Maintenance and Servicing of a Condensing Boiler with DHW: What and How Often to Check

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Maintenance and servicing of a condensing boiler with DHW: what and how often to check

A condensing boiler with instantaneous domestic hot water (DHW) heating is today the most common type of heating appliance in Slovak households and smaller commercial premises. Its advantages are high efficiency (typically 97–109% when calculated based on calorific value), compactness, quiet operation, and convenient access to hot water without the need for a storage tank. But like any device with moving parts, heat exchangers, and electronics, the boiler requires regular attention. Neglected maintenance will quickly catch up with you: higher gas consumption, faults, shorter lifespan, and in extreme cases, even a safety risk.

In this article, we will systematically go through all the areas that need to be monitored, at what intervals, what you can handle yourself, and what belongs exclusively in the hands of a certified service technician. We draw on experience from real jobs – from boiler rooms in apartment buildings, family houses, and guesthouses, where we have seen absolutely everything: from clogged heat exchangers to cracked seals and incorrect settings that the owner ignored for years.

Why regular maintenance of a condensing boiler is a necessity, not a luxury

A condensing boiler works on the principle of using heat from the condensation of water vapor in the flue gases. This means that water (not only in the heating circuit but also in the DHW circuit), gas, flue gases, and condensate are constantly moving inside it. The combination of these media in one device places high demands on cleanliness, tightness, and correct chemical water parameters.

According to STN EN 13384 and the recommendations of manufacturers such as Protherm or Vaillant, an annual professional inspection combined with cleaning is recommended. Insurance companies and gas suppliers (SPP Distribúcia) also condition the validity of the warranty and insurance claims on documented regular servicing. Boilers such as the Protherm Gepard Condens 18/25 MKV or the Vaillant VUW 236/5-3 ecoTEC pro have it directly stated in their warranty that service inspections must be carried out by an authorized technician – otherwise the warranty terms become invalid.

From a purely economic point of view: a neglected boiler consumes 5–15% more gas per year. With an average consumption of 1,500 m³ of gas per year and a price of around €0.70/m³, this means an unnecessary €52–157 per year. Over ten years, you would therefore pay for several complete services, not just one.

Maintenance areas: overview and frequencies

Checking the heating water pressure: simple but critical

This is the one thing the owner of a condensing boiler should do themselves regularly – ideally once a month, or even every two weeks during the heating season. The system pressure is shown either on an analog manometer (a dial with a needle) or digitally directly on the boiler's display. The correct value for a cold system (heating water temperature up to 30°C) is 1.2–1.8 bar. In a warm system, it may be 0.3–0.5 bar higher – this is normal thermal expansion.

If the pressure drops below 1.0 bar, the boiler will usually shut down and display an error. Refilling – so-called system top-up – is necessary. Procedure: open the filling valve (usually a small brass valve with a nut under the boiler, connected by a hose to tap water), let the pressure rise slowly, and close the filling valve at around 1.5 bar. Never fill the system above 2.5 bar – there is a risk of triggering the safety valve.

Beware of repeated pressure drops: if you have to top up several times a month, there is a leak somewhere in the system. That is not a normal condition – call a technician. Frequent topping up also means new raw water containing calcium and oxygen is continuously being added to the system, which accelerates corrosion and scaling.

Filters and strainers: small components with a big impact

Heating water filter

Most condensing boilers have a strainer or magnetic filter built in before the return heating water inlet. Its task is to catch impurities – sludge, rust, deposits from old pipes and radiators. A clogged filter causes the water to stop circulating properly, the pump becomes overloaded, and the heat exchanger becomes clogged with sediment. The result: the boiler overheats, drops into a safety limit, or simply heats unevenly.

Cleaning the filter is simple: shut off the inlet and outlet, loosen the filter cap, remove the strainer, rinse it under running water, possibly clean it with a soft brush, and put it back. On a magnetic filter, wipe off the collected ferromagnetic sludge (dark black deposit – magnetite) dry with a cloth. It is recommended to do this twice a year – at the beginning and end of the heating season.

DHW filter (instantaneous heating)

A condensing boiler with instantaneous DHW heating has sensitive small-diameter channels in its flow heat exchanger. Water supplied from the mains passes through these channels, and if it is hard (which is the case in many areas of Slovakia – hardness above 3 mmol/l), limescale deposits quickly form. In addition, impurities from pipes can clog the DHW inlet filter.

This filter is usually located at the cold water connection of the boiler. It is a small strainer in a brass fitting. Removing, cleaning, and replacing it takes five minutes – you just need pliers and a small wrench. We recommend cleaning it every three months, or even monthly in areas with hard water.

Bleeding the system and radiators

Air in the system is the enemy of a condensing boiler. It causes gurgling and noise in the pipes, hinders water circulation, reduces radiator output, and has a long-term corrosive effect on internal parts. Bleeding should be done at the start of each season, or after topping up a larger amount of water.

Procedure for bleeding radiators: turn on the boiler, let the water heat up and circulation start. Then, starting from the highest point of the system (typically the radiator on the top floor), gradually open the bleed valves (a small square or hex screw in the corner of the radiator) and wait until the air stops escaping and water starts dripping. Then close it. After bleeding, check the pressure – since air was replaced by water, the pressure may have dropped slightly and may need topping up.

Modern boilers such as the Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect have an automatic air vent directly in the boiler. This makes things easier, but it will not bleed the radiators for you manually – only the boiler circuit.

Condensate siphon: an underrated component

A condensing boiler produces condensate – acidic water (pH 3–5) formed when the flue gases cool down. This liquid drains through a siphon into the sewer. The siphon has two functions: it prevents flue gases from entering the room through the sewer (similar to a sink siphon) and it captures any impurities.

A problem occurs when the siphon dries out (for example, after a longer summer shutdown of the boiler) or becomes clogged. A dried-out siphon allows flue gases to enter the room – this is a safety risk. A clogged siphon can cause condensate to accumulate in the boiler and damage the heat exchanger or trigger error codes.

Cleaning the siphon: unscrew the siphon container under the boiler (usually a plastic bowl with a thread), empty the contents, clean the inside of deposits, rinse with clean water, and reinstall. Before restarting the boiler, pour about 0.5 dl of water into the siphon so that the water seal is functional again. Do this at least once a year, ideally before and after the season.

Expansion tank: checking the pre-charge pressure and membrane

The expansion tank is a pressure vessel, usually a red sphere or cylinder, located either directly in the boiler (internal) or on the pipework (external). Its task is to compensate for changes in water volume during heating. Inside is a rubber membrane – on one side there is air or nitrogen under pressure (the so-called pre-charge pressure, typically 0.75–1.0 bar), and on the other side is the heating water.

When the membrane bursts or the pre-charge pressure changes, the expansion tank stops working properly. The system then responds with repeated pressure losses (the safety valve blows off water) or, conversely, with pressure that is too low. The pre-charge pressure is checked with a standard pressure gauge (as with tires) via the air valve on the tank – always with the system depressurized (boiler cold, heating circuit pressure released). If the pre-charge pressure is off, top it up with a regular foot pump. If the membrane has burst, the tank must be replaced.

This is a job for a technician, as it involves depressurizing the system. Most manufacturers (including Protherm and Vaillant) include checking the expansion tank as part of the mandatory annual inspection.

Cleaning the DHW heat exchanger and the primary flue gas heat exchanger

This is the heart of the condensing boiler and also the most costly repair if neglected. The DHW flow heat exchanger (typically a stainless steel or copper plate heat exchanger) has very narrow channels where limescale deposits build up faster than anywhere else. A boiler with a clogged DHW heat exchanger does not deliver sufficiently hot water, is slower, consumes more energy, and in extreme cases the heat exchanger can crack from overpressure or thermal shock.

Cleaning is done either mechanically (disassembly and flushing with acids – typically a citric acid solution or a special descaler), or by chemical in-situ flushing using a circulation pump. This procedure requires disassembling the inlets, connecting a pump, and flushing for 30–60 minutes. After flushing, the heat exchanger is neutralized and rinsed with clean water.

A similar rule applies to the primary flue gas heat exchanger (aluminum or stainless steel, located above the burner). In addition, soot and products of incomplete combustion can build up on its surface – especially if the boiler has been poorly adjusted for a long time (excess gas, dirty air filter). The heat exchanger is cleaned with soft brushes and vacuumed. This work belongs exclusively to a technician.

For these tasks, we recommend entrusting appliances such as the Protherm Panther Condens 20/26 KKV or the Protherm Puma Condens 18/24 MKV exclusively to an authorized technician, since disassembling the heat exchanger requires knowledge of the specific model and correct tightening of the seals.

Checking the burner, electrode, and combustion process

The burners in condensing boilers are premix burners – air and gas are mixed before combustion, ensuring clean and efficient combustion. Nevertheless, deposits can form on the burners, or they can be mechanically damaged. The ionization electrode (a rod sensed by the flame) gradually wears out and loses sensitivity – this can lead to "flame not detected" error codes even though the boiler is burning normally.

Combustion quality is verified during the annual inspection by flue gas analysis – the technician connects a flue gas analyzer to the exhaust outlet and measures the CO, CO₂, and O₂ content. A properly adjusted condensing natural gas boiler should have CO₂ around 8–9%, O₂ around 5–7%, and CO below 50 ppm. Deviations indicate the need to readjust the gas-air ratio or clean the burner. Boilers equipped with IoniDetect technology (such as the Vaillant ecoTEC plus) can automatically correct combustion in real time based on the signal from the ionization electrode – but this also requires the electrode to be in good condition.

Safety components: what to check and what not to test yourself

Safety valve

The safety valve is the last line of defense against overpressure in the system. Usually set to 3 bar. You should test the safety valve (by turning the lever) no more than once a year, and only if you know the valve is new or was tested less than 5 years ago. Old valves can have their seal damaged after opening and start to drip – in that case, they need to be replaced. Recommendation: it's better to leave this to a technician who has a replacement valve ready.

Gas leak detector

Although not directly part of the boiler, at every service the technician should check the gas train (gas valve, gas filter, pressure regulator) for tightness using manometric pressure. Never check gas seals with soapy water yourself if you lack experience and equipment. An (electronic) gas detector is a good investment in any case – we recommend having one in every boiler room.

Non-return flap and flue gas exhaust

You should visually inspect the flue gas exhaust (plastic or stainless steel coaxial pipes) once a year: look for cracks, loose joints, and accumulation of condensate in places where it should not collect. The outer end of the exhaust must not be overgrown or blocked. The non-return flap in the exhaust (if fitted) must move freely.

Water quality: the silent destroyer of a condensing boiler

This is a topic that is too rarely discussed in routine maintenance. The heating system is a closed circuit, which is an advantage – but if the water in the system is chemically unbalanced, it causes enormous damage over the years. What to watch out for:

pH of the system water should be 7.5–9.0. Water that is too acidic (pH below 7) corrodes aluminum and copper, while water that is too alkaline (pH above 9) damages seals and inhibitors.
Hardness of the filling water: ideally below 2 mmol/l (soft water). In areas with hard water (e.g., around Bratislava, Nitra) we recommend adding corrosion and scale inhibitors, or using a water softener.
Oxygen content: oxygen in a closed system is a source of corrosion. Therefore, the system should never be unnecessarily opened and regularly topped up with new water. If topping up is necessary, use demineralized or at least softened water.
Inhibitors: chemical additives (e.g. Sentinel X100, Fernox F1) extend the system's lifespan. Their concentration should be checked and, if necessary, topped up every 3–5 years.

You can find out more about this and other aspects of installation in our article Installing a condensing boiler with instantaneous DHW heating: procedure and requirements.

Software, firmware, and electronics: modern boilers also need updating

Modern condensing boilers such as the Vaillant ecoTEC plus or Protherm Gepard/Panther are equipped with control electronics that manage not only combustion but also the pump, the three-way valve (heating/DHW switch), diagnostics, and communication with a smart thermostat. This electronics may have firmware that the manufacturer occasionally updates.

In practical terms, this means: if the boiler communicates via eBUS (Vaillant) or OpenTherm (Protherm/Vaillant) with a smart thermostat, the technician should also check the control settings, communication parameters, and functionality of all sensors during servicing. The return temperature sensor, the DHW temperature sensor, the flow sensor (flowmeter) – if one of them is outdated or shows deviations, the boiler will not work optimally.

You can read more about optimal settings in the article Setting the temperature and pressure in a condensing boiler with DHW for optimal savings.

What is included in the annual professional inspection: complete list

So you know what to expect from the technician during the annual inspection and what to request in the report, here is a standard list of tasks for a complete service inspection of a condensing boiler with DHW:

Visual inspection of the boiler, its external condition, mounting, labeling, and documentation
Inspection and cleaning of the combustion chamber, burner, and burner seals
Inspection and possible replacement of the ionization and ignition electrode
Flue gas analysis (CO, CO₂, O₂, flue gas temperature, degree of condensation)
Cleaning and inspection of the primary heat exchanger (flue gas/water)
Cleaning and inspection of the secondary DHW heat exchanger (or plate heat exchanger)
Inspection and cleaning of the condensate siphon, checking condensate drainage
Checking the expansion tank pre-charge pressure, topping up if necessary
Checking the safety valve
Checking the system pressure, topping up if necessary
Inspection and cleaning of the heating and cold water (DHW) filters
Checking the function of the pump, three-way valve, and flow sensor
Checking the flue gas exhaust, joint seals, and non-return flap
Checking the gas train – seals, gas filter, shut-off valves
Functional test – start-up, heating operation, DHW heating, safety shutdown
Entry into the service book, issuance of a service report

Always have the report signed and stamped – it is your proof for the insurance company and for warranty terms. If the technician skips any of these points, you have the right to request it or to change service providers.

When to call a technician outside of the regular inspection: signals you shouldn't ignore

From experience, we know that most customers call a technician too late – when the boiler no longer reaches temperature, or when an error code persists even after a restart. Here are the signals that require immediate action:

Repeated pressure drops (several times a month) – a leak in the system
Noise and gurgling from the boiler or radiators – air in the system, or a pump fault
Yellow or orange flame instead of blue (visible through the inspection window) – incorrect gas/air ratio, risk of CO
Smell of combustion around the boiler – a leak in the flue gas duct
DHW is cold or lukewarm even at full output – a clogged DHW heat exchanger or a faulty flow sensor
The safety valve repeatedly discharges water – a faulty expansion tank or an oversized heater
Error codes that recur after a restart – do not leave them undiagnosed, even if the boiler works in the meantime
Uneven heating – some radiators cold even at full temperature – hydraulic imbalance or clogged valves

A detailed explanation of error codes can be found in the article Common faults of condensing boilers with instantaneous DHW heating and their solutions.

Seasonal shutdown and restarting the boiler

If you completely turn off the boiler for the summer (which we do not recommend for instantaneous DHW heating, since you need DHW year-round), a few steps must be followed:

When shutting down for the summer: switch the boiler to summer mode (DHW only, heating off), or if you are really shutting it down completely – close the gas valve and disconnect it from the power supply. Check the condensate siphon and top it up with water. Do not leave the system depressurized (air would corrosively attack the system) – keep the pressure at 1.0–1.5 bar.

When starting up in the autumn: before the first start-up, check the pressure, bleed the radiators (starting from the highest point), inspect the condensate siphon, and check the outdoor exhaust (bird nests, insects, debris). Start the boiler at minimum output and let it perform several short cycles before switching to automatic control.

Contract servicing vs. one-off servicing: what's worth it?

Many manufacturers and service organizations offer annual service contracts – a flat fee covers the inspection, cleaning, and possibly the replacement of standard consumable parts (seals, electrodes). From a long-term cost perspective, this is usually more advantageous than one-off servicing, and you also have the assurance that the inspection will take place on time without you having to remember it.

A service contract also usually guarantees priority intervention in the event of a breakdown – something you will appreciate in December at minus ten degrees. Prices for annual servicing of a standard condensing boiler with DHW (excluding parts replacement) range from €80–150 depending on the region and service organization.

Frequently Asked Questions (FAQ)

How often do I need to have my condensing boiler serviced to keep the warranty valid?

The standard requirement of manufacturers such as Protherm and Vaillant is an annual professional inspection by an authorized technician. The inspection must be recorded in the boiler's service book with the technician's signature and stamp. Without this document, you may lose your warranty in the event of a claim. Some models (for example the Vaillant ecoTEC pro) allow online registration and warranty extension up to 5 years precisely thanks to regular servicing.

Can I clean the condensate siphon myself, or do I need to call a technician?

Yes, cleaning the siphon is one of the few things a handy owner can do themselves. The siphon is usually located under the boiler, is made of plastic, and can be unscrewed without tools or with a standard wrench. It is important to add a little clean water back into the siphon after reinstalling it so that the water seal works. If you are not sure of the siphon's location on your specific model, check the manual or ask during a service visit.

What should I do if the boiler's safety valve is dripping water?

If the valve only drips when the boiler is hot and then stops, this is normal thermal expansion (the valve opens slightly at a pressure close to 3 bar). If it drips continuously or water repeatedly flows from it, this indicates a problem with the expansion tank (a burst membrane, lost pre-charge pressure). Shut off the water supply, reduce the pressure, and call a technician immediately. Also get a discharge hose for under the valve – drips must not fall directly onto hot parts of the boiler.

Why do I have less hot DHW than before, even though the boiler is working?

The most common cause is limescale build-up in the plate or tube-type DHW heat exchanger. The boiler generates heat but cannot transfer it efficiently through the clogged heat exchanger. A second possibility: a faulty flow sensor (flowmeter) – the boiler does not detect water flow and does not start DHW heating at full output. The solution is a chemical flush of the heat exchanger or replacement of the sensor – both require a technician. More in the article Instantaneous DHW heating in a condensing boiler: how it works and what affects comfort.

Is it necessary to replace the ionization electrode at every annual service?

Not necessarily every year – it depends on the condition of the electrode. The technician visually inspects the electrode and measures the ionization current (the value should typically be 2–8 µA, exact values depend on the model). If the current is at the lower limit or the electrode shows erosion damage to the tip, preventive replacement is recommended. The electrode is inexpensive (€5–20), but its failure will prevent the boiler from igniting and will trigger a safety lockout – which is not pleasant in winter.

Which is better – antifreeze (non-freezing mixture) or plain water in the system?

For most family houses with a condensing boiler, clean (treated) water with a corrosion inhibitor is recommended – not antifreeze. Antifreeze has lower thermal capacity, reduces system efficiency, and its pH needs to be checked regularly, since after years it degrades into an acidic solution that corrodes aluminum. Antifreeze is only justified if there is a real risk of freezing (e.g. a boiler room in a holiday property without permanent heating). If you use it, it must be compatible with aluminum heat exchangers (most condensing boilers have them) – always read the data sheet.

Conclusion: Regular care always pays off

A condensing boiler with instantaneous DHW heating is a sophisticated appliance that, with proper care, can last 15–20 years of reliable operation with high efficiency. The key is a combination of daily attention (monitoring pressure, noise, error codes) and regular professional inspections once a year. The technician's job is not only to clean the boiler, but also to preventively detect part wear before it causes a fault.

If you are considering a new condensing boiler with DHW, take a look at our products – for example the reliable Protherm Gepard Condens 18/25 MKV or the premium Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect. Our article How to choose a condensing boiler with instantaneous DHW heating: what to focus on discusses choosing the right model, including a comparison of key parameters.

Do you have a question about this topic?

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