Worcester
The Worcester Greenstar 28 CDi Compact ErP is a 24 kW A-rated combi boiler, offering a 10 lpm domestic hot water flow rate and compact dimensions for flexible installation.
The Worcester Greenstar 28 CDi Compact ErP is a wall-hung condensing combi boiler designed for domestic heating and hot water provision in UK properties. Introduced in 2015, this model is available for both natural gas and LPG installations. It features a compact design, making it suitable for installation in various locations, including kitchen cupboards. The appliance provides a maximum central heating output of 24 kW and a domestic hot water output of 28 kW, alongside an ErP A-rating for both heating and hot water.
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Full technical specifications extracted from the manufacturer brochure. Empty sections are hidden.
Bottom connections via plumbing manifold
pH value of system water must be less than 8. Artificially softened water must not be used to fill the central heating system.
The Worcester Greenstar 28 CDi Compact ErP operates as a combi boiler, providing both central heating and instantaneous domestic hot water. It delivers a maximum central heating output of 24 kW and a domestic hot water output of 28 kW, with a flow rate of 10 litres per minute at a 30°C temperature rise. The appliance achieves a seasonal efficiency of 89.8% and holds an ErP A-rating for both heating and hot water, with an XL domestic hot water profile. Physical dimensions are 680mm high, 390mm wide, and 280mm deep, with a dry weight of 39.5 kg. The unit incorporates a modulating pump and offers compatibility with a range of Worcester controls, including options for weather and load compensation, as well as smart connectivity. Safety features include frost protection and a flue overheat thermostat.
Every fault code listed in the manufacturer brochure, with likely causes and engineer actions. Each code has a permalink — paste a URL like https://www.boilertag.com/boilers/worcester/36cdi-compact-erp#fault-9u 233 to jump straight to the entry.
| Code | Meaning | Details |
|---|---|---|
| 9U 233Lockout | HCM (Heat Control Module) problem Show causes & actionsProblem with the code plug Likely causes
Reset procedure Press reset button | Details |
| b7 257Lockout | Internal error Show causes & actionsInternal error Likely causes
Reset procedure Press reset button | Details |
| C6 215Lockout | Fan problem Show causes & actionsFan running too fast Likely causes
Reset procedure Press reset button | Details |
| C6 216Lockout | Fan problem Show causes & actionsFan running too slow Likely causes
Reset procedure Press reset button | Details |
| C7 214Lockout | Fan problem Show causes & actionsFan not running Likely causes
Reset procedure Press reset button | Details |
| C7 217Lockout | No airflow after a defined period of time Show causes & actionsNo airflow after a defined period of time Likely causes
Reset procedure Press reset button | Details |
| D1 240Lockout | Return sensor error Show causes & actionsSensor wet or damaged (the boiler will block for 300 secs before this condition). Likely causes
Reset procedure Press reset button | Details |
| D1 241Lockout | Return sensor error Show causes & actionsSensor disconnected or damaged (the boiler will block for 300 secs before this condition). Likely causes
Reset procedure Press reset button | Details |
| D1 286Lockout | Return temperature too high Show causes & actionsBoiler overheat Likely causes
Reset procedure Press reset button | Details |
| E2 222Lockout | Flow sensor short circuit Show causes & actionsFlow sensor shorted or damaged. Likely causes
Reset procedure Press reset button | Details |
| E2 223Lockout | Flow sensor open circuit Show causes & actionsFlow sensor disconnected or damaged. Likely causes
Reset procedure Press reset button | Details |
| E5 218Lockout | Flow temperature too high Show causes & actionsPrimary flow sensor overheated Likely causes
Reset procedure Press reset button | Details |
| E5 332Lockout | Primary flow sensor has exceeded 110°C Show causes & actionsPrimary flow sensor overheated Likely causes
Reset procedure Press reset button | Details |
| E9 219Lockout | Safety sensor temperature too high Show causes & actionsThe main heat exchanger has overheated. Maximum temperature 105C° Likely causes
Reset procedure Press reset button | Details |
| E9 220Lockout | Safety sensor short circuit Show causes & actionsMaximum temperature sensor on the main heat exchanger has failed. Likely causes
Reset procedure Press reset button | Details |
| E9 221Lockout | Safety sensor open circuit Show causes & actionsMaximum temperature sensor on the main heat exchanger has not been recognised. Likely causes
Reset procedure Press reset button | Details |
| E9 224Lockout | Max temperature thermostat activated Show causes & actionsFlue gas thermostat overheat Likely causes
Reset procedure Press reset button | Details |
| EA 227Lockout | No flame detected after ignition Show causes & actionsFailure of four ignition attempts. The boiler waits 30 seconds before another attempt is made. Likely causes
Reset procedure Press reset button | Details |
| EA 229Lockout | Loss of flame signal during operation Show causes & actionsAn established flame has been extinguished. This could be caused by: moisture in the sump, due to a blocked sump/condensate trap. a strong wind into the flue. Likely causes
Reset procedure Press reset button | Details |
| EA 234Lockout | Gas valve error Show causes & actionsGas valve coil disconnected Likely causes
Reset procedure Press reset button | Details |
| EA 261Lockout | Ignition timing error Show causes & actionsFlame not established during the first 4 seconds ignition period. Likely causes
Reset procedure Press reset button | Details |
| FO 237-239, 242-256, 258, 259, 262, 263, 267, 272, 279, 290 | Internal error Show causes & actionsInternal error Likely causes
Engineer action Call Worcester, Bosch Group Technical helpline 0330 123 3366 | Details |
| FO 278 | Sensor test failed Show causes & actionsOn power up all safety sensors are checked. Check has failed. Likely causes
Reset procedure Press reset button | Details |
| FO 280 | Recycle time error Show causes & actionsAfter the burner has stopped, the recycle mode purges gas from the heat exchanger before attempting to burn again. The software has 3.1 seconds to intervene before the boiler goes into this fault condition. Likely causes
Reset procedure Press reset button | Details |
| FO 338 | Too many short burner starts without confirmation Show causes & actionsThere is an error where the pump is not detecting water. Likely causes
Reset procedure Press reset button | Details |
| F7 228 | Flame error Show causes & actionsFalse flame. Flame detected before burner start. Likely causes
Reset procedure Press reset button | Details |
| F7 328 | Internal error Show causes & actionsInternal error Likely causes
Reset procedure Press reset button | Details |
| FA 306 | False flame error Show causes & actionsFlame detected after burner stop Likely causes
Reset procedure Press reset button | Details |
| FD 231 | Mains power error Show causes & actionsElectrical power interruption. Likely causes
Reset procedure Press reset button | Details |
| FA 364 | Gas valve EV2 leak test failed Show causes & actionsGas valve leak. Likely causes
Reset procedure Press reset button | Details |
| Fb 365 | Gas valve EV1 leak test failed Show causes & actionsGas valve leak. Likely causes
Reset procedure Press reset button | Details |
| A1 281 | Pump stuck or running in air Show causes & actionsPump stuck or running in air Likely causes
| Details |
| C1 264 | Airflow stopped during operation Show causes & actionsAirflow stopped during operation Likely causes
| Details |
| C4 273 | Airflow present during last 24 hours Show causes & actionsAirflow present during last 24 hours Likely causes
| Details |
| D1 240 | Return sensor short circuit Show causes & actionsWet sensor. Blocking for 300 secs before going to Locking error (→ table 33). Likely causes
| Details |
| D1 241 | Return sensor disconnected Show causes & actionsBlocking for 300 secs before going to Locking error (→ table 33). Likely causes
| Details |
| D4 271 | Temperature difference between flow and safety sensors exceeds limit Show causes & actionsThe temperature of the primary flow and the safety sensor on the WB7 should be within 15°C of each other. Likely causes
| Details |
| E9 224 | MAX thermostat activated Show causes & actionsFlue gas thermostat overheat. Likely causes
| Details |
| E9 276 | Flow temperature exceeds 95°C Show causes & actionsPrimary overheat - boiler waits for temperature to reduce; if not it becomes a Locking error. Likely causes
| Details |
| E9 277 | Safety temperature exceeds 95°C Show causes & actionsMain heat exchanger safety sensor overheat - boiler will wait for the temperature to reduce; if not it becomes a Locking error 219. Likely causes
| Details |
| E9 285 | Return temperature exceeds 95°C Show causes & actionsThe boiler will stop the burn and wait to see if it cools down. If after 2 seconds there is an increase in temperature the boiler will go to Locking error 286. Likely causes
| Details |
| EA 227 | No ionisation detected after ignition Show causes & actionsNo flame is detected. The boiler will block during 4 attempts before going to Locking fault. Likely causes
| Details |
| EF 349 | Central heating, boil detect. Large delta T despite minimum burner. Show causes & actionsThe boiler is operating at minimum burner load and there is a greater than 18°C Flow & Return temperature difference. Likely causes
| Details |
| No code 212 | Safety or flow temperature rises too fast Show causes & actionsSafety or flow temperature rises too fast Likely causes
| Details |
| No code 213 | Temperature difference between supply and return exceeding limit Show causes & actionsTemperature difference between supply and return exceeding limit Likely causes
| Details |
| No code 260 | No temperature rise after burner starts Show causes & actionsNo temperature rise after burner starts Likely causes
| Details |
| No code 380 | Inlet DHW temperature higher than the DHW outlet temperature. Show causes & actionsThe output is blocked until the inlet temperature drops below the output temperature. Likely causes
| Details |
Patterns that group across the manufacturer's fault-code table. Each entry links to the full code listing further down the page.
Triggered by fault code 9U 233.
Triggered by fault code b7 257.
Triggered by fault code C6 215.
Triggered by fault code C7 217.
Triggered by fault code D1 240.
Diagnostic checklists from engineer-submitted Worcester faults, with step-by-step checks and safety notes.
Frequent pressure drops on a Worcester Bosch Greenstar boiler typically indicate a leak within the sealed system, a faulty pressure relief…
The Worcester Greenstar CDi is a condensing combination boiler.
The Worcester Greenstar 12i System displaying a "Heat exchanger overheat" fault code indicates the boiler's safety systems have activated…
The Worcester Greenstar 12i System displaying an 'Ignition lockout' fault indicates that the boiler has attempted to ignite but failed to…
Common questions from engineers and homeowners about the Worcester 36CDi Compact ErP.
Parts referenced in the manufacturer brochure for routine service and repair.
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Specifications on this page are extracted from the manufacturer brochure. Always cross-check against the latest datasheet before commissioning or quoting. Last updated 28 June 2026.