Overview
D.15 on a Vaillant EcoTEC is a diagnostic display value that reports the actual speed or running status of the boiler's circulating pump. It is not a single fault code that always means a failed part by itself; instead it shows what the boiler is reading for pump speed during operation. Technicians compare the pump speed actual (D.15) with the pump speed target (D.14) and other diagnostics to see if the pump is running appropriately for the current demand. This condition can be caused by several things: the pump not receiving a control signal, an electrical or wiring fault to the pump, mechanical seizure or blockage in the pump or pipework, airlocks in the heating system, closed isolation valves, or a problem with the boiler's electronics that control pump speed. Severity ranges from low (if it’s an intermittent reading or caused by air in radiators) to high (if the pump is not circulating water at all, risking overheating, poor heating performance or safety lockouts). If the pump is not circulating correctly the boiler can cycle, shut down on safety limits, or cause damage to the heat exchanger over time. Homeowners can perform a few safe checks (reset, pressure, valves, bleeding radiators) but any electrical, gas, or internal pump work should be left to a Gas Safe registered engineer. If the D.15 reading shows a persistent mismatch with the target speed or the pump is silent when it should be running, call a qualified heating engineer to fully diagnose and repair the boiler.
Possible Cause: Pump speed actual value
Troubleshooting Steps
Safety precautions:
- Before doing anything, turn off the boiler at the mains isolator and let it cool. Do not open the boiler casing or attempt internal electrical or gas work. If you are unsure, stop and call a Gas Safe engineer.
- When topping up water or opening drain/fill valves, be careful around hot pipes and moving parts. Use basic PPE: gloves and eye protection.
- Do not attempt to dismantle or replace the pump, PCB, gas valve or other sealed components yourself.
Initial homeowner checks you can do safely:
1. Note exactly when the D.15 appears (on hot water demand, central heating demand, startup). Write down any other F or S codes that appear at the same time.
2. Reset the boiler once using the reset button and observe if the D.15 returns. A one-off reading that does not reappear after reset may be transient.
3. Check the system water pressure on the boiler display or gauge. Pressure should normally read around 1–1.5 bar when cold. If pressure is low (close to 0.5 bar or less) top up the system to the recommended level, following the boiler manual.
4. Confirm external isolation valves are open: check any flow/return isolation valves near the boiler and the external pump isolating valves (if visible) are in the open position.
5. Bleed radiators to remove airlocks—start with the highest radiators in the house. Airlocked systems can prevent circulation and change pump readings.
6. Listen for the pump when the boiler is calling for heat: when there is a heating or hot water demand you should hear or feel the pump run (a soft hum or vibration). If the pump is silent and there is a demand, that indicates no circulation.
Specific diagnostic steps and fixes to try (homeowner-friendly where safe):
1. Compare diagnostic values if you can access service diagnostics: view D.14 (pump speed target) and D.15 (pump speed actual). If the target shows a value but actual is zero or much lower, circulation is not occurring despite the request.
2. If pressure was low, top up to the correct level and re-check D.15 after a restart. Low pressure can prevent the pump from building flow or cause lockouts.
3. After bleeding radiators and topping up pressure, restart the boiler and observe whether D.15 matches D.14 more closely and whether heating performance returns to normal.
4. If there is a visible magnetic filter or system strainer on the return to the boiler and you are comfortable doing so, isolate the boiler power, close valves, drain down the isolated section and inspect/clean the filter following the manufacturer’s guidance. Only do this if you are confident—otherwise leave to an engineer.
5. Check for signs of debris, corrosion, leaking joints, or unusual noises in the pipework around the pump. Blockages or sludge in the system can reduce flow and change pump readings.
6. If the pump hums but does not move water, it may be seized or clogged—this requires a professional to inspect or replace the pump.
7. If wiring to the boiler or external controls looks damaged or wet, turn the appliance off and call an engineer. Moisture or wiring faults can cause incorrect diagnostic readings.
When to call a professional and what they will do:
- Call a Gas Safe registered heating engineer if D.15 continues to show a mismatch with the target after the homeowner checks above, if the pump does not run when it should, if you detect electrical fault signs, or if the boiler repeatedly locks out.
- A qualified engineer will safely remove the casing, electrically test the pump, measure voltages and signals from the PCB, check and test the pump motor and rotor, inspect wiring harnesses and connectors, test the pump control electronics (including speed control and any capacitors), check for blockages or a seized impeller, and test the PCB if needed.
- The engineer can also perform a system flush or magnetic filter clean if sludge is present, and will replace any faulty components (pump, wiring, NTCs or PCB) safely and in compliance with gas safety regulations.
Do not attempt internal repairs, electrical testing inside the boiler, or gas component work yourself. If you are unsure at any stage, isolate the boiler and contact a Gas Safe registered engineer for safe diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant EcoTEC Gas Boiler.