Worcester Bosch [H27] fault codewhat it means and how to fix it

H27 is the gentlest thing the Heatslave II can show you. Table 13 of the 2022+ manual lists two H codes for this boiler, and both come with a warning triangle rather than a flashing blue light: H27, "DCW in sensor defect (appliance operation not affected)", and H29, weather compensation activated but no weather sensor detected. DCW is domestic cold water, and the sensor in question reads the temperature of the mains water coming into the boiler. It matters more on this boiler than on most because of what the Heatslave does with the reading.

The manual's solar compatibility section describes the logic: if the incoming water is already warm — from a solar pre-heat cylinder, say — the burner is held off for hot water above a certain flow, and if it arrives above 80 °C the burner and pump are stopped altogether with 380 recorded in the service menu. Without a working inlet sensor the boiler cannot make those judgements, so it falls back to running as though the water were cold, which is why Worcester can say operation is not affected: you lose an efficiency, not a function.

Because it is an H code the boiler carries on; a reset will not clear a sensor that has failed, and the code is there to be noted rather than acted on urgently. The repair is a sensor on the cold inlet pipe inside the case, and it belongs with the next service or a visit at your convenience rather than an emergency call. The codes the Heatslave shares with Worcester's gas boilers behave the same way — the Greenstar i manual describes its H family as a situation that will need some attention while the boiler continues — and H29 on the Heatslave is the same H29 the Greenstar i shows when weather compensation is switched on with no outdoor sensor connected.

1. 1

   Confirm what you are seeing: a warning triangle with H27 and the blue light steady, not a flashing blue light with a letter pair and a three-digit number. If the blue light is flashing, look up that code instead — it is the one stopping the boiler.

2. 2

   Check that the heating and the hot water are both working normally. Worcester say H27 does not affect operation, so if either has stopped there is a second fault to find, and that is the one to report.

3. 3

   Note the code and book the sensor replacement with your next service, or at a convenient time. It is a small sensor on the cold inlet inside the case — an OFTEC registered technician's job, but not an urgent one.

4. 4

   If solar-heated water feeds this boiler, mention it when you book. The boiler uses this sensor to decide when the burner can stay off for pre-heated water, so that saving is lost until the sensor is replaced.