Overview
F.11 on a Vaillant Turbomax Plus 824/828E is not absolutely consistent across all Vaillant documentation, but it most commonly points to either a problem with temperature sensors (NTC thermistors) or a hydraulic flow/blockage issue inside the appliance. On some Vaillant ranges F.11 is logged where the return or flow NTC is shorted or faulty; on others it is used where the appliance senses restricted flow or blockage that prevents normal circulation. Either type of fault will cause the boiler to go to lockout or reduced operation to protect the appliance. Severity: moderate to high. The boiler will usually stop providing heating and/or hot water and can lock out. If the cause is a sensor fault, the boiler may be prevented from controlling temperatures correctly; if the cause is a flow/blockage or dry-fire condition, there is risk of overheating or damage to the heat exchanger and pump if the boiler keeps trying to run. Because this can involve gas, electrical wiring, and internal hydraulic components, the diagnosis and repair are normally best left to a Gas Safe registered heating engineer. DIY or professional: some simple external checks and a basic reset are suitable for a competent homeowner (pressure, visible leaks, frozen condensate pipe, basic reset). Anything that requires opening the boiler, testing or replacing sensors, working on the gas valve, replacing the pump/heat exchanger, or any electrical wiring must be done by a qualified engineer.
Troubleshooting Steps
Safety precautions:
1. If you smell gas, evacuate the property and call the gas emergency number immediately. Do not attempt inspections or resets.
2. Do not open the boiler casing or work on gas/electrical components unless you are a qualified and competent person. Isolate mains power before opening the cover if you are doing any internal checks and only do so if you know what you are doing.
3. Beware of hot surfaces and scalding when checking pipes or radiators. Wear eye protection and gloves when appropriate.
Initial checks a homeowner can safely perform:
1. Record the exact fault code and any other display information or LEDs. Note whether the boiler is in lockout (won’t reset) or simply showing the code while running.
2. Try a safe reset: follow the boiler manual reset procedure (many Turbomax units allow a manual reset by turning the control knob to the flame symbol and releasing). If unsure, switch the boiler off at the programmer and isolate from mains for 30–60 seconds, then turn back on and observe whether the fault returns.
3. Check system pressure on the pressure gauge. Typical cold system pressure is around 1–1.5 bar. If pressure is significantly low (<1.0 bar), the boiler may fault. If you know how to use the filling loop safely and the system requires topping up, you may do so following the manual; if unsure, call an engineer.
4. Look for obvious signs: visible leaks, water on the condensate drain/pan, frozen condensate pipe in cold weather (thaw with warm—not boiling—water), or error codes that change when you operate hot water or heating demand.
5. Listen: can you hear the pump running when heating is demanded? Are radiators heating unevenly (possible blockage or air lock)?
Specific diagnostic and next steps (do not attempt if not competent):
1. If the error suggests a sensor fault (NTC): isolate mains power, remove the boiler cover and visually inspect the flow and return NTC connections and wiring for loose plugs, chafing, corrosion or obvious damage. Re-seat connectors if secure and undamaged.
2. With power isolated, a competent person may measure the resistance of the flow and return NTC sensors with a multimeter. An open circuit (infinite resistance) or a near-zero short circuit indicates a failed sensor or wiring short. Note: NTC values change with temperature; if you are not experienced with NTC testing, leave this to an engineer.
3. If the error suggests blocked flow or hydraulic restriction: check the magnetic filter/strainer on the system (if you have one fitted) for sludge/iron deposits and clean it out. Check TRV settings and bleed one or two radiators to remove trapped air that could prevent circulation.
4. Check the pump: with the cover off (power isolated while accessing wiring but the mains can be temporarily restored to run the pump tests if you know what you’re doing), ensure the pump impeller is not seized. Do not attempt to dismantle the pump unless qualified. If a pump fault is suspected, call an engineer.
5. If you suspect heat exchanger blockage, diverter valve failure, or internal scaling, these are specialist jobs requiring stripping parts and should be performed by a Gas Safe registered engineer.
When to call a professional and what to tell them:
1. Call a Gas Safe registered heating engineer if the F.11 fault does not clear after the safe reset and the simple external checks above, or if you find damaged wiring, faulty sensors, a seized pump, low pressure you cannot correct, evidence of internal leaks, or repeated lockouts.
2. When you call, tell them: the exact model (Vaillant Turbomax Plus 824/828E), the displayed fault code F.11, what initial checks you have done (reset, pressure, any visible leaks, condensate thawing), and whether the boiler is in permanent lockout.
Important warnings:
1. Do not attempt to adjust or replace gas valves, ignition components, PCB, or internal hydraulic parts yourself. These are gas/electrical safety critical and must be handled by a qualified engineer.
2. Do not keep forcing resets repeatedly—this can cause further damage. If a reset does not clear the fault, arrange a professional inspection.
Summary recommendation: perform the safe external checks listed above. If the code persists, or if you find wiring damage, an open or shorted sensor, pump seizure, internal leaks, or low pressure you cannot rectify confidently, stop and call a Gas Safe registered heating engineer to diagnose and repair the fault.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Vaillant Turbomax Plus 824/828E.