Worcester Bosch [D5] fault codewhat it means and how to fix it

Not every boiler has a condensate pump, but a good many Worcester installations do — wherever the appliance sits below the drain it has to discharge into, such as a basement, a converted loft, or a kitchen where the only available drain is on the far side of the room. In those cases a small electric pump lifts the condensate to where gravity can take over. D5 means that pump has failed or cannot discharge. This matters more than it might sound. A condensing boiler produces a surprising volume of water — several litres a day in cold weather — and it is mildly acidic, typically around pH 3 to 4. With nowhere to go it backs up into the appliance and eventually into the room. The appliance shuts down to limit the damage, which is the correct behaviour. Because a condensate pump is often an add-on fitted alongside the boiler rather than part of it, it is also the component most likely to be forgotten at service time.

1. 1

   Find the condensate pump. It is a small plastic box, usually white or grey, sitting below or beside the appliance with a thin discharge pipe leaving it.

2. 2

   Check whether it has power. Some are plugged into a socket or fed from their own fused spur, and a tripped switch is a free fix.

3. 3

   Look for standing water in or around the unit, and check whether the floor beneath is wet. If water has escaped, mop it up and put a towel down — it is mildly acidic and will mark flooring over time.

   Do not dismantle the pump or the appliance. Condensate is acidic enough to irritate skin and eyes; if you get any on you, rinse it off.
4. 4

   In cold weather, check where the discharge pipe ends outside for ice, and thaw it with hand-hot water as you would a normal condensate run.

5. 5

   If none of that resolves it, book an engineer. A replacement condensate pump is one of the cheaper repairs on a boiler and it is not worth living with.