Maintenance and Servicing of a Low-Temperature Boiler with Storage Tank: What to Check and How Often

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Why regular maintenance of a low-temperature boiler with a tank is so important

A low-temperature boiler with a built-in DHW tank is a complex appliance that combines two functions: space heating and domestic hot water preparation. It is precisely this combination that makes it more prone to several types of wear at once. While with a condensing boiler without a tank you mainly deal with the condition of the heat exchanger and the condensate system, here you also have the entire volume of the tank, the magnesium anode, the tank coil, and the tank thermostat. Each of these components has its own lifespan, its own weak points, and requires regular oversight.

From experience, I know that most failures of these boilers don't happen suddenly – they are slowly developing processes that an experienced eye would spot months in advance. Limescale, which builds up over years, eventually insulates the tank coil so much that the boiler starts having trouble heating water to the required temperature. A rusted and depleted anode, unchecked for three years, allows corrosion to attack the steel shell of the tank. Contact pressure in the expansion vessel, which nobody bothered to check, causes the safety valve to start dripping every day.

The article Rusting, limescale, and the tank anode – how to extend the life of your boiler in this Knowledge Center goes deep into the physical nature of these phenomena. Here I focus on a concrete service schedule – what to do, when, and how – so you know what to expect from a technician and what you can check yourself.

Overview of components to monitor

Before moving on to the schedule, it's useful to be clear about what a boiler with a built-in tank actually consists of and where its critical points are. The following diagram shows a simplified cross-section of the appliance:

Critical components that a technician (or you yourself) must regularly monitor:

Magnesium (or titanium) anode – protects the steel tank from corrosion; it wears out and needs replacing
Tank heat exchanger (coil) – transfers heat from the boiler circuit to the tank; limescale builds up on it
Tank safety valve – protects the tank from overpressure; tested manually
Expansion vessel (boiler and tank) – maintains system pressure; its pre-charge pressure is checked
Primary boiler heat exchanger – with low-temperature boilers, deposits can build up if water quality is poor
Circulation pump – risk of seizing after summer shutdown
Tank thermostat and controls – correct setting affects energy efficiency and safety (Legionella)
Burner assembly, igniter, ionization electrode – burner service points
Flue and exhaust gas outlet – cleanliness and sealing

Annual service schedule: what, when, and by whom

The easiest way not to forget any maintenance is to plan it over time. Below is a schedule divided into three levels: things you can check yourself (marked "owner"), things where it's enough to occasionally call a technician, and things that are strictly for an authorized service technician.

Monthly checks (done by owner)

System pressure. Every boiler has a pressure gauge – either analog or digital. A cold boiler (in the morning before start-up) should show 1.0 to 1.5 bar. When the boiler is warm (fully heated), the pressure rises to about 1.5 to 2.5 bar – that is normal. If the pressure is repeatedly below 1 bar, the system is losing water and you should look for the cause (leak, faulty expansion vessel). Conversely, if the safety valve regularly drips pressurized water, the expansion vessel has probably failed.

Visual check under the boiler. Once a month, simply check whether there are any traces of water, limescale, or moisture under the boiler. Dried water stains on the floor are the first sign of a leak.

Sounds and vibrations. The boiler should operate quietly – a soft hum from the burner and pump. Knocking, cracking, or "banging" in the tank (usually during heating) indicates heavy limescale deposits on the tank coil. Squealing or humming from the pump can mean air in the system or a damaged bearing.

Semi-annual checks (spring and autumn)

Venting the system. Before the heating season (September/October) and after it (April/May), check whether there is air in the system. Air in radiators shows up as noisy water flow and uneven heating (top of radiator cold). Bleeding radiators is a common owner task – you'll need a bleed key and a container to catch the water.

Expansion vessel pre-charge pressure. This is a technical matter, but a handy owner with a tire pressure gauge can manage it. The expansion vessel should have a pre-charge pressure corresponding to the static height of the system – generally 0.5 bar + 0.1 bar for each meter of height above the boiler. For a typical family home this is usually 0.8 to 1.0 bar. A membrane expansion vessel is checked as follows: depressurize the boiler, and measure pressure at the vessel's valve (looks like a car valve). If it is zero or much lower than it should be, the membrane is probably perforated and the vessel needs replacing.

Tank safety valve. Once every six months, manually test the DHW tank's safety valve (usually set to 6 bar). Turn or lift the lever, water should flow out, and after releasing the lever the valve should close. If the valve doesn't drip or keeps dripping after the lever is closed, it is worn and should be replaced – it's not an expensive part, but neglecting it can lead to the tank bursting.

Annual boiler service: what the technician does

Once a year the boiler should be checked by an authorized service technician. For a low-temperature boiler with a tank, the service usually goes deeper than for a standalone condensing boiler, since it also covers the tank part. The ideal time is late summer (August/September) – before the heating season, when the boiler isn't under load and any findings can be addressed without the risk of being left without heating over winter.

Boilers like the Thermona THERM PRO 14 KX or the Thermona THERM PRO 14 TKX are popular precisely for their clear construction and accessible service points, which makes the job easier for both the technician and the owner.

What the technician checks and cleans during the annual service

Burner and burner chamber: Check the condition of the burner nozzle, ignition electrode, and ionization electrode. The electrodes are visually inspected and, if needed, their position and air gap are adjusted. Over longer operation, soot deposits build up that disrupt flame stability.
Primary heat exchanger: Inspection from the accessible side, possibly flushing. In low-temperature boilers (unlike condensing ones), the heat exchanger doesn't come into contact with condensate, but with hard water, limescale can build up here too.
Combustion setting: The technician checks the gas-to-air ratio using a flue gas analyzer. CO₂ in the flue gases should be within the range specified by the manufacturer (for natural gas typically 8–10%). Poorly set combustion increases gas consumption and can produce CO.
Return line filter (dirt trap): Cleaning the strainer, flushing. A clogged filter reduces flow, overloads the pump, and can distort control operation.
Circulation pump: Check operation, possibly vent the pump. After summer shutdown, the pump sometimes "seizes" – a technician can manually rotate the shaft without disassembly, unless the situation is too severe.
Flue and exhaust gas outlet: Check sealing, patency, correct slope of the condensate line (for low-temperature boilers the flue is mostly free of condensate, but seals can age). Check the protective grille at the flue outlet (birds, debris).
Operational documentation: Service report, possibly resetting the service indicator in the control unit.

DHW tank: special service points

The tank part of the boiler deserves its own chapter. It is essentially a pressure vessel with a capacity (depending on type) of 80 to 200 liters that is in contact with drinking water. Here, not only thermal protection plays a key role, but also protection against corrosion and biological contamination.

Magnesium anode: when and how to replace it

The magnesium anode is a rod (20–50 cm long, 20–33 mm in diameter) that sacrifices itself in place of the tank's steel shell. Electrochemically it is less noble than steel, so corrosion "eats away" at it preferentially. Once it's used up, the tank starts rusting rapidly from the inside. Depending on water hardness and anode quality, its lifespan is 2 to 5 years.

The anode should be checked at every second annual service, i.e. once every 2 years, or sooner if you have very hard water (above 3 mmol/l, i.e. above 300 mg CaCO₃/l). The technician depressurizes the tank, opens the access opening (usually a threaded cap on top of the tank), and removes the anode. If its diameter at the thinnest point is less than 10 mm (the original was about 22–26 mm) or it is broken, it needs replacing. A new anode costs 15–40 euros, plus another 30–60 minutes of the technician's work.

Practical experience: with very soft water (pH below 7, hardness below 1 mmol/l), the anode paradoxically corrodes faster – soft water is more aggressive than hard water. In such areas it is sometimes better to switch to a durable, actively powered titanium-magnesium or pure titanium anode with external power supply (so-called impressed current protection).

Protection against Legionella pneumophila bacteria

This is an area customers tend to forget about, but it is a health-critical one. Legionella bacteria multiply in DHW tanks at temperatures of 25–50 °C. They die at temperatures above 60 °C. A low-temperature boiler typically heats the tank to 55–65 °C, which is a safe operating temperature if the thermostat is set correctly.

A problem arises when, for the sake of saving energy, the owner lowers the tank temperature below 55 °C (e.g. to 45 °C) – while saving a bit of energy, this creates ideal conditions for Legionella growth, especially in the lower part of the tank, where the temperature can be even lower.

Recommended practice: Maintain a permanent tank operating temperature of at least 55 °C. Once a week (some controllers do this automatically), run a "thermal disinfection" cycle – heating the tank to 70–75 °C. On boilers such as the Thermona THERM 28 LXZ.A 5, this function is set directly in the control unit and runs automatically.

Flushing and sediments in the tank

Over the years, sediments build up at the bottom of the tank – a combination of limescale, corrosion products, and biological buildup. These sediments reduce heating efficiency, can cause an unpleasant smell in the hot water, and are a breeding ground for bacteria.

Recommended tank flushing procedure (once every 2–3 years under normal operation, once a year with hard water above 400 mg/l):

Turn off the boiler, let the tank cool down (below 40 °C, for safety reasons)
Shut off the cold water supply to the tank
Open the tank's drain valve (usually at the bottom, G ¾") and let the tank drain into the sewer
Briefly open the cold water supply and flush – until the water coming out is clear
Fill the tank and start the boiler

In case of heavy sediment and limescale buildup on the tank coil (tank heat exchanger), chemical descaling is needed – using a special agent (e.g. citric acid or phosphoric acid at an appropriate dilution) circulated through the tank circuit. This is a job for a technician, not the owner.

Exhaust gas outlet and flue

Unlike a condensing boiler, a low-temperature boiler operates with higher flue gas temperatures (usually 120–200 °C) and does not produce condensate, which is an advantage on one hand (no worries about a condensate line), while on the other hand the flue gases are drier and pass better through an old stone or brick chimney. Nevertheless, regular inspection of flues is a legal obligation (Decree No. 401/2007 Coll. on technical requirements).

The frequency of flue cleaning and inspection depends on the output and duration of operation:

Boiler up to 50 kW (the vast majority of domestic boilers): inspection and cleaning once a year
Boiler above 50 kW: twice a year

With modern plastic (PP or PPs) system flue pipes for low-temperature boilers, inspection is simpler – it's enough to check the sealing of individual components, the flue outlet (grille cleanliness, integrity), the slope of the condensate line (even though there's minimal condensate), and distances from combustible structures.

Checking and replacing water filters and conditioners

If your system has a magnetic sludge filter or return-line filter installed (which we recommend for every new build), it needs cleaning. Most magnetic sludge filters are cleaned easily – you turn it, remove the magnet, and rinse off the accumulated sludge. The frequency depends on the water quality in your area – sometimes once a year is enough, but with old radiators or steel piping, even once every 3 months.

If you have a water softener or a corrosion inhibitor doser (e.g. Fernox or Sentinel systems), regularly check the level of regeneration salt (for softeners) or the level of the dosed agent. Depletion of the corrosion inhibitor is treacherous – the system gives no warning, but without the inhibitor, the heat exchanger and steel piping start to corrode.

Tank and DHW preparation – energy efficiency in the service context

One of the less visible but economically important areas of service is verifying that the boiler is operating efficiently. A low-temperature boiler with a tank is less energy efficient than a condensing one – condensing boilers can utilize up to 109% of fuel calorific value, while low-temperature boilers range around 88–92%. We write about this topic in more detail in the article Condensing vs. low-temperature boiler with tank: which pays off more.

In the service context, this means that any deviation from the optimal condition shows up as increased consumption. A 1 mm thick layer of limescale increases energy consumption by 6–8%. Poorly set combustion (excess or lack of air) adds another 5–15% of unnecessary consumption. A clogged return-line filter extends the tank heating time. Adding it all up: a neglected boiler can consume 20–30% more natural gas than a properly maintained one.

That's why it pays to have a service protocol where the technician records the measured CO₂ level in the flue gases and the flue gas temperature before and after service. If you see gas consumption rising year after year without a change in weather or usage pattern, that's a clear sign that something isn't right.

Special situations: summer shutdown and restarting operation in winter

Summer shutdown (DHW-only operation)

In summer, most low-temperature boilers with a tank operate only for domestic hot water preparation – heating is switched off. This is a different kind of stress for the boiler: burners switch on and off in much shorter and more intense cycles (starts and stops), which is more taxing on the burner than a long continuous run. This is especially pronounced with boilers with a small tank (80 l or less), where the tank empties quickly and the boiler has to ignite frequently.

So the summer mode requires attention: let the boiler operate at a comfortable, not minimal, tank temperature setting. The temperature hysteresis should be sufficient (e.g. the tank heats from 50 °C to 60 °C, not from 55 °C to 58 °C), so that the ignition frequency remains reasonable.

Restarting heating operation in autumn

Before the first start of heating for the season, we recommend:

Visual check under the boiler (leaks)
Check system pressure
Bleed the radiators – air accumulates over summer
Test the thermostat/controller function – temperature settings
Observe the first ignition – the boiler should start smoothly without repeated attempts

You can find more on the technical requirements for the space where the boiler is located in the article Low-temperature boiler with tank – requirements for the boiler room, dimensions, and space.

Warranty, service records, and their importance

Most manufacturers (including Thermona) make warranty validity conditional on regular service by an authorized technician. The warranty on the boiler is usually 2 years from installation (statutory), but with product registration and a documented service record, the manufacturer extends it to 5 years. The DHW tank (shell) usually has a separate warranty – typically 5 years, and up to 10 years with proven care (anode replacement on schedule).

Practical conclusion: always get every service confirmed in writing – a service card, a digital record in the service organization's system. Without this, you'll be at a disadvantage in the event of a claim, even if you took exemplary care of the boiler. When buying used boilers (e.g. from a renovation), always ask for the service history – it's the first indicator of the appliance's condition.

Frequently Asked Questions (FAQ)

How often should the tank's magnesium anode be replaced?

Typically every 2 years with average water quality (hardness 2–3 mmol/l). With soft water (below 1 mmol/l) or aggressive water, it's advisable to check the anode every year, or switch to an active titanium-magnesium anode with impressed current. With hard water above 4 mmol/l, there's a risk of rapid scale incrustation on the anode – this should also be checked sooner. Best approach: at the first service, pull out the anode and assess its condition – then set an individual interval for your specific home based on that.

The boiler is discharging through the safety valve – what does this mean?

The most common cause is a depleted expansion vessel – a membrane losing pre-charge pressure. The pressure in the system has nowhere to expand during heating, rises above 3 bar, and the valve opens. Another possibility is that the valve itself is worn and leaking. In any case, the situation needs to be addressed, not just have a container placed under the valve – it's a symptom of a problem that won't resolve itself.

Can I set the tank to 45 °C to save gas?

Technically yes, but it's a health risk – at 45 °C, Legionella pneumophila bacteria actively multiply. A compromise solution is to set normal operation to 55 °C and automatically activate a thermal disinfection cycle once a week (heating to 70–75 °C, which kills the bacteria). Most modern controllers, including those in the Thermona THERM PRO 14 KX and THERM PRO 14 TKX boilers, support this function directly. Setting 45 °C without a disinfection cycle in an apartment building or larger property can have legal consequences.

Is service needed even if the boiler works without problems?

Yes, for three reasons: (1) Warranty terms – without annual service, the manufacturer's warranty terms typically become void. (2) Safety – a non-functional safety valve, a loose flue seal, or a neglected burner with high CO are safety risks you won't detect without a professional inspection. (3) Economics – a boiler that seems to work fine but has a clogged heat exchanger and poorly set combustion can consume 20% more gas without it being visible at first glance.

What to do when the tank gives off an unpleasant hydrogen sulfide (rotten egg) smell?

This smell arises from a reaction between sulfate-reducing bacteria and the magnesium anode under certain conditions – especially with prolonged water stagnation, low tank temperature, and the presence of sulfate ions in the water. Solution: heat the tank to 70 °C (thermal disinfection), flush it, check the anode (sometimes it's better to switch to a titanium-magnesium or aluminum anode). If the problem persists, consult a technician – sometimes the solution is installing sulfate bacteria reduction on the cold water inlet.

How do I know if there's limescale in the tank?

The most direct sign: the boiler needs longer to heat the tank than it did a few years ago, even though the temperature setting is the same. Another symptom: cracking and crackling in the tank during heating (limescale cracks due to thermal expansion). Visually, limescale can only be detected by opening the tank or with an endoscope through the service opening. A technician can estimate the degree of incrustation through indirect measurements too (tank heating time, temperature delta between the coil's inlet and outlet).

Conclusion: a service plan as an investment, not an expense

A low-temperature boiler with a built-in tank – whether it's the compact Thermona THERM PRO 14 TKX or the more powerful Thermona THERM 28 LXZ.A 5 – is an appliance with a potential lifespan of 15–20 years if properly cared for. Conversely, a neglected boiler can fail after 7–8 years, and replacing the entire appliance today costs several thousand euros.

Annual service costs (including the technician's labor, common spare parts, and any anode) range from 80–200 euros depending on the region and scope of service. Against this stands the extended lifespan of the appliance, preserved warranty, 15–25% savings on fuel, and above all the certainty that the boiler will reliably heat the house even in January at three in the morning.

If you're interested in related topics, we also recommend reading the articles Common failures of low-temperature boilers with tanks and how to recognize them and What DHW tank capacity do I need for a family home or apartment – both are available in this Knowledge Center.

Do you have a question on this topic?

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