Servicing and Maintenance of Boiler and Cylinder Sets – What and How Often

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Servicing and maintaining a boiler and cylinder set – what to do and how often

A boiler and domestic hot water cylinder set is the heart of home heating and hot water preparation. It is a technically sophisticated system in which every component depends on the correct functioning of the others – the boiler, the cylinder, the circulation pumps, the expansion vessel, the safety valves, the hydraulic connections and the control electronics. If you only take care of this system half-heartedly, or "don't notice it until it stops working", you can expect several unpleasant and costly surprises. In this article we break down the complete service cycle of such a set: what to check, what to replace, what to clean, how often – and, above all, why it matters.

We base this on real-world practice, not just on manuals. Over the years, the same scenarios keep repeating: a customer calls because the cylinder "isn't heating up" or the boiler "keeps faulting". On inspection, it turns out the anode in the cylinder hasn't been changed in five years, the heat exchanger is clogged with limescale, or the pressure gauge shows 0.5 bar instead of 1.5. It all adds up, and the result is an expensive repair instead of a cheap preventive check.

Why servicing a set is different from servicing the boiler alone

Many owners have their boiler serviced every year (which is correct), but never deal with the cylinder – or vice versa. Yet the set works as a whole. The boiler may be in perfect condition, but if the cylinder has a damaged protective coating and a corroded anode, contaminated water will get into the entire system. Or if the cylinder's safety valve leaks, it drips unnecessarily and increases the pressure in the pipework.

Another difference compared to a standalone boiler is the hydraulic scheme. A set typically has two circulation pumps (one for heating, one for the cylinder), sometimes a three-/four-way valve, a cylinder thermostat, a cylinder temperature sensor, and potentially a solar connection as well. Each of these components requires attention and has a different replacement interval.

Annual service – the basics you shouldn't skip before winter

Once a year, a comprehensive service of the entire set should be carried out. The ideal time is late summer or early September – before the heating season, when service technicians are more readily available and a potential fault won't catch you off guard in sub-zero temperatures.

What the annual boiler service in a set should include

Visual inspection of the combustion chamber and burner – for atmospheric boilers (e.g. Vaillant atmoTEC plus + VIH CK 70), the burner is cleaned of deposits and the nozzle openings are checked; for fan-assisted boilers, the combustion chamber and condensate (if it's a condensing type) are also checked
Cleaning the heat exchanger – the primary heat exchanger is the boiler's most expensive component. A 1 mm layer of limescale reduces heat transfer by 10–15%, and a 3 mm layer can mean a 20% increase in gas consumption and a significant reduction in the exchanger's service life
Checking the flue gas discharge – for fan-assisted boilers (e.g. Vaillant turboTEC plus + VIH R 120), the coaxial flue is checked – whether it is blocked, whether the joint seals fit properly, and whether there is any flue gas back-suction
Checking and adjusting combustion parameters – measuring CO₂ and CO in the flue gases, checking the air/gas ratio, adjusting burner modulation
Checking the system pressure – correct operating pressure is 1.0–2.0 bar (typically 1.5 bar in a cold system). If the boiler repeatedly needs topping up with water, the problem is not the boiler but the expansion vessel or a leak
Checking and servicing the circulation pumps – both the cylinder and heating pumps: unblocking the shaft (usually with a screw on the side), checking the flow rate and head
Testing safety components – the boiler's safety valve (should release at 3 bar) and the emergency thermostat

What the annual cylinder service should include

Checking the magnesium anode – the most important item for stainless steel or enamelled cylinders. The anode sacrifices itself to corrosion instead of the tank. If it has worn down by more than 50% of its original mass, it needs to be replaced. Replacement frequency depends on water hardness – in soft water the anode can last 3 years, in hard water (over 20 °dH) realistically just 1 year
Checking the cylinder's safety valve – it should open at 6 bar (the standard value for DHW cylinders). If the valve drips during normal operation, it is either worn or there is no expansion vessel for the DHW circuit in the system
Visual inspection of the inside of the tank – when unscrewing the anode, it's a good opportunity to inspect the bottom of the cylinder with a flashlight. Look for sludge and limescale deposits, possible brown discolouration (corrosion), or cracked enamel
Flushing the cylinder – draining sludge from the bottom through the drain valve. Sludge in the cylinder reduces the effective volume, worsens heat transfer, and is a breeding ground for Legionella bacteria
Checking the cylinder's thermal insulation – the surface of the cylinder should not feel warm to the touch. If it does, the jacket insulation has been damaged or has come loose, increasing heat losses both at night and during the day
Checking the temperature sensor (thermostat/NTC) – the cylinder temperature sensor should be dry inside its immersion sleeve, free of corrosion, and its resistance should match the expected temperature characteristic

The two-year cycle – less frequent but critical tasks

Some tasks don't need to be carried out every year, but neglecting them will catch up with you in the long run. These include:

Checking and possibly replacing the expansion vessel

The expansion vessel (EV) compensates for the thermal expansion of water. It has two chambers separated by a membrane – an air chamber (filled with nitrogen or air, usually 0.5–1.5 bar) and a water chamber. If the membrane bursts, the EV stops performing its function and the boiler's safety valve starts opening every time the water heats up. The boiler reports high pressure even though you've just topped it up to the correct value.

Checking it: with the system cold, disconnect the EV from the hydraulics (close the ball valve in front of the EV), drain the water chamber and check the air chamber pressure with a hand pressure gauge. The correct value is the system's static height pressure + 0.2 bar (e.g. for a two-storey house, approx. 0.7–1.0 bar). If the air chamber shows 0, or water leaks out of the valve (burst membrane), the EV needs to be replaced. Membrane service life is 8–15 years, but depends on water quality and the number of cycles.

Important: a system with a DHW cylinder typically has a second expansion vessel for cold/hot domestic water as well (if the cylinder is in a closed circuit). This one is even more often forgotten during servicing.

Flushing and chemically cleaning the primary circuit

Every two years, the system should be flushed with a corrosion inhibitor or chemically cleaned of iron oxides and sludge. Indicator: if the water in the system is dark (reddish-brown) when drained, it needs flushing right away. Sludge in the circuit damages circulation pumps (increases friction), fouls the heat exchanger, and causes noise.

Cleaning procedure: drain the system, fill with a magnetic cleaning agent (e.g. Fernox F3), let it circulate for 24 hours at operating temperature, drain, flush with clean water, then fill with a protective inhibitor (e.g. Fernox F1). After chemical cleaning, it's a good idea to check the seals and joints – the chemical treatment may loosen old deposits that were sealing minor leaks.

Servicing the three-way/four-way valve

In sets where the boiler alternates between heating the cylinder and heating the house, a diverter valve is fitted. This valve has a motor (actuator) and a mechanical body. The typical service life of the motor is 5–8 years; the body lasts longer but can get stuck. Symptom of failure: the cylinder doesn't heat up (the valve stays in the heating position), or conversely, heating doesn't work and the cylinder keeps heating constantly. Servicing: removal, checking the movement of the flap, lubricating moving parts with silicone grease, testing the correct position.

Protection against Legionella – thermal shock and when to set it

Legionella pneumophila – a bacterium causing severe pneumonia – multiplies optimally in DHW cylinders at temperatures of 20–45 °C. Any cylinder that hasn't been used for a while, or that has been operating at a lower temperature for a long time, poses a potential risk. This isn't just marketing scaremongering – thousands of legionellosis cases are recorded in Europe every year, and home cylinders are one of the sources.

Protective measures in a set:

Cylinder operating temperature of at least 60 °C – at this temperature, Legionella dies within a few minutes. Many modern sets (e.g. Protherm Aqua Complet Panther 25KOO + FE 120 BM) have an automatic thermal shock program – once a week or every few days, the cylinder heats up to 70 °C
Enabling the anti-Legionella function in the boiler control – this needs to be activated in the controller menu (it's not always switched on from the factory). Interval: once a week, temperature: 70 °C, time: typically 3:00–4:00 am, when water consumption is lowest
After a holiday – if the house was empty for more than 2 weeks and the cylinder cooled down, manually run a thermal shock or let hot water flow through the whole tank
Disinfection with chlorine or a UV lamp – for cylinders installed in areas with problematic water, as additional protection

Water hardness and its impact on sets – what to watch out for in Slovakia

Slovakia has very varied water hardness. Western Slovakia (Bratislava, Trnava) and the northeast have water with a hardness of 15–30 °dH – that's medium-hard to hard water. Central Slovakia (Banská Bystrica, Ružomberok) has softer water, 5–12 °dH. Every additional degree of hardness means faster limescale build-up in both the cylinder and the heat exchanger.

Practical implications for sets:

In hard water (over 20 °dH): replace the cylinder anode every year, clean the boiler heat exchanger every 2 years, and consider a water softener or a magnetic/physical water conditioner before the cylinder inlet
In medium-hard water (10–20 °dH): anode every 1.5–2 years, boiler heat exchanger cleaning every 3 years
In soft water (below 10 °dH): the anode can last 3 years, check the heat exchanger every 4–5 years, but beware – soft water is aggressive towards metals (higher CO₂ content), so an inhibitor in the primary circuit is important

If you don't know the water hardness in your area, call your local water utility – they provide this information free of charge. Or get a test kit (approx. €5–10) and you'll have the result in 5 minutes. This information is key to setting the service intervals for the whole set.

Servicing a cylinder with two heat exchangers (with a solar connection)

If your set includes a cylinder with two heat exchangers (lower for solar, upper for the boiler – e.g. model Vaillant turboTEC plus + VIH R 120 with backup heating option), the servicing procedure extends to the solar circuit as well. This operates with a propylene glycol mixture (antifreeze), which degrades over time. During every annual service you should:

Measure the pH and density of the solar fluid (using a refractometer). The correct density is approx. 1.04 g/cm³ at a 40% glycol ratio, and the pH should be 7.5–8.5. If the pH is below 7 or the density is incorrect, the fluid needs to be replaced
Check the pressure in the solar circuit (typically 1.5–2.5 bar when cold). A pressure drop indicates a leak or a burst expansion vessel in the solar circuit
Check the solar collectors – pipe joints, seals, cleanliness of the collector glass. A 10% dirtier glass surface reduces output by 10%
Check the solar circulation pump and controller – correct activation at the temperature difference (typically ΔT 5–8 °C between the collector and the bottom of the cylinder)

Practical scenarios from the field – what actually happens

Scenario 1: The cylinder "isn't heating properly" even though the boiler is running

A customer calls saying the hot water is only lukewarm. The boiler is working and shows no fault. On site it turns out: the cylinder thermostat is set to 45 °C (set by a technician years ago to save energy), the three-way valve got stuck in the heating position, and the cylinder only gets heat randomly. On top of that, the cylinder's NTC sensor has slipped out of its sleeve and is measuring the temperature of the jacket, not the water. Result: an apparent fault, but actually just a setting issue plus a mechanical fault. Repair takes 1–2 hours, no spare parts needed.

Scenario 2: The boiler keeps topping up water, pressure keeps rising

A classic every winter. The customer calls saying the boiler reports low pressure every day and needs topping up. Diagnosis: the expansion vessel membrane has burst. The boiler takes on water, when it heats up the pressure spikes, the safety valve opens and releases water, and the boiler needs topping up again. Replacing the expansion vessel (approx. €60–120 for the EV plus labour) solves the problem immediately. The customer had insisted for a year that "it's always been like that."

Scenario 3: Smell from the hot water (hydrogen sulphide)

Less common, but real. In the cylinder, the anode combined with the presence of sulphates in the water and sulphate-reducing bacteria becomes a source of an unpleasant smell (eggs, rot). Solution: replace the magnesium anode with an aluminium one (less reactive), run a thermal shock at 70 °C, and if it recurs, consider a titanium electric (external) anode. The Protherm Aqua Complet Panther 25KTO + FE 120 BM cylinder comes with a magnesium anode that can be replaced with an alternative type depending on water quality.

Scenario 4: The cylinder is leaking from the bottom

This is a serious situation. If it's leaking from the safety valve, that's manageable. If it's leaking directly from the cylinder shell or from the anode flange, it could be corrosion of the bottom part (anode failure). A cylinder that has been perforated by corrosion cannot be repaired – the whole cylinder needs replacing. Prevention: regular anode replacement – otherwise the anode sacrifices itself and the cylinder corrodes instead, leading to exactly this situation.

What the owner can do themselves and what requires a professional

It's important to clearly separate what falls within the scope of an ordinary DIY-minded owner and what requires a certified service technician (gas engineer, manufacturer's service certificate).

You can do yourself (without special certification):

Check the system pressure on the boiler's pressure gauge (once a month, a quick glance is enough)
Drain sludge from the cylinder through the drain valve (once a year, in spring)
Check whether the cylinder's safety valve is dripping or not
Check that the cylinder's insulation is undamaged and the cylinder isn't warm on the outside
Unscrew and visually inspect the anode (if you have the right tool and the confidence)
Run the manual thermal shock program (via the controller menu)
Top up the system pressure via the boiler's filling valve (if you know the correct target value)
Check that the circulation pumps can be heard running (a gentle hum is normal, loud knocking is not)

Requires a professional (gas technician / service engineer):

Any intervention in the combustion section (burner, burner plate, heat exchanger)
Measuring and adjusting combustion parameters (CO, CO₂, lambda)
Replacing the boiler's safety valve (if the pressure is incorrect)
Replacing a circulation pump
Servicing and adjusting the three-way/four-way valve
Replacing or topping up glycol in the solar circuit
Chemical cleaning of the circuit with neutralisation and inhibitor
Checking the gas valve assembly, gas pressure regulator, and gas manifold

Warranty conditions and service records – why not to underestimate them

Manufacturers such as Protherm and Vaillant make warranty coverage conditional on regular professional servicing by an authorised technician. Specifically: Vaillant standardly provides a 2-year warranty, which is extended with each annual service performed by an authorised partner. Protherm has similar conditions. This isn't just a formality – if something breaks down in the third year of operation and you don't have service records, the manufacturer will refuse the claim.

A service record should include: the date of the service, the technician's name and certification, the measured values (pressure, temperatures, combustion parameters), the work performed, and the parts replaced. Keep these records together with the boiler and cylinder documentation – they are documents that also increase the value of the property when selling.

Estimated service life of individual set components

Component	Typical service life	Note
Boiler (whole unit)	15–20 years	With regular servicing
DHW cylinder (enamel)	15–20 years	Depends on the anode and water quality
Magnesium anode	1–3 years	Depends on water hardness
Expansion vessel (membrane)	8–15 years	Check every 2 years
Circulation pump	10–20 years	EC pumps last longer, old ON pumps shorter
Cylinder safety valve	5–10 years	Replace preventively if it keeps dripping
Three-way valve (motor)	5–10 years	The valve body lasts longer than the actuator
Cylinder NTC temperature sensor	10–15 years	A cheap part, its check is often overlooked
Boiler primary heat exchanger	15–25 years	Depends on limescale and servicing

How to organise a service plan – a simple model for the average owner

From experience, most owners don't keep any written record of servicing. A simple notebook or a document on your phone is enough:

Every time you top up water in the boiler: note the date and how many bars you added. If you're topping up more than once a month, contact a service technician.
Once a year (September): boiler service + cylinder flush + anode check + SV test + EV pressure check
Once every 2 years: anode replacement (even if it looks OK, in hard water), chemical flush, solar circuit (if you have one)
Once every 5 years: comprehensive diagnostics of the whole system – heat exchanger, expansion vessel (membrane replacement), circulation pumps, valves

One more practical tip: when installing a new set, ask the technician for a list of service intervals specific to your particular boiler and cylinder. Protherm and Vaillant issue service sheets listing all the intervals for a specific model. For sets like the Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH CK 70, the myVAILLANT app is also available, where you can set reminders and track error codes.

We cover the topic Common faults in boiler and cylinder sets and how to recognise them in a separate article in our Knowledge Centre, where you'll find extended diagnostics based on error codes. If you're planning your first set installation, we also recommend reading How to connect a boiler to a cylinder – installation procedure and Correctly sizing boiler output to cylinder volume.

Frequently Asked Questions (FAQ)

How do I know the anode in the cylinder is worn out if I can't see it?

The best way is to physically unscrew and check it once a year. The anode (a threaded rod, G6/4" or G1-1/4") is usually found under the top cover of the cylinder or under an insulating cap. If it has worn down to less than 50% of its original diameter, or is covered with a white substance (magnesium carbonate) along its entire length, replace it. Indirect signs of a worn anode: smelly water, browning of the hot water, or impurities. A new anode costs €15–40 depending on type, plus €20–40 for labour. Compared to the price of a new cylinder (€300–1000), that's nothing.

Do I need to service the cylinder even if I only use it for hot water and heat the house another way?

Yes, servicing the cylinder is independent of whether it's connected to a heating boiler or just a boiler for DHW preparation. The anode wears out through contact with water, not with heat from the heating system. Legionella can multiply in any cylinder with hot water. The safety valve, the DHW circuit's expansion vessel, and sludge flushing are needed regardless of the hydraulic setup.

The boiler in the set signals "low pressure" every day – is that a cylinder fault?

Probably not the cylinder, but the expansion vessel of the primary heating circuit. Daily topping up usually indicates a burst EV membrane – the system's pressure rises when it heats up, the safety valve releases a little water, and when it cools down the pressure drops below the minimum. The DHW cylinder has its own closed circuit and its own expansion vessel (if it's a closed system), which is a separate one. For a definitive diagnosis, have a technician check both expansion vessels.

After ten years, is it worth doing a major cylinder service, or is it better to buy a new one?

It depends on the cylinder's condition. If the enamel is undamaged, the anode is clean, and there's just accumulated sludge – a flush, a new anode, and the cylinder can easily last another 10 years. If the enamel is cracked or the cylinder shows shell corrosion (reddish-brown water even after flushing, rust stains on the shell), replacement is the only sensible choice. As a rough guide: repairing a cylinder for under €150 is worth it. Above €300 (e.g. replacing the heat exchanger, the whole shell), consider a new cylinder.

What does it mean when water drips from the cylinder's safety valve every time it heats up?

This is normal behaviour in a closed system WITHOUT a DHW expansion vessel. Water expands as it heats and the excess has to go somewhere – and since there's nowhere for it to go, it opens the SV. Solution: install an expansion vessel for the DHW circuit (a "drinking water" type with a membrane certified for potable water, e.g. Reflex 10 DT). If there is an expansion vessel but the SV still drips, either the EV is under-pressurised or the SV is worn. Never block a cylinder's safety valve – in an emergency it protects the cylinder from bursting.

Can I use any inhibitor fluid in the boiler's primary circuit with a cylinder?

No. Boiler manufacturers (Vaillant, Protherm) specify approved additives, and some models explicitly prohibit certain types of inhibitors – typically those containing nitrites (old-type nitrate inhibitors). Safe choices are organic-phosphate inhibitors such as Fernox F1, Sentinel X100, or Adey MC1+. These are compatible with copper, aluminium, and steel components and won't void the manufacturers' warranty conditions. Always check the boiler documentation for which additives are explicitly approved.

Conclusion

A boiler and cylinder set is not an appliance that "just runs" after installation. It's a hydraulic system with dozens of moving and static components, each with its own service life and service interval. The good news: regular maintenance genuinely extends the system's service life, saves energy, and prevents costly breakdowns. The bad news for those who neglect it: repairing a boiler and cylinder after long-term neglect is significantly more expensive than the sum of a whole series of preventive services.

If you're considering a new set or want to compare what different models offer in terms of servicing demands, take a look at our articles Protherm Panther vs. Vaillant turboTEC – comparing sets and How to choose a boiler and cylinder set – what to look out for, where we cover these aspects in detail.

Have a question on this topic?

Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.