Vaillant F23 fault code: circulation fault explained

What does the Vaillant F23 fault code mean?

The Vaillant F23 fault code means the temperature difference between the flow and return pipes is too great. In plain terms it is a circulation fault: the water is not moving through your system properly, so the heat leaving the boiler and the heat coming back are too far apart. The usual causes are trapped air, a weak or seized pump, sludge blocking the flow, or a faulty flow or return temperature sensor. Topping up low pressure is safe to do yourself; the pump, the sensors and any work inside the boiler are a Gas Safe registered engineer's job.

Vaillant's own fault-code list describes F.23 as "the temperature spread between the flow/return is too great". The flow pipe carries freshly heated water out to your radiators, and the return pipe brings the cooler water back to be reheated. When the gap between the two becomes too large, the boiler shuts down as a safety measure. Heating engineers commonly explain that the boiler locks out once that gap goes beyond roughly 35 degrees, which is why F23 almost always points at how well the water is circulating rather than at the burner.

Key facts

The quick version, before the detail.

• Meaning: the temperature difference between the flow and return pipes is too great (Vaillant's official wording).
• DIY-fixable? Only partly: a pressure top-up and a single reset. The pump and sensors are engineer-only.
• Gas Safe job if: the pump, the flow or return sensors, a system flush or the expansion vessel are involved, or the code keeps coming back.
• Most common on: the Vaillant ecoTEC range.
• Smart Plan boiler & central heating cover: parts and labour up to £500 a year if the boiler is under 7 years old, or up to £200 a year if it is over 7.
• A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Outside those hours you are only seen sooner for a genuine emergency breakdown, so a Friday-evening or weekend fault can wait until Monday.
• Using a module starts a 12-month agreement period the first time you use a service. There is a 14-day cooling-off period, but it ends once a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Why does the F23 code appear?

F23 points at how water moves around your heating system, not at the ignition or the gas supply. The common causes are trapped air (an airlock), a weak, seized or failing circulation pump, sludge or debris restricting the flow, a faulty flow or return temperature sensor, low system pressure, or a defective internal expansion vessel. It shows up most on the Vaillant ecoTEC range.

The table below runs through the likely causes, what you'll notice, and who can safely deal with each one.

F23 causes and who fixes them

Can I fix a Vaillant F23 myself?

There are only a couple of checks you can safely make before booking anyone. F23 is mostly a job for an engineer, because the pump, the sensors and any system flush are all beyond safe DIY. Work through these first.

1. Check the boiler's pressure gauge. If it reads below the normal band on the dial, top it up using the filling loop, following the steps in your Vaillant manual.

2. Check the boiler has power and its display is on.

3. Reset the boiler once, following the steps in your Vaillant manual. Reset only once.

Stop there. Freeing or replacing the pump, cleaning or flushing the system, changing a temperature sensor or working on the expansion vessel all mean opening the boiler or the sealed heating system, which is a Gas Safe registered engineer's job. If F23 comes back after a top-up and a single reset, book an engineer rather than resetting again.

When to call a Gas Safe registered engineer

If the pressure is in the normal band and a single reset has not cleared it, F23 needs an engineer to check the circulation: the pump, the flow and return sensors, and whether the system needs cleaning. That is diagnostic work on the sealed heating system and the boiler's internals, not a DIY job.

Clearing an airlock, freeing or replacing a pump, or flushing a sludged-up system can range from a quick visit to a bigger job, so the parts and labour can add up. That's worth weighing against how you'd rather cover it: a one-off repair or an ongoing module.

You can book a one-off repair with us and a Gas Safe registered engineer will come and fix it, or call us on 0333 772 6247. A Smart Plan boiler and central heating module covers parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it's over 7. A larger repair such as a new pump or a power flush can cost more than that, so you would pay anything above the limit yourself. Cover is modular, so you only pick what you want.

A few things worth knowing before you set up cover. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, if the engineer can't get access, and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00; outside those hours you're only seen sooner for a genuine emergency breakdown as set out in your terms, so a fault on a Friday evening or over the weekend can wait until Monday. And using any module starts a 12-month agreement period from the first time you use a service: there's a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Smart Plan is a service plan, not insurance. In plain terms, it's a membership that arranges and funds covered repairs up to the limits on your plan, rather than a regulated financial product: it doesn't pay out a cash sum, and cover is limited to the modules you've chosen and the limits set out in your terms. It's provided by UK Boiler Company Ltd, which has traded since 2014 and looked after over 15,000 customers.

One safety note: if you ever smell gas or suspect carbon monoxide, don't wait for an engineer: call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F23 fault code FAQs

Seeing an F23 fault code? We'll come and fix it for you.

Book a one-off repair and a Gas Safe registered engineer will check the pump, the sensors and the circulation, or set up a Smart Plan boiler and central heating module so covered repairs are handled up to your cover limit: up to £500 a year for a boiler under 7 years old, or up to £200 a year if it's older. It's a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, no-access visits and early annual-service requests. Cover-plan call-outs are Monday to Friday, 08:00 to 18:00 unless it's a genuine emergency. Using a module starts a 12-month agreement period once you first use a service; leaving early costs the remaining months or 75% of the outstanding balance.

Helpful links

• Vaillant F27 fault code
• Vaillant F29 flame-loss fault
• Vaillant F54 gas supply fault
• All Vaillant fault codes
• Boiler not working? Book a repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register