Common Faults of Condensing Boilers with Instantaneous DHW Heating and Their Solutions
Why condensing boilers with instantaneous DHW heating fail differently from classic boilers
A condensing boiler with instantaneous domestic hot water (DHW) heating is a technically much more complex machine than it might seem at first glance. It combines two functional circuits – heating and sanitary – both of which must work reliably, quickly, and simultaneously. When you add condensing technology with a stainless steel heat exchanger, flue gas recirculation, burner modulation, and a whole range of sensors and electronics, you get a device that has excellent efficiency but also several potential points of failure.
From experience, most service calls for boilers of this category tend to repeat. Same error codes, same symptoms, same causes. If you know these patterns, in many cases you can identify the problem yourself and either fix it yourself (where regulations allow) or give the service technician an accurate description, which saves time and money.
In this article, we will systematically go through the most common faults, their symptoms, causes, and solutions. We will focus on boilers such as those found in the category condensing boilers with instantaneous DHW heating – i.e. combi boilers without a tank, where cold water is heated directly on demand via a plate or tube DHW heat exchanger.
Basic boiler architecture – what can go wrong and where
Before we move on to the faults, it's useful to understand the main components such a boiler contains and how they relate to each other. Instantaneous DHW heating works so that when the cold water tap is opened, a flow sensor (flowmeter) detects the flow, the boiler switches to DHW heating, and heat from the heating circuit is transferred to the sanitary water via a plate heat exchanger. The whole cycle takes a few seconds.
Key components where faults concentrate:
- DHW flow sensor (flowmeter) – triggers heating when water is drawn
- Three-way diverter valve – switches between heating and DHW heating
- Plate heat exchanger DHW – transfers heat to the sanitary water
- Condensing heat exchanger (primary) – heats the boiler water
- Circulation pump – circulates water in the heating circuit
- Expansion vessel – compensates for pressure changes
- Heating and DHW NTC sensor – measures temperatures
- Ionization electrode / ignition electrode – monitors the flame
- Fan + flue gas pressure sensor – flue gas and condensate discharge
Fault #1: The boiler does not heat DHW or heats only weakly
Symptoms
You open the hot water tap, the boiler does start up (you hear the fan, the burner), but the water is lukewarm or cold. Or the boiler doesn't react to opening the tap at all. Heating, meanwhile, works normally.
Most common causes
1. Dirty or non-functioning flow sensor (flowmeter). The flowmeter is a small turbine in the cold water inlet that generates an electrical pulse for the control unit when there is flow. If limescale or dirt builds up, the turbine gets stuck and the boiler "doesn't know" that water is flowing. Result: DHW heating does not start.
This is one of the most common faults we see in areas with hard water. In Slovakia, most areas have water hardness of 15–25 °dH, a value at which limescale builds up relatively quickly – especially in components with low flow and higher temperature.
2. Non-functioning three-way valve. If the three-way valve gets "stuck" in the heating position, hot water from the boiler never flows into the DHW plate heat exchanger. The valve can fail mechanically (worn sealing surface), electrically (non-functioning servo motor), or get stuck due to sludge.
3. Scaled DHW plate heat exchanger. With significant scaling of the plate heat exchanger, heat transfer decreases and pressure drop increases. This shows up as gradual cooling of the DHW – the boiler is firing, but the water gets progressively more lukewarm. In extreme cases, complete clogging can occur.
4. Low system pressure. Below 0.5–0.8 bar, most boilers automatically block both DHW heating and space heating. The pressure gauge on the boiler will reveal this immediately.
Solution
An experienced layperson can clean the flowmeter – close the valve, unscrew the flowmeter, flush the turbine, and reinstall it. The plate heat exchanger is descaled with citric acid in a pressurized circulation loop (chemical descaling) or replaced. The three-way valve usually requires a service technician, since replacing it requires draining the circuit.
Models such as Protherm Gepard Condens 18/25 MKV have a DHW plate heat exchanger designed so it can be removed without draining the entire boiler – which significantly shortens the descaling process.
Fault #2: Unstable DHW temperature – water alternating between hot and cold
Symptoms
The hot water fluctuates while showering – hot for a while, then cold, then hot again. Sometimes the boiler repeatedly turns on and off during DHW operation. Customers describe this as "cold clumps" while showering.
Causes and solutions
Sandwich effect of the three-way valve. If the three-way valve does not seal perfectly – i.e. it leaks slightly into the heating circuit during DHW heating – the temperature will be unstable. Solution: replace the seals or the entire valve.
The boiler's thermal safety cutout keeps activating. When the boiler overheats (for example due to a dirty primary heat exchanger or a non-functioning pump), the safety thermostat shuts off the burner. During DHW heating, this shows up as intermittent heating. The error code depends on the manufacturer (F.75, F.22, E5, etc.).
Low DHW flow (below 2 l/min). Most boilers in this category start at a minimum flow of 1.8–2.5 l/min. If the flow fluctuates (for example due to simultaneous draw at another tap), the boiler repeatedly turns on and off – so-called "cycling". This is not a fault of the boiler itself, but a problem with the house's hydraulics.
Real-world example: A customer in an apartment block complained about cold clumps during morning showers. Upon investigation, it turned out the neighbor upstairs was showering at the same time and the pressure in the pipes dropped below the flowmeter's trigger threshold. The solution was not in the boiler, but in replacing the pressure-equalizing insert in the distribution manifold.
Fault #3: The boiler fails to ignite, repeatedly locks out
Symptoms
The boiler attempts to ignite – you hear the igniter clicking, the fan starts up – but no flame appears. After two to three attempts, the boiler goes into lockout (error code on the display). Typical codes: F.28 (Vaillant), E0 or E2 (Protherm), A01, etc.
Causes
1. Dirty ionization electrode. The ionization electrode detects the presence of a flame by measuring ionization current. If it is covered with carbon deposits or oxide, it will not report a flame even when one is present. The boiler interprets this as "no flame" and locks out. A visual inspection of the electrode will reveal grey or black coatings.
2. Incorrect gas pressure. The nominal natural gas pressure upstream of the boiler should be 20 mbar (±2 mbar). If the pressure is low (for example due to simultaneous demand from multiple appliances in the house or issues at the gas meter), the boiler will not ignite reliably. Measurement with a pressure gauge is mandatory during a service inspection.
3. Non-functioning igniter or electrode cable. The ignition electrode erodes over time, and the sparking voltage decreases. Cracked insulation on the electrode cable causes spark leakage that fails to reach the ignition point.
4. Clogged air filter / blocked air supply. Boilers with a sealed combustion chamber (which is most of them today) are very sensitive to resistance in the intake duct. Bird nesting or a clogged fan wheel can prevent startup. The flue gas differential pressure sensor (pressure switch) detects the problem and blocks startup.
Solution
Cleaning the ionization electrode is a minor service task – the electrode is removed, gently sanded with 400-grit sandpaper, reinstalled, and tested. Gas pressure may only be measured by a certified gas technician. Replacing the electrode or igniter is a standard service job, usually costing 30–60 euros per part plus labor.
Fault #4: Drop in system pressure – the boiler shuts off, the pressure gauge shows below 0.8 bar
Symptoms
The pressure gauge on the boiler shows less than 0.8–1.0 bar after cooling down. You have to regularly (every 2–4 weeks) top up water in the system via the filling valve. The boiler shuts off during heating due to excessive pressure rise (the safety valve drips) or, conversely, the pressure drops below the minimum.
Causes
1. Discharged or non-functioning expansion vessel. The expansion vessel has a membrane that separates the air (on the Schrader valve side) from the boiler water. If the air leaks out (defective membrane or valve), the vessel fills with water and loses its dampening function. System pressure then rises quickly and the safety valve drips.
Checking is simple: release the pressure on the boiler down to 0 bar, then press the Schrader valve on the vessel – if water comes out (not air), the membrane is damaged and the vessel must be replaced or recharged with air, or replaced entirely.
2. Expansion vessel too small for the system volume. The vessel built into the boiler as standard has a capacity of 8–12 liters. For larger systems (long pipe runs, more radiators, underfloor heating), this isn't enough. An external expansion vessel must be added.
3. Actual water leak from the system. A slow leak from a joint, radiator valve, or safety valve (if it drips at pressures above 3 bar). The water evaporates, and an unnoticed leak can be the cause of a repeated pressure drop.
Solution
Adding air to the expansion vessel via the Schrader valve (like a bicycle tire valve) can be done by a layperson using a common pump. The air pressure should be 0.5–0.75 bar with the system depressurized. If the vessel takes air pressure but quickly loses it, the membrane is punctured and the vessel needs replacing. An external vessel is added to the return pipe and must be sized according to the system volume.
Fault #5: Noisy operation – knocking, bubbling, whistling
Symptoms and causes
Knocking or cracking during heating – the so-called "kettling" is caused by water boiling on the surface of the heat exchanger where limescale has built up. Limescale has very low thermal conductivity (about 0.6 W/mK compared to 15–20 W/mK for steel), so the areas under the deposit locally overheat, the water starts to boil, and bubbles form. The boiler sounds like a boiling kettle.
Bubbling in radiators – air in the heating system. Water in newly filled or refilled systems contains dissolved air that is released upon heating. Solution: bleed the radiators, check the function of the automatic air vent on the boiler.
Whistling during DHW operation – high inlet water pressure (above 6 bar) or cavitation in the pump due to insufficient filling. We recommend installing a pressure reducing valve on the inlet water, set to 3.0–3.5 bar.
Pump rattling – either air in the pump (bleed via the screw on the pump head), or the pump bearings are at the end of their service life (typically after 8–15 years of operation). Replacing the circulation pump is a routine service job.
Fault #6: Condensate does not drain – the boiler locks out, alarm
Causes
Condensing boilers produce condensate from the water vapor in the flue gases. At a temperature gradient of 80/60 °C there is little of it, but under proper condensing operation (60/40 °C) a boiler with 20 kW output can produce 1–3 liters of condensate per hour. Condensate must drain freely into the sewer.
Problems occur when:
- The drain pipe freezes (installed in a freezing environment or without insulation)
- The condensate trap is clogged with dirt or has dried out during a longer period without operation
- The condensate level float sensor is stuck or dirty
Some boilers (e.g. Vaillant VUW 26CS/1-5 ecoTEC plus IoniDetect) have an internal condensate trap with easy access for cleaning. It's good practice to flush the drain trap once a year during the service inspection.
Solution
Cleaning the trap is straightforward – remove the cover, take out the float chamber, and rinse with water. If the pipe is frozen, warm it up gently (with warm water, NOT an open flame!). If the trap keeps drying out repeatedly and the condensate doesn't drain freely, check the slope of the drain pipe (minimum gradient of 2%).
Fault #7: Error code – what the Vaillant and Protherm displays reveal
Error codes are only the first step in diagnostics – the same code can have several causes. For example, code F.75 on a Vaillant appears with a non-functioning pump, but also with a faulty water pressure sensor, or even with air trapped in the primary heat exchanger. A good technician interprets the code in the context of other symptoms, not just by looking it up in a table.
Models such as Vaillant VUW 236/5-3 ecoTEC pro have access to a service menu where the technician can view the fault history, current sensor parameters, and the number of lockout events – which significantly speeds up diagnostics.
Fault #8: The boiler heats but does not bring DHW to the required temperature – power limitation
Instantaneous DHW heating in a condensing boiler is directly dependent on the available heat output and the cold water flow rate. If a customer wants 45 °C at the outlet with a flow of 10 l/min (a typical shower + sink used simultaneously), they need at least the following from the boiler:
P = Q × c × ΔT = (10/60) × 4186 × (45-10) ≈ 24,400 W ≈ 24.4 kW
A boiler with a nominal DHW output of 18 kW (for example Protherm Puma Condens 18/24 MKV) is physically not enough for such a draw – the temperature will drop. This is not a fault, but a matter of sizing. This is covered in more detail in the topic What condensing boiler output with DHW do I need for my house in our Knowledge Center.
A genuine fault is a situation where a boiler with 24–26 kW DHW output fails to heat even at low flow. Causes:
- Dirty DHW plate heat exchanger (scaling)
- The control unit limits output due to a fault (e.g. dirty flue gas sensor)
- Incorrectly set maximum DHW temperature in the boiler parameters (sometimes set to 40 °C instead of 55–60 °C)
- Low gas pressure – the boiler cannot reach full output
Fault #9: Freezing of the condensate trap or the outdoor coaxial flue
This is a seasonal fault, but in practice we see it every winter. The coaxial flue pipe (air/flue gas) exiting through the wall sometimes forms condensate droplets which, at temperatures below –5 °C, freeze and block the flue gas outlet. The boiler detects overpressure (pressure switch) and locks out.
Boilers with longer horizontal runs and a north-facing orientation are more prone to this. Long-term solution: correct outward slope of the pipe (min. 2–3%), or an insulated route. Immediate solution: carefully de-ice the outdoor outlet (with warm water, NOT an open flame).
The condensate trap inside the boiler can only freeze if the boiler room is unheated and the temperature drops below 0 °C. In that case, the correct solution is to ensure minimal heating of the boiler room space (min. +5 °C) or install heating tape on the condensate drain pipe.
Preventive maintenance – how to avoid most faults
Long experience shows a simple statistic: boilers that receive a regular annual service inspection break down rarely, and the faults that do occur are cheaper to fix. Boilers without a service history usually come in for repair with a combination of several problems at once – a scaled heat exchanger, a discharged expansion vessel, a dirty electrode, a clogged trap.
An annual inspection should include:
- Checking and, if necessary, cleaning the ionization and ignition electrodes
- Measuring gas pressure and analyzing flue gases (CO, CO₂, efficiency)
- Checking and cleaning the condensate trap
- Checking the pressure in the expansion vessel
- Visual inspection of the DHW plate heat exchanger (flow, fouling)
- Checking system water pressure and topping up
- Checking the safety valve (not dripping, not seized)
- Checking the DHW flowmeter
- Updating parameters according to the operating season
You can find more on proper regular maintenance in the topic Maintenance and servicing of a condensing boiler with DHW: what and how often to check in this Knowledge Center.
When to call a service technician and when you can handle it yourself
It's important to know the line between what a layperson may and can do themselves, and what belongs exclusively to a certified technician:
| Task | Layperson | Certified technician |
|---|---|---|
| Topping up system pressure (filling valve) | ✔ | |
| Bleeding radiators | ✔ | |
| Checking and resetting an error code (without gas) | ✔ | |
| Cleaning the condensate trap | ✔* | |
| Topping up air pressure in the expansion vessel | ✔* | |
| Cleaning the ionization electrode | ✔ | |
| Measuring and adjusting gas pressure | ✔ | |
| Replacing the three-way valve, pump, heat exchanger | ✔ | |
| Flue gas analysis and burner adjustment | ✔ | |
| Descaling the heat exchanger (chemical) | ✔ |
* Provided the procedure and safety rules are followed. Always turn off the boiler and close the main gas valve before any intervention.
Protherm Panther Condens – specific faults of this series
Protherm Panther Condens 20/26 KKV is a more powerful model in the Protherm condensing combi boiler range, commonly found in family homes with higher DHW consumption. From experience, we see several recurring issues in this series:
Stuck three-way valve (code P.07 / E7). After a longer summer break (the boiler was only heating DHW, not space heating), the three-way valve can get stuck in the DHW position. Solution: a service technician performs a manual cleaning or replaces the valve actuator. As a preventive measure, we recommend running a short heating cycle once a month in summer (the "chimney draft" function or a manual test).
Excessive fluctuation in system pressure. With underfloor heating with a larger water volume (20 liters or more), the original expansion vessel is not sufficient. Adding an external vessel of 18–25 liters on the return pipe usually solves the problem.
Frequently Asked Questions (FAQ)
The boiler displays an error code and neither heating nor DHW work – what should I do first?
The first step is to check the pressure on the gauge (it should be 1.0–1.5 bar with a cold system). If it's low, top up water via the filling valve. Then reset the code according to the manual (usually by holding the reset button for 3 seconds). If the code reappears within an hour, call a service technician – resetting without eliminating the cause only postpones the fault repeatedly.
DHW is lukewarm in the morning when I'm the first to use it – during the day it's fine. Why?
This is a classic case of heat loss in the pipework – the water in the pipe cooled down overnight and in the morning you're waiting for the boiler to deliver new hot water. The solution is a DHW circulation pump or a circulation loop with a timer switch. This is an installation solution, not a boiler fault. More on this topic is covered in the article Instantaneous DHW heating in a condensing boiler: how it works and what affects comfort.
Why does the safety valve drip even though the pressure gauge shows normal pressure?
The safety valve (usually set to 3 bar) can open for two reasons: either the pressure really does briefly rise above 3 bar during heating (non-functioning expansion vessel), or the valve itself is worn and does not seal properly even at lower pressure. An old safety valve becomes contaminated with sludge after dripping and stops sealing – at that point it drips even at normal pressure. Replacing the safety valve costs less than 20 euros per part and is common after 8–10 years of operation.
The boiler works, but it's getting louder than it used to be. Is this serious?
Noise that gradually increases over the years is almost always a symptom of scaling in the primary or secondary heat exchanger. "Kettling" (crackling sounds) isn't immediately dangerous, but it reduces efficiency and stresses the heat transfer surfaces. If detected early, chemical descaling is enough; in advanced cases, the heat exchanger needs to be replaced, which is a more expensive repair. Regular cleaning before winter is a much cheaper alternative.
How long does a DHW plate heat exchanger last and when does it need replacing?
Lifespan depends mainly on water hardness. At a hardness above 20 °dH without softening, the heat exchanger can clog up in as little as 5–8 years. With soft water (below 10 °dH) or an installed water softener, it can last 15–20 years. A signal for replacement is a significant drop in DHW temperature at an unchanged flow rate, or unsuccessful chemical descaling (the heat exchanger has corroded too much). The cost of replacing a plate heat exchanger is 80–200 euros per part plus labor.
Can I use chemical descaling agents in the heating circuit myself?
Chemical descaling of the primary circuit (with citric acid or special products) is not strictly forbidden for a layperson, but it requires precise adherence to the procedure – correct dilution, correct exposure time, and thorough rinsing. An incorrect procedure can damage seals or, with leftover acid, accelerate corrosion even further. For the DHW heat exchanger, we recommend leaving this task to a certified technician, since an unsuitable chemical product could contaminate the drinking water.
Conclusion: Boiler faults can be prevented and handled
Condensing boilers with instantaneous DHW heating are reliable devices – if properly cared for. Most of the faults described here are not the result of a poor product, but a combination of hard water, neglected maintenance, and a lack of preventive checks. From experience, we know that a customer who calls a service technician once a year and invests 80–120 euros in an inspection saves, on average, several times more on repairs and part replacements.
If you are considering a new boiler and want to avoid these issues from the start, also read the topics How to choose a condensing boiler with instantaneous DHW heating: what to focus on and Protherm vs. Vaillant condensing boilers with DHW: comparison of models and parameters in this Knowledge Center – and if you have any questions, we're here for you.
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