Central heating (CH) return sensor overheating
The E21 error code on Ravenheat boilers indicates central heating (ch) return sensor overheating.
Ravenheat E21 means: central heating (ch) return sensor overheating. The usual causes are circulation pump failure or air lock in the system, blocked or restricted heat exchanger (sludge or limescale) or system valves closed restricting water flow. You can safely reset the boiler once — if E21 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does E21 Mean
The E21 code is shown by the boiler’s control board when it detects central heating (ch) return sensor overheating. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows E21 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 1 technical sources.
| Cause |
|---|
| Circulation pump failure or air lock in the system |
| blocked or restricted heat exchanger (sludge or limescale) |
| system valves closed restricting water flow |
| faulty thermistor giving incorrect temperature reading |
| thermostat wiring fault |
| insufficient water flow through the boiler |
Safety Warning
E21 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
Check the circulation pump — verify it is running and water is flowing (feel the pipes either side of the pump for temperature difference)
Check system valves — ensure all isolation valves and zone valves are fully open
Check the system for air locks — bleed radiators and check for gurgling noises
Inspect the heat exchanger — check for limescale build-up or sludge
Test the flow and return thermistors — compare the temperature reading with an external thermometer
Check pump speed setting — increase if necessary
If the system has a filter (magnetic or inline), check if it needs cleaning
How to Fix It
Bleed the system of air
flush the heating system if sludge is present (consider a power flush)
replace the pump if it is not circulating
descale or replace the heat exchanger if blocked with limescale
open any closed valves
replace faulty thermistors
Refer to the Ravenheat service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Ravenheat models that can display fault code E21:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Ravenheat models | — | Possible |
Frequently Asked Questions
Bleed the system of air. According to the technical documentation, this is the first check on the list for E21.
E21 is not classified as a high-severity fault, so the boiler can be reset once. If E21 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.