Quick answer: The F23 fault code (shown as F.23 on some displays) means your Vaillant boiler has detected too big a temperature difference between the flow pipe and the return pipe. The boiler has locked out on purpose to stop the heat exchanger cooking itself. In almost every case we attend across Edinburgh and central Scotland, the root cause is poor circulation, not a broken boiler.
Written by the engineering team at Edinburgh Boiler Company, Scotland’s number one boiler installer. We fit and repair Vaillant ecoTEC boilers every working day, and F23 is one of the five codes we see most often once the weather turns.
What does the F23 fault code mean on a Vaillant boiler?
F23 means the temperature spread across the boiler is too wide. The boiler measures water leaving through the flow pipe and water coming back through the return pipe. When the gap between those two readings climbs past roughly 35°C, the control board shuts the burner down and displays the code.
Think of it this way. The burner is producing heat at a fixed rate. If water is moving through the heat exchanger properly, that heat gets carried away and spread around your radiators. If the water is crawling, the same amount of heat goes into a much smaller volume of water, so the flow temperature shoots up while the return stays cool. That widening gap is what triggers the lockout.
| Reading | What a healthy system looks like | What a system throwing F23 looks like |
|---|---|---|
| Flow temperature | 70°C | 80°C+ (climbing fast) |
| Return temperature | 50°C | 40°C or lower |
| Difference (Delta T) | 15–20°C | 35°C or more |
| Boiler behaviour | Runs steadily, radiators warm evenly | Fires briefly, gets hot, locks out |
A Delta T of about 20°C is what a modern condensing combi is designed around. Anything much wider tells you the water isn’t moving as it should.
Why does F23 keep coming back on my Vaillant ecoTEC?
F23 keeps returning because resetting the boiler clears the code but doesn’t clear the blockage, the airlock, or the tired pump behind it. The lockout is a symptom. Until something restores proper flow through the system, the boiler will hit the same threshold again, usually within minutes of firing up.
Here’s what we find when we strip the job back on site:
| Cause | How common | Typical symptoms alongside F23 | What fixes it |
|---|---|---|---|
| Magnetite sludge restricting flow | Very common | Cold spots at radiator bottoms, black water when bleeding, dirty filter | Powerflush or chemical clean, then inhibitor |
| Weak, seized or wrongly set circulating pump | Common | Boiler hot to touch, radiators barely warm, humming from boiler | Pump replacement or speed setting change |
| Air trapped in radiators or pipework | Common | Gurgling, cold radiator tops, recent system work or top-up | Bleed radiators, purge system |
| Closed or partly closed valves | Common | One zone or floor cold, recent decorating or new flooring | Open lockshields and isolation valves |
| Faulty flow or return NTC sensor | Occasional | Fault appears with no other symptoms, radiators actually fine | Replace NTC thermistor pair |
| Blocked plate heat exchanger (combis) | Occasional | Hot water lukewarm as well as heating fault | Descale or replace plate exchanger |
| Failed expansion vessel or diverter valve | Less common | Pressure swings wildly, or heating comes on with hot water | Recharge or replace component |
| Low system pressure | Less common | Gauge below 1 bar, may show F22 first | Top up via filling loop, find the leak |
Sludge deserves its own note, because it’s the cause we confirm most often. Magnetite is the black iron oxide that forms when steel radiators corrode from the inside. It settles at the bottom of radiators, coats the inside of narrow pipework, and eventually chokes the heat exchanger’s waterways. A boiler can be two years old and perfectly healthy while the twenty-year-old system it’s bolted onto is full of it.
Is F23 the same as F22, F24 or F75?
No. They’re related circulation faults, but each one points somewhere slightly different, and mixing them up sends you down the wrong path.
| Code | What the boiler is reporting | Most likely cause |
|---|---|---|
| F22 | Dry fire protection, not enough water | Low pressure, leak, failed pump, empty system |
| F23 | Temperature spread too wide | Restricted circulation, sludge, pump underperforming |
| F24 | Temperature rising too quickly | Very poor flow, seized pump, closed valve |
| F75 | No pressure change detected on pump start | Pressure sensor fault, expansion vessel, pump fault |
If you’re seeing F22 alongside F23, start with pressure. If it’s F24 or F75 alongside it, the pump is the prime suspect. Note the exact code before you reset anything, because that history is genuinely useful to the engineer who turns up.
Can I fix a Vaillant F23 fault myself?
You can safely carry out four external checks, and they resolve a decent share of F23 call-outs before anyone needs a van. What you must not do is take the casing off. Everything inside a Vaillant is gas or mains work and is legally the preserve of a Gas Safe registered engineer.
1. Reset the boiler, once. Write the code down first. On most ecoTEC models you hold the reset button marked with a crossed-out flame for around three seconds, or turn the mode dial to the reset position. Check your model’s manual. One reset is diagnosis. Repeated resets are just stressing a heat exchanger that’s already flagging a problem.
2. Check the pressure gauge when the system is cold. You’re looking for somewhere between 1.0 and 1.5 bar on most models. If it’s low, top up through the filling loop and recheck. If it drops again over the following week, you have a leak somewhere and that needs looking at properly.
3. Bleed the radiators, ground floor first. Work upwards through the property. In an Edinburgh tenement or a Glasgow four-in-a-block that might mean coordinating with neighbours if the system spans levels. Recheck the pressure once you’re done, because bleeding drops it.
4. Open every valve fully. Radiator valves all the way open. Lockshields turn clockwise to the stop, then back off half a turn, or return them to wherever they were set during balancing. Then find the flow and return isolation valves under the boiler and confirm both are fully open. We’ve attended F23 call-outs where the only fault was a valve someone nudged shut while fitting a new carpet.
While you’re doing this, turn your heating flow temperature down to around 65°C. It won’t fix anything, but it buys you a bit of heat while you arrange a repair.
Stop and call an engineer immediately if you smell gas. Leave the property, then ring the National Gas Emergency Service on 0800 111 999 from outside.
Why is F23 so common in Scottish homes?
Scotland’s housing stock and heating patterns make this fault more likely than the UK average, and the reasons aren’t the ones people expect.
Most guides written for an English audience blame limescale. That advice doesn’t travel well. Scottish Water supplies soft water to the large majority of the country, so scale is rarely the culprit here. What we do get is soft, slightly acidic water drawn from peaty upland catchments, and that’s perfectly capable of corroding steel radiators from the inside. The end result is the same restricted flow, just by a different mechanism.
Add to that:
- Long heating seasons. A boiler in Aberdeen or Inverness runs far more hours per year than one in Surrey. More running hours means more circulation cycles through whatever is sitting in the system.
- Older pipework. Tenement flats, terraces and pre-war semis across Edinburgh, Leith and the Lothians often still run on microbore or narrow 8mm and 10mm pipe. Narrow pipe blocks quicker and blocks harder.
- Modern boiler, ancient system. A new ecoTEC plus fitted onto a system that was never flushed properly will pull decades of debris straight into its heat exchanger. This is why we insist on a system clean and a magnetic filter on every installation we carry out.
- Rooms shut off in winter. Closing valves in a spare bedroom to save money reduces the total flow path available to the pump, which pushes Delta T up.
How much does it cost to fix an F23 fault in Scotland?
Most F23 repairs land between £150 and £450. Sludge removal on a large or badly neglected system is the outlier that can run higher. The figures below are guide prices for Scotland in 2026 and assume a Gas Safe registered engineer with parts fitted.
| Repair | Guide cost (fitted) | Time on site | How long it lasts |
|---|---|---|---|
| Diagnostic visit | £70–£120 (often deducted from repair) | 30–45 mins | — |
| Pump speed adjustment or reconnection | £70–£150 | 30–60 mins | Permanent if that was the fault |
| Circulating pump replacement | £200–£420 | 1.5–3 hours | 8–12 years |
| NTC flow and return sensor pair | £150–£230 | 1–1.5 hours | Lifetime of boiler, usually |
| Chemical clean plus magnetic filter fit | £220–£400 | Half day | 3–5 years, longer with inhibitor upkeep |
| Full powerflush | £350–£750 depending on radiator count | Full day | 5–8 years with correct dosing |
| Plate heat exchanger descale or swap | £190–£300 | 1.5–2 hours | 6–10 years |
| Expansion vessel replacement | £180–£350 | 1.5–2 hours | 8–10 years |
Prices vary by location, by boiler model, and by how much of your system needs attention. Always get the quote in writing before work starts, and ask whether the price includes inhibitor and a filter. A powerflush without fresh inhibitor afterwards is money down the drain.
Will my Vaillant warranty cover an F23 repair?
Sometimes, and the distinction matters. Vaillant guarantees, which run up to 10 years on the ecoTEC plus range when installed by an approved installer, cover the boiler’s own components. A failed NTC sensor or a boiler-mounted pump inside the guarantee period should be covered.
What isn’t covered is damage caused by the system around the boiler. If the engineer opens up and finds the heat exchanger packed with magnetite, that’s classed as a system fault, and the bill comes to you. Two things protect you here: keep your annual service up to date, because a lapsed service invalidates most guarantees, and keep the paperwork showing your system was cleaned and dosed with inhibitor at installation.
How do I stop F23 coming back for good?
Fix the circulation properly once, then protect the system. Six things do the heavy lifting:
- Fit a magnetic filter if you don’t have one. A MagnaClean or equivalent catches magnetite before it reaches the heat exchanger. It’s the single best value item on this list.
- Get the filter emptied at every service. A full filter stops catching anything.
- Keep inhibitor at the right concentration. BS 7593:2019 recommends testing it annually. Inhibitor depletes over time and after any drain-down.
- Book an annual service with a Gas Safe registered engineer. It keeps the guarantee live and catches a struggling pump before it strands you in January.
- Leave unused radiators partly open or on the frost setting rather than fully shut.
- Balance the system after any radiator is added, removed or replaced.
If your system has never been flushed and the boiler is more than a decade old, a clean is worth doing on its own merits. You’ll get quicker warm-up, lower gas use, and a boiler that stops locking out.
When is it worth replacing the boiler instead of repairing it?
Replace rather than repair when the boiler is past 12 years old, parts are getting scarce, and the F23 diagnosis points at the heat exchanger itself. At that point you’re spending several hundred pounds on a component in a boiler that’s near the end of its service life, on a system that will likely need cleaning anyway.
An A-rated Vaillant replacement, fitted onto a properly cleaned system with a filter and correct inhibitor, ends this fault permanently and cuts your gas consumption at the same time. If your existing boiler is under ten years old and otherwise sound, repair almost always makes better financial sense.
The bottom line
F23 on a Vaillant is a circulation warning, and the boiler is doing exactly what it should by shutting down. Run the four safe checks first: reset once, check pressure cold, bleed the radiators, and open every valve. If the code returns after that, get an engineer out promptly. A restricted system left running will damage the heat exchanger, and that turns a £250 repair into a £2,000 conversation.
Need it sorted? Edinburgh Boiler Company covers Edinburgh, the Lothians and central Scotland with Gas Safe registered engineers, Vaillant-approved installations, and clear written pricing before any work begins. Get in touch and we’ll get your heating running properly again.
This guide is general advice. Always refer to your specific Vaillant model’s user manual for reset and pressure instructions, and never remove the boiler casing. Gas work must be carried out by a Gas Safe registered engineer.
