What F23 means, and which boilers this guide covers
Vaillant's ecoTEC exclusive system installation manual, document 0020193966_04, identifies F.23 as a safety shutdown caused by an excessive temperature spread. Its possible causes include inadequate pump circulation, air and incorrectly connected flow and return temperature sensors. Those are diagnostic possibilities for an engineer, not a shopping list of parts to replace.
The ecoFIT pure system manual, document 0020230533_07, gives the same general temperature-spread interpretation for F.23. Match your actual model and manual revision before relying on any code table. A brand name on the front panel is not enough to establish that instructions for another generation apply.
You may see the code written F23 or F.23 online. Photograph the display exactly as it appears, including any additional text or number. If the display instead shows F.22, F.24 or a longer code, do not round it to the nearest familiar search result. The distinction changes the first questions an engineer should investigate.
F23 is not an instruction to add water. Look at the pressure indication if it is visible through the normal user controls, and record it. Do not open filling valves, bleed every radiator or reset the boiler repeatedly simply because those actions helped someone with a different code. Each can change the evidence the attending engineer needs.
Vaillant's homeowner fault-code guidance directs F.23 towards an installer because the problem can involve how the boiler interacts with the heating system. Its list includes air, sensor connections and circulation problems. A code returning after a reset needs the cause checked rather than a routine of restarting the appliance until it stays on.
Keep the casing in place. Pump, internal sensor, wiring and combustion-related work belong with a suitably qualified heating engineer. If you smell gas, suspect fumes or a carbon-monoxide alarm activates, stop treating this as a routine code query. Follow the emergency guidance in Gas Safe Register's safety guide; in Great Britain the gas emergency number is 0800 111 999.
Pressure and circulation answer different questions
The pressure display tells you something about the state of water in the heating circuit. It does not measure how effectively that water is moving through every part of the installation. A circuit can have a plausible pressure reading while heat is not being transported as intended. That is why a normal-looking gauge does not rule out an F23 investigation.
Think of the observations separately. “The pressure is lower each cold morning” concerns a changing pressure trend. “The boiler locks out soon after a heat demand” concerns behaviour during operation. “Only two radiators warm” concerns heat distribution. They may be related, but none should erase the others from the report.
Likewise, a temperature-spread fault does not establish where a water leak is. There may be no water leaving the system at all. Before ordering a hidden-leak survey, ask the heating engineer whether there is independent evidence of ongoing loss and whether internal boiler and circulation causes have been considered.
If the actual issue is persistent pressure loss, our Vaillant losing-pressure guide deals with that pattern. If the display says F22, use the separate Vaillant F22 page. Keeping those questions on their own pages avoids treating every fault as a reason to repressurise.
Write down what happens before the shutdown
Use ordinary observations; you do not need to enter an installer menu. Record whether F23 appeared at the first morning demand, after the system had been running, following a hot-water demand, or immediately after maintenance. “Happens every day” is less informative than a clear description of the operating stage.
If you have a separate hot-water cylinder, say so. A combi providing hot water directly and a system boiler heating a cylinder have different arrangements. “Hot water still works” can mean different things: a cylinder may contain previously heated water even though the boiler is not currently heating it. Describe what the controls and boiler actually did, not just whether one tap felt warm.
| Observation | Record it like this | Do not infer |
|---|
| Display | Photograph F.23 and any accompanying text | That another similar code has the same cause |
| Timing | Appeared after the heating schedule started | That the programmer itself must be defective |
| Pressure | Reading and whether the system was cold or warm | That pressure proves circulation is adequate |
| Heat distribution | Which rooms warmed during normal operation | That the coldest radiator marks a leaking pipe |
| Recent work | Date a radiator, valve or boiler component was changed | That the last contractor necessarily caused the fault |
| Other symptoms | Visible drip, noise or additional code | That all symptoms have one explanation |
Do not keep trying to produce the fault for a video. If it has shut down, save the display photograph and arrange help. The engineer can decide which controlled operating checks are appropriate for that appliance and installation.
Why recent radiator or heating work is relevant
Tell the engineer about draining, refilling, radiator replacement, valve changes or work on the boiler even if it seemed minor. A useful history includes what was changed, whether the system worked normally afterwards and when the code first appeared. Send the previous invoice or work record if available.
This is not an accusation against the earlier installer. It provides a point in the timeline from which the present arrangement can be checked. A fault appearing after work may be connected with the changed hydraulic conditions, or it may be coincidental. The physical tests need to decide that.
Do not attempt to return concealed valves to an assumed “original” position. If you remember that something was adjusted, describe it or show a photograph. Turning several valves without a record can make an installation harder to assess, particularly where heating zones or an underfloor manifold are involved.
If a radiator was removed for decorating, report whether it was subsequently refitted and whether the heating has been commissioned since. Our guide to leaks after radiator removal covers visible water-loss checks, but an F23 investigation also needs attention to system operation, not just dry joints.
What a useful professional diagnosis should establish
You are not paying merely for someone to translate F23 into a phrase. Ask for an explanation of the finding: what was tested, what was abnormal and how it supports the proposed repair. An engineer should distinguish a confirmed component defect from a possible cause which still needs investigation.
For example, “pump issue suspected” and “pump output checked and found inadequate under the relevant operating conditions” are different levels of conclusion. The second has evidence behind it. You need not demand raw technical measurements you cannot interpret, but you can reasonably ask what established the diagnosis rather than accepting the code as the diagnosis.
Depending on the model and installation, the professional investigation may address circulation, air, temperature sensing and the relationship between the boiler and heating circuit. That does not mean every job needs every test or a full system clean. The scope should follow the observed fault and the applicable manual.
If cleaning or flushing is proposed, ask what evidence points towards a restriction or contamination, what part of the system is affected and how the result will be assessed. F23 alone does not prove that a powerflush is required. If a replacement part is proposed, ask why that part rather than another possible cause is implicated.
When a hidden leak becomes a separate line of investigation
A leak investigation becomes more relevant when there is a reliable pattern of pressure dropping between comparable conditions, repeated replenishment, visible water escape or other supporting findings. Keep records at similar system temperatures because comparing a hot reading with a cold reading can be misleading.
Do not continue topping up to build a longer dataset if the boiler is faulting or an engineer has advised you to leave it off. Existing records are useful; safe operation takes priority over collecting more. If a competent engineer has already checked the appliance, request the written findings so a leak specialist knows what has and has not been excluded.
Possible water loss at a visible valve or discharge point should be described before anyone assumes pipework is leaking beneath the floor. Conversely, the absence of a stain does not establish that a buried heating circuit is sound. These are reasons for a staged diagnosis, not reasons to start opening floors at random.
If the evidence points to concealed heating pipework, a specialist can consider suitable circuit tests and locating methods. Heating leak detection and repair should have a defined purpose: establish and locate water loss, then discuss repair. It is not a substitute for diagnosing an internal boiler circulation fault.
Prepare for the visit without changing the system
Photograph the model information accessible without removing covers. Gather the user manual, service record and any recent work reports. Clear ordinary access to the boiler and controls, but do not remove fitted panels, stored-water-cylinder insulation or safety covers to make the visit appear easier.
Tell the booking team about access restrictions, pets, parking limitations and whether the property is a flat with managed services. If an external pump or zone controls are in a locked cupboard, arrange access with the person responsible. An engineer cannot assess a system component which nobody can reach during the appointment.
State whether there are vulnerable occupants or whether loss of heating or hot water creates an urgent practical problem. That helps the firm arrange an appropriate response; it does not change the fault-code meaning. Ask what can safely remain in use while you wait and follow appliance-specific advice.
For a rented home, notify the landlord or managing agent and preserve the time of your report. Avoid appointing multiple firms independently without clarifying who is instructing and paying. A shared factual record is more useful than several contractors receiving different versions of the symptoms.
Resetting is not the same as repairing
A restart which restores heating temporarily does not show that the cause has gone. Record whether a reset has already been attempted and how long normal operation lasted. Consult the correct user instructions and an engineer's advice before further attempts; do not use repeated resetting to override recurring shutdowns.
There is no universal number of safe resets to prescribe across every Vaillant model and every symptom. A guide which offers one without knowing the appliance and conditions is skipping the diagnostic question. What matters is the manufacturer's instructions and why this installation is stopping.
If the boiler is under warranty or a maintenance agreement, contact the relevant provider before commissioning chargeable repairs. Ask what attendance covers and how it treats system faults outside the appliance. Do not assume that every component connected to a boiler is included in the same warranty.
After the repair, verify the behaviour which caused the call
Ask the engineer to document the work and explain what operating checks were completed. If the original fault occurred during space heating, a brief hot-tap check alone does not describe whether that heating behaviour was assessed. The appropriate test depends on the appliance and findings, but the report should connect the repair to the original complaint.
Keep the model-specific part details and service record, especially where a component has been replaced. Ask what symptoms would justify contacting the firm again. “Call if F23 returns” is useful, but so is knowing whether a change in pressure or newly observed discharge should also be reported.
Use normal household operation for follow-up rather than stress-testing the system. Note any recurrence with the same headings you used before the visit. If a second professional becomes involved, send the first report so they do not start by repeating assumptions which have already been checked.
Book the right specialist for the evidence you have
For an F23 code without evidence of escaping water, start with a qualified heating engineer or the manufacturer's service route. A fault code alone is not a reason to buy a water-leak search. Tell the engineer you need a model-specific diagnosis of the temperature-spread shutdown, and share the observations above.
If that investigation identifies unexplained heating water loss, send Leak Fix London the report and pressure history. We can discuss a targeted investigation of the leaking circuit, repair options and reinstatement where access is required. The useful handover states what was tested at the boiler and why concealed pipework now needs checking.
Our rate is £150 per hour, and the total depends on the time involved. If we do not find the leak, you do not pay for the time we spent looking. Read our pricing and scope; this water-leak search commitment is not a free boiler fault-code diagnosis. A precise booking brief helps keep your first paid visit focused on the actual problem.
