Common Faults of Vaillant atmoTEC and turboTEC Boilers and How to Fix Them

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Common Faults of Vaillant atmoTEC and turboTEC Boilers and Their Solutions

Vaillant atmoTEC and turboTEC boilers are among the best-selling combined low-temperature boilers with instantaneous domestic hot water heating on the Slovak market. They are structurally well-built, reliable, and with proper maintenance can run without major issues for 15 to 20 years. Nevertheless, in service practice we encounter a whole range of faults that recur in almost every second customer's case – and most of them have a fairly simple cause, if you know where to look.

In this article, we will go through all the typical faults encountered with atmoTEC and turboTEC boilers, including error codes, their causes, and practical troubleshooting procedures. The text is intended for DIY enthusiasts, apartment building administrators, and service technicians who want to understand the fault patterns of these boilers before calling a technician (or, conversely, to know when you really must call one).

How to Read Error Codes on a Vaillant Boiler Display

Before we get into specific faults, it's important to understand how the diagnostic system of Vaillant boilers works. The atmoTEC and turboTEC boilers display faults as a combination of a letter and a number – for example F.22, F.28, S.30, and so on. The letter F (Fehler – fault) means the boiler has locked out and is not heating. The letter S (Status) means the boiler is in an operating state that is not a fault, but a waiting or limiting state.

A fault is reset by pressing and holding the flame-symbol (reset) button on the control panel for about 3 seconds. If the fault reappears shortly after, it cannot be solved by a reset – the cause needs to be found.

F.22 – Insufficient Water in the System (Low Pressure)

This is by far the most common fault encountered with both atmoTEC and turboTEC boilers. Code F.22 indicates that the water pressure in the boiler's primary circuit has dropped below the minimum value, which is typically set at 0.5 to 0.8 bar. The boiler automatically locks out to protect the heat exchanger from running dry.

Causes of F.22 and How to Fix It

In the vast majority of cases, the cause is simple – the system is under-pressurized because someone forgot to top up water, or there is a leak somewhere in the system. The correct cold operating pressure is 1.2 to 1.5 bar; some technicians recommend topping up to 1.5 bar for a new system. When hot (system at operating temperature), the pressure typically rises to 1.8 to 2.2 bar – this is normal.

Procedure for F.22: Open the filling valve (sometimes inside the boiler, sometimes external on the filling loop), watch the pressure gauge and top up to 1.5 bar. Close the valve and reset the boiler. If the pressure holds and the fault does not return, everything is fine. If the pressure drops again – you have a leak somewhere in the system. Look for wet spots near radiators, pipe joints, under the boiler, and at bleed valves.

Warning: If you need to top up water too often (once a month or more), this is definitely not normal. Every top-up increases the hardness of the water in the system and accelerates fouling of the heat exchanger. If the pressure drops with no visible leak, check the expansion vessel – the membrane may be punctured.

F.28 – Ignition Fault (Boiler Fails to Ignite)

Code F.28 means the boiler attempted to ignite but did not detect a flame after the standard number of attempts (usually 2–3 attempts). This is the second most common fault and has several causes depending on whether it's an atmoTEC or turboTEC model.

F.28 on atmoTEC (Atmospheric Boiler)

Atmospheric boilers (atmoTEC) draw air directly from the room and discharge combustion gases through a flue. The most common causes of F.28 on an atmospheric boiler:

Closed or partially closed gas valve – check that the gas valve upstream of the boiler is fully open. This happens after a gas inspection or service work.
Low gas pressure in the network – during severe frost, when hundreds of households in an area depend on gas, the distribution pressure can drop. Call the gas utility or ask a neighbor if they have the same issue.
Contaminated ignition electrode – the electrode gets coated with combustion deposits, weakening the spark. A service technician will clean or replace the electrode (the electrode itself is fairly inexpensive).
Contaminated ionization electrode – the ionization electrode "senses" the presence of a flame. If it is fouled, the boiler determines that no flame formed, even though it did. The symptom is that during ignition the flame briefly lights but the boiler locks out immediately.
Gas valve (solenoid) problem – the gas block may be worn and fail to open correctly. This already requires a service intervention.

F.28 on turboTEC (Turbo/Fan-Assisted Boiler)

Turbo boilers (turboTEC) have a sealed combustion chamber, with both air intake and flue gas discharge routed through a coaxial duct. In addition to the causes listed above, there are also:

Blocked air intake or flue outlet – especially in winter, the end of the coaxial pipe can freeze or become blocked by snow, a bird's nest, or leaves. Check the outdoor termination of the duct.
Fan failure – the turboTEC has a fan that forces air into the combustion chamber. If the fan does not start, the boiler will not ignite and will usually display F.28 or F.36.

F.36 – Fan Fault (turboTEC Only)

Code F.36 occurs exclusively on turbo boilers (turboTEC) and indicates a problem with the fan or its signal. The fan is critical for turbo boilers – without it, the gas valve must not open (this is a safety measure). The boiler monitors fan speed via a Hall sensor, and if the speed does not match the required value, the boiler locks out.

Causes of F.36: Worn fan bearing (the fan spins slowly or stutters), a faulty Hall sensor, a broken cable to the fan, or a fan stuck due to dirt. This is a fault that requires a service technician – the fan needs to be cleaned, speed checked by measurement, or the entire fan motor replaced. Replacement parts for Vaillant are readily available, but replacing the fan is a fairly labor-intensive job.

F.75 – Pressure Sensor (Pressostat) Fault

F.75 is a fault that can have two very different causes, which is exactly why it confuses even experienced technicians. Code F.75 occurs when the pump starts circulation but the pressure sensor does not detect a change in pressure – which can mean either a pump fault or a fault in the pressure sensor itself.

Stuck Pump – the Classic Post-Summer Problem

This is by far the most common scenario. After the boiler's summer shutdown (3–5 months of inactivity), the pump "seizes" – the rotor shaft gets stuck in the bearing due to limescale deposits or rust. The boiler tries to start the pump, fails, the pressure sensor detects no pressure rise, and F.75 appears.

Solution: The pump has a small screw (usually flat or Phillips head) – the so-called bleed screw. With the boiler powered off and locked out, carefully turn it a quarter turn; this can help free up the pump shaft. Watch out for escaping water – place a cloth underneath. If this doesn't help, the pump may need to be replaced.

F.62 and F.63 – Gas Valve Faults

Codes F.62 and F.63 indicate a problem with the combined gas valve. F.62 is a fault when closing the valve (the valve fails to close when it should), F.63 is a faulty EEPROM (the main circuit board memory). Both are more serious in nature and require servicing. The gas valve is a safety-critical component – its replacement must be carried out by a certified service technician and must not be improvised.

The Boiler Heats DHW but not Central Heating – or Vice Versa

This is a fault customers often complain about, and it's not always a fault of the boiler itself. Combined boilers with instantaneous DHW heating (such as the Vaillant atmoTEC pro VUW SK 200/3-3 or Vaillant turboTEC pro VUW SK 202/3-3) prioritize DHW – meaning that when hot water is flowing, the boiler switches to DHW heating and temporarily suspends central heating. If the flow valve or flow sensor is not working properly, the boiler may have trouble switching between modes.

The Flow Switch – Small but Important

Inside the boiler there is a small flow switch that detects when someone opens a hot water tap. If it is dirty or stuck (again, limescale), the boiler either fails to detect DHW flow at all (and does not heat the water), or, conversely, it keeps detecting flow even when the tap is closed (and keeps heating DHW at the expense of central heating). Cleaning or replacing the flow switch is a relatively simple service task.

Another cause: the customer has too low a cold water inlet pressure. Instantaneous DHW heating requires a minimum inlet cold water pressure – usually at least 1 bar, ideally 2 bar. If the pressure is lower, the flow is small and the DHW is either cold or only lukewarm.

Boiler Dripping or Water Leaking Underneath

A water leak from or under the boiler is a problem that must be dealt with immediately – both because of property damage and because falling pressure leads to F.22 and eventually dry running. The most common leak points on atmoTEC and turboTEC boilers:

Pump seals – the o-rings around the pump harden and crack over the years. A visible drip under the pump.
Safety valve – opens under excess pressure. If it opens regularly, the system pressure is too high (the expansion vessel is not working) or the safety valve is worn and leaks even when closed. The discharge pipe from the safety valve should be routed to a drain – if it drips, the valve is releasing pressure.
Primary heat exchanger – corrosion or mechanical damage can cause a direct leak in the exchanger. This is a serious and expensive fault.
DHW plate heat exchanger – on models with a plate heat exchanger (some versions), a leak can occur between the primary and secondary circuits, which shows up as heating water entering the DHW circuit or vice versa.

Boiler Hissing, Gurgling, or Whistling – What Does It Mean?

Boiler noises are an excellent diagnostic tool. An experienced technician can quickly narrow down the cause based on the sound alone:

Gurgling or bubbling – air in the system. Bleed the radiators, and check that the automatic air vent on the boiler is working.
Whistling or howling – a dirty filter (on models with a filter), a fouled primary heat exchanger (the so-called "kettle noise" – like in a kettle). Limescale buildup in the exchanger causes water in the narrow channels to locally overheat and form steam bubbles. Solution: chemical descaling of the exchanger.
Loud roaring from the fan – fan bearing on the turboTEC. Preventive fan replacement before complete failure is cheaper than an emergency winter callout.
Knocking when the pump starts – air in the pump, water hammer. Bleed the pump (bleed screw on the front of the pump).

Faults Related to Venting and the Expansion Vessel

The expansion vessel is the silent hero of every heating system and also the most frequently overlooked component during service inspections. The atmoTEC and turboTEC boilers have a built-in expansion vessel with a typical volume of 8 liters (on the 200/3 and 202/3 models) or 12 liters (on the 240/3 and 242/3 models). The correct membrane pre-charge pressure is 0.75 bar – exactly the same as the system's minimum operating pressure.

If the membrane in the expansion vessel bursts or tears, water fills the entire volume of the vessel. The system pressure then jumps to 3 bar when heated, and the safety valve opens. After cooling, the pressure drops well below 1 bar and the boiler reports F.22. If the pressure changes dramatically between cold and hot states (for example, from 0.8 to 3.2 bar), this is an almost certain sign of an expansion vessel fault.

Checking the expansion vessel: Shut down the system, reduce the system pressure to zero (by draining), then measure the pressure with a gauge at the valve on the expansion vessel (it looks like a bicycle tire valve). If water comes out of the valve instead of air – the membrane is punctured and the vessel needs replacing.

Control and Thermostat Issues

The boiler "runs constantly" or "doesn't run at all" – these complaints are often not related to the boiler itself, but to the controls. The atmoTEC and turboTEC boilers have a built-in controller, but most customers use an external room thermostat or a programmable controller.

Common control issues:

The room thermostat has a dead battery – the boiler receives an incorrect signal or no signal at all.
Incorrect switch setting on the boiler – the switch for with/without external controller must match the actual wiring. If it is set to "internal thermostat" but an external thermostat is connected, the boiler will ignore the external signal.
Damaged or broken thermostat cable – cables are often damaged during apartment renovations.
Incorrectly set weather compensation curve – with weather-compensated control (based on outdoor temperature), the curve must be correctly set for the given house. If the curve is too steep, the boiler will overheat the system; if too flat, the house will be cold.

Vaillant atmoTEC pro vs. turboTEC pro – Do They Differ in Reliability Too?

Customers ask us this fairly often. The answer is yes – but not dramatically. The Vaillant atmoTEC pro VUW SK 240/3-3 and the Vaillant turboTEC pro VUW SK 242/3-3 are structurally very similar boilers. The biggest difference lies precisely in the extra components the turboTEC has: the fan and the coaxial duct. These are also the most common "unique" faults of turbo boilers (F.36, blocked coaxial terminal). Atmospheric boilers are simpler in this respect, but on the other hand require a functioning chimney, which brings its own issues (smoke backdraft, condensate in the chimney during under-firing, etc.).

If you're interested in comparing these two types from a different perspective, read the article Atmospheric vs. Fan-Assisted Boiler with DHW Preparation: Which Type Is Right for You in our Knowledge Center, where you'll find a more detailed comparison including installation requirements.

Boiler Ignites, but DHW Heating Is Slow or Unstable

This is a classic customer complaint typical of older boilers with significant scale buildup in the DHW heat exchanger. The instantaneous heat exchanger for DHW in atmoTEC and turboTEC boilers is usually a coiled copper or stainless steel exchanger, which is fairly sensitive to hard water. Limescale deposits build up on the inner walls, narrowing the flow diameter and reducing heat transfer.

Symptoms: DHW is warm only at first, then cools down; DHW temperature fluctuates; DHW flow is lower than before. In extreme cases, fouling of the DHW exchanger can even cause F.75, when the boiler determines the flow has been interrupted.

Solution: Chemical descaling of the DHW exchanger. Special products exist for this (e.g. Fernox DS40, Betz Dearborn and similar), which a technician circulates through the exchanger using a pump. On boilers with a hard-to-access exchanger (e.g. Vaillant atmoTEC plus VUW CZ/SK 240/3-5) the DHW exchanger is removable and can be taken out and soaked in the solution. With very hard water (above 25°FH), we recommend installing a filter or softener before the cold water inlet to the boiler.

S.xx Status Codes – When It's Not a Fault

Many customers call for service because of codes S.30, S.31, S.32, or S.41 – yet these are not faults at all. They are status codes that inform about the boiler's current operating state:

S.0 – Boiler in standby, no call for heat or DHW.
S.1 – Boiler waiting for a call from the thermostat.
S.30 – DHW heating in progress (instantaneous mode).
S.31 – Central heating in progress.
S.41 – Boiler in frost protection mode (heating at a minimum because the outdoor temperature dropped below the set value).
S.51 – Boiler in pump overrun phase after the burner switches off (pump continues running without combustion).

If you see S.xx on the display, the boiler is operating normally. If the temperature in the house still doesn't rise, look for the cause in the controls (thermostat), not in the boiler itself.

Preventive Maintenance: How to Avoid Most Faults

From years of experience, we know that Vaillant atmoTEC and turboTEC boilers that receive a regular annual service inspection have a dramatically lower incidence of breakdowns. An annual inspection should include:

Cleaning the burner and combustion chamber (combustion deposits)
Checking and, if needed, cleaning the ignition and ionization electrodes
Checking the pressure and condition of the expansion vessel
Bleeding the system and radiators
Checking filters (magnetic filter, mesh filter at the water inlet)
Checking flue paths and chimney draft (atmoTEC)
Trial ignition and emissions measurement (CO/CO₂)
Checking the tightness of gas connections (with a detector)
Updating the service record

For more detailed information about recommended service intervals and inspection content, see the article Maintenance and Servicing of Combined Low-Temperature Boilers: What and How Often in our Knowledge Center.

When to Call a Service Technician and When You Can Try a DIY Fix

It's important to know what you can safely do yourself as a layperson, and what belongs exclusively in the hands of a certified service technician. In Slovakia, there is a legal requirement that any intervention on gas appliances must be carried out by a person holding a valid certificate (OPP – a person authorized to work on gas pipework and appliances).

You can do yourself:

Topping up the water pressure (filling valve)
Resetting the boiler after a fault (reset button)
Bleeding the radiators
Replacing the battery in the room thermostat
Checking that the gas cock is open or closed
Trying to free up the pump via the bleed screw

Exclusively for a certified technician:

Any work on the gas fittings (valve, burners, electrodes)
Replacing the pump, heat exchanger, or expansion vessel
Setting the gas pressure at the valve
Cleaning the combustion chamber and burner
Measuring flue gases and setting the combustion ratio
Electrical work on the boiler's control electronics

Step-by-Step Diagnostics: What to Do When the Boiler Stops Working

When a Vaillant atmoTEC or turboTEC boiler stops and displays a fault, we recommend the following procedure:

Note the error code – take a photo or write it down. If it changes after a reset, record both codes.
Check the pressure gauge – if it's below 1 bar, top up the water to 1.5 bar and reset.
Check the gas cock – is it fully open? Does gas work elsewhere in the house (e.g. the stove)?
Check the outdoor coaxial terminal (turboTEC only) – is it blocked by snow or ice?
Reset the boiler – press reset for 3 seconds. Observe whether the boiler starts up and how long it keeps running.
Watch the behavior at startup – do you hear an ignition attempt? Does the fan start? Does a flame appear and then immediately go out?
If the fault persists or recurs within an hour – call a service technician and pass on all the information you have recorded.

Typical Service Costs for Common Faults

Customers always ask us about prices, so here are indicative costs. Actual prices depend on the region, the service company, and current parts prices.

Fault / Task	Indicative price (€ incl. VAT)	Note
Annual service inspection	80 – 130	Excluding replacement parts
Replacing the ionization/ignition electrode	50 – 90	Including labor
Replacing the pump (complete)	180 – 280	Depends on the pump model
Replacing the expansion vessel	120 – 200	Including refilling the system
Replacing the fan (turboTEC)	200 – 350	A more labor-intensive job
Cleaning/replacing the DHW exchanger	80 – 220	Cleaning cheaper; replacement more expensive
Replacing the gas valve	250 – 400	Original Vaillant part

When Repair Is Not Worthwhile and What to Consider Instead

Vaillant atmoTEC and turboTEC boilers older than 15 years can reach a stage where repair costs are disproportionate to the boiler's remaining value and efficiency. General recommendation: if the repair cost exceeds 40–50% of the price of a new boiler and the boiler is older than 12 years, it's better to invest in a new unit. Modern models have a better output modulation range, lower gas consumption, and a longer warranty.

When choosing a new boiler, read our articles How to Choose a Low-Temperature Boiler with Instantaneous DHW Preparation: What to Look Out For and Vaillant atmoTEC pro vs. atmoTEC plus: What's the Difference and Which to Choose, where you'll find practical comparisons of current models including output and DHW heating capacity.

Frequently Asked Questions (FAQ)

My Vaillant boiler shows F.22 every week – do I need to call a technician?

If the pressure drops regularly (once a week or more), the cause is not the boiler itself, but a leak in the system or a fault in the expansion vessel. Top up the pressure yourself to 1.5 bar and monitor how quickly it drops. If the pressure drops again within 3–4 days with no visible leak, the expansion vessel is most likely damaged. Call a technician – it's not an urgent fault, but it does need to be resolved. Every unnecessary top-up of fresh water into the system accelerates corrosion and fouling of the heat exchangers.

The boiler ignites but immediately goes out. What's the cause?

This is almost always a problem with the ionization electrode or its signal. The boiler "doesn't know" that the flame is burning because the electrode is fouled, misaligned, or broken. Another possible cause is too low a gas pressure – the flame lights weakly and doesn't ionize the air sufficiently. Both causes are addressed by a service technician during an inspection. Leave this to a professional – working with gas electrodes and adjusting gas pressure is not a job for a layperson.

Hot water runs cold even though the boiler is running. Where should I look for the cause?

The first thing to check: t