What d5 means
The error-code table in Worcester Bosch's Greenstar Highflow user instructions gives d5 a single description: condensate pump fault. Worcester's instruction next to that table is equally short, and it applies to every code in it. Do not try to rectify the listed faults yourself, and contact a Worcester Bosch service engineer or another Gas Safe registered engineer.
So this page is not going to talk you through a repair. It is going to explain what the water is, why the code sometimes arrives with a wet floor, and how to tell that water apart from a heating leak, because the two get confused and they need different people.
The water in question is not your heating water
Condensate is the waste product of a condensing boiler. Burning gas produces water vapour, the boiler extracts heat from that vapour until it condenses back to liquid, and the liquid drains away. It is mildly acidic, roughly the pH of vinegar, which is why it runs to a drain in plastic rather than copper pipe. A working boiler produces a steady trickle of it whenever it fires.
Where the boiler sits below or far from a drain, gravity will not do it, so a small condensate pump is fitted to lift the water to the waste. That pump has a reservoir and a float switch, and d5 is the boiler reporting that the pump or its float switch is not doing its job.
This matters for one reason. Your sealed heating circuit is unaffected. The pressure gauge does not fall because of d5, no water is leaving the radiators or the buried pipework, and no amount of leak detection on the heating system will find anything. The problem is a few litres a day of waste water with nowhere to go.
Which is still water on your floor
A condensate pump that has failed does not simply stop. The reservoir fills, and then it overflows, usually into the boiler cupboard, and from there into the floor, the ceiling below, or a flat below that. In a first-floor airing cupboard or a kitchen wall unit that is enough to stain a ceiling within a day and to soak a chipboard floor within a week.
The signs that go with it: a boiler that locks out and restarts, a gurgling noise as it tries to run, water pooling under or behind the boiler that is clear and slightly sharp-smelling rather than the dark, metallic water that comes out of a heating system, and a damp patch that grows only while the boiler is firing.
Telling condensate from a heating leak in two minutes
| Test | Condensate | Heating water |
|---|
| Pressure gauge over a day | Steady. Condensate is not drawn from the sealed circuit. | Falls. That is the definition of the fault. |
| Colour of the water | Clear. | Usually dark grey or black from magnetite sludge, and it stains. |
| When it appears | Only while the boiler is running, and worst on long heating runs. | Continuously, or whenever the circuit is under pump pressure. |
| Where it appears | At or immediately below the boiler, near the condensate pipe or pump. | Anywhere on the pipe run, often a long way from the boiler. |
If the gauge is falling as well, you have two separate things happening and the heating loss is the one that does structural damage. Worcester boilers losing pressure deals with that.
What you can safely check yourself
Very little, and that is the honest answer. The pump sits inside or alongside the appliance, and Worcester's own instruction on this table of codes is not to rectify the fault yourself. What you can reasonably do is stop the damage while you wait: turn the heating off so the boiler stops producing condensate, put a tray or towels under the leak, and move anything stored in the cupboard. If the boiler discharges to an outside condensate pipe rather than a pump and the weather has been below freezing, check whether that pipe is frozen instead, because a frozen pipe on a pumped system loads the pump against a blockage.
When to call us instead
Book a Gas Safe registered engineer for the d5 itself. Call us for what the water has done, or when you are not convinced the water is condensate at all:
- A ceiling below the boiler is stained, sagging or dripping and you need the source proved before repairs.
- The floor around or below the boiler cupboard is wet and you do not know how far the water has travelled.
- The pressure gauge is falling as well as the boiler showing d5, so there is a heating leak underneath the condensate problem.
- You are making an insurance claim and need a written trace report and the damage made good.
Moisture mapping is the useful tool here. It shows how far water has spread through a floor or ceiling void, which is almost always further than the visible stain, and it tells you what has to come out and what can dry. Where the source is genuinely unclear, thermal imaging and tracer gas separate a heating pipe from a condensate overflow without opening anything.
We charge £150 per hour, fixed before we attend, with no find, no fee on the time spent looking for a leak. Once the source is settled, our own handymen and builders repair the ceiling, floor or plaster rather than handing you a report. See ceiling leak repair, leak damage repair and reinstatement, and insurance claims.
