it can no longer regulate system pressure effectively, contributing to the conditions that lead to overheating. A PRV that is visibly weeping water or failing to hold pressure is a sign it needs attention.
How to fix it
- Check for a kettling or rumbling noise DIY safe
Stand near the boiler while it is running (before lockout silences it). A persistent kettling sound — similar to a boiling kettle — strongly suggests limescale hotspots in the heat exchanger and will help the engineer diagnose the problem quickly.
- Note the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler fascia. It should sit between 1.0 and 1.5 bar when cold. If pressure is unusually high (above 2.5 bar) or has dropped below 0.5 bar, note this down as it is useful diagnostic information for the engineer.
- Top up system pressure if it has dropped low DIY safe
If the gauge reads below 1.0 bar, you can top up via the filling loop — consult your boiler's user guide for the correct procedure. Do not pressurise above 1.5 bar. This alone is unlikely to clear the E29 fault but may allow a reset attempt.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (or use the on/off switch as described in your user manual) for approximately three seconds. If the boiler fires up and runs normally, monitor it over the next hour. If the E29 code returns promptly, do not keep resetting — repeated resets without addressing the cause can mask worsening problems.
- Do not attempt to open the boiler casing or touch internal components Gas Safe engineer
Diagnosing overheating faults requires testing the thermistor, inspecting the heat exchanger, and assessing the pump — all of which involve internal boiler components. This work must only be carried out by a Gas Safe registered engineer.
- Book a Gas Safe registered engineer to carry out a full diagnosis Gas Safe engineer
An engineer will use a flue-gas analyser and temperature probes to confirm whether the fault lies with the thermistor, heat exchanger, pump, or PRV. Based on findings, they may carry out a chemical descale, power flush, or component replacement. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- NTC thermistor (flow or return) · from £25
- Circulation pump · from £85
- Pressure relief valve (PRV) · from £20
- Heat exchanger · from £500
- Magnetic system filter (e.g. Adey MagnaClean) · from £70
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 £200–£400, depending on the underlying cause.
Frequently asked questions
Can I keep resetting the boiler to get hot water while I wait for an engineer?
It is best not to. The E29 lockout exists to prevent the boiler from running at unsafe temperatures. Repeatedly resetting without addressing the cause puts extra thermal stress on the heat exchanger and other components, potentially turning a relatively affordable repair (such as a thermistor replacement) into a much costlier one. One reset to check whether the fault clears is reasonable; beyond that, leave the boiler off and arrange an engineer visit promptly.
How much does it cost to fix a Ravenheat E29 fault?
For the most common repairs — thermistor replacement or pump replacement — most UK homeowners pay in the region of £200–£400 including diagnosis, parts, and labour. A power flush to clear sludge typically adds £300–£500 depending on system size. If the heat exchanger is severely scaled or damaged, the part alone can cost £400–£600 before labour, which is often the point where replacing an older Ravenheat boiler becomes more cost-effective than repairing it.
Is the E29 fault common on the Ravenheat CSI 85T?
Yes. The CSI 85T has a known history of NTC thermistor issues, and the E29 overheating lockout is one of the more frequently reported faults on this model. If your boiler is a CSI 85T and is more than eight to ten years old, it is worth asking the engineer for an honest assessment of whether repair or replacement represents better value, particularly if other components are also showing wear.
Will a power flush fix the E29 fault?
If sludge or limescale is the primary cause of restricted flow and overheating, a power flush carried out by a Gas Safe engineer — often combined with a descaling chemical such as Sentinel X200 — can resolve the fault and significantly extend the boiler's working life. However, if the heat exchanger has already sustained damage from prolonged overheating, or if a thermistor or pump has failed independently, a flush alone will not be sufficient and those components will still need replacing.