What the D5 code means
Condensing boilers produce water, called condensate, as they burn gas. Most wall-hung boilers let this drain away by gravity. Floor-standing Greenstar boilers, such as the Greenstar FS CDi range and the Highflow CDi storage combis, are often fitted where gravity drainage is not possible, so they include a small internal pump that lifts the condensate out to a drain.
On these models, Worcester's user instructions list D5 as a condensate pump fault, and the Highflow CDi manual describes it as condensate overflow. In simple terms, the boiler has detected that condensate is not being cleared, and it has stopped to avoid water building up inside the appliance.
D5 does not mean the same thing on every Worcester boiler. Some other Greenstar models use D codes for different faults, and older models may not use it at all. If your boiler is a wall-hung combi rather than a floor-standing model, check the fault table in your manual before going further.
Common causes of D5
Frozen condensate pipe
If the pipe carrying condensate away runs outside, it can freeze in a hard frost. The pump then has nowhere to push the water, and the boiler reports a fault. This is the most common cause in winter and one you can often deal with yourself.
Blocked condensate pipe or drain
Debris, sludge or a kinked pipe can restrict the flow even in mild weather. The outlet end is sometimes blocked by leaves or soil, especially where it runs into a garden or a gully that fills with debris in autumn.
Condensate pump failure
The pump has a motor and a float switch. Either can wear out or stick, and when that happens the condensate is not lifted away.
Blocked condensate trap
The trap inside the boiler can fill with sludge over time. Clearing it means opening the boiler, so it is an engineer job.
Electrical or control faults
Less often, the wiring to the pump or the boiler control board is the problem. An engineer can test the pump and float switch separately to find out which part is at fault before replacing anything.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Frozen condensate pipe | Freezing weather, ice at the pipe end | You, by thawing the pipe |
| Blocked pipe outlet | Leaves or soil over the end of the pipe | You, if easy to reach at ground level |
| Condensate pump failure | Pipe clear, fault keeps returning | Gas Safe engineer |
| Blocked condensate trap | Gurgling, water near the boiler | Gas Safe engineer |
| Wiring or control fault | Fault with no sign of a blockage | Gas Safe engineer |
Safe checks you can do yourself
- Find where the condensate pipe leaves the house. It is usually a white or grey plastic pipe running to a drain or soakaway.
- In freezing weather, look for ice at the end of the pipe or at bends. Thaw it by pouring warm (not boiling) water along the outside of the pipe, or hold a hot water bottle against it. Our frozen condensate pipe guide explains how.
- Check the outlet end is not buried or blocked by leaves, as long as you can reach it safely at ground level.
- Check the fused spur and power to the boiler are on.
- Press and hold the reset button for about five seconds, as the Greenstar user instructions describe, and wait for the boiler to restart. Stop after two or three attempts.
Never try to thaw a condensate pipe that is at height, and never use boiling water, which can crack the plastic. Do not remove the boiler casing to reach the condensate pump or trap. Worcester's own instructions say to contact a Gas Safe registered engineer for a condensate pump fault.
When to call a Gas Safe engineer
If the pipe is clear and not frozen, or D5 keeps returning after two or three resets, call a Gas Safe registered engineer. You should also call one if you see water collecting around the base of the boiler, as our guide on a boiler leaking water explains.
The engineer will test the condensate pump and its float switch, clear the trap, check the route and fall of the pipework, and check the wiring. When you book, tell them your exact model name, which is usually printed on the front or underside of the boiler, and mention if the fault began during a cold snap.
Typical repair costs in the UK
If thawing the pipe clears the fault, there is no cost. If a part or pipework change is needed, these ranges are typical. Prices vary by region and engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Clearing a blocked condensate trap | Typically £80 to £150 |
| Condensate pump replacement | Typically £150 to £300 |
| Rerouting or insulating the condensate pipe | Typically £100 to £250 |
How to stop D5 coming back
If your condensate pipe runs outside, ask an engineer whether it can be insulated, increased in size or rerouted inside, as these are the usual ways to cut down on winter freezing. Keep the outlet clear of leaves and garden debris. A yearly service gives an engineer the chance to clean the condensate trap and check the pump before it fails. See our boiler service cost guide for what to expect. If your boiler shows a flame fault in cold weather instead, see our page on Worcester EA 227.