Common Issues with DAB Circulation Pumps and Their Solutions

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Common DAB circulation pump failures and their solutions – a complete guide for households and plumbers

The circulation pump is the heart of every heating system. When it works flawlessly, no one notices it – but when it breaks down, the whole house loses heat, usually exactly when it's cold outside. DAB pumps are among the most reliable on the Slovak market, but even they are not immune to failures. Over years of experience with the installation and servicing of heating systems, I repeatedly encounter the same problems – and most of them can be resolved without calling a service technician, if you know where to look.

This article thoroughly covers the most common DAB pump failures, their causes, diagnostics, and specific solutions. You will also find practical tips from real customer cases, which will help you avoid recurring problems.

Why DAB circulation pumps fail – a basic look at the construction

Before we get into specific failures, it is important to understand how these pumps work and what makes them prone to certain types of problems. Modern DAB circulation pumps – for example, the EVOSTA2 or EVOTRON series – are electronically controlled devices with a wet-rotor motor, which means that the rotor and bearings are directly cooled and lubricated by the circulating liquid. They have no mechanical seals between the rotor and the hydraulic part.

This design brings advantages (quiet operation, long service life, low consumption), but also specific weak points: the quality of the liquid in the system directly affects the pump's lifespan. Contaminated water, air in the system, pipe corrosion, or unsuitable pH – all of these will eventually show up on the pump.

The DAB.EVOTRON 40/130 series belongs to class A electronically commutated pumps, which means they contain advanced electronics for speed regulation. This electronics is equally sensitive to voltage fluctuations, humidity, and mechanical shocks, which are further potential sources of failures.

Failure No. 1 – The pump does not work, makes no sound

Symptoms and diagnosis

This is one of the most common situations I encounter during service calls: the customer calls saying the heating is not working, the radiators are cold, but the boiler is burning. When you approach the pump, it is silent and the display (if it has one) does not light up or shows an error code.

The first step is always to check the power supply. It may sound trivial, but I have encountered cases where the problem was a tripped circuit breaker or a loose terminal in the distribution board. Check the following:

Voltage at the pump terminals (230 V AC, tolerance ±10 %)
Status of the fuse in the electrical box
Contacts in the pump terminal block – humidity in the boiler room can cause oxidation
External thermal fuse (some older installations have one)

Locked rotor – the most common cause

If the power supply is fine, but the pump does not respond, it is almost certainly a locked rotor. The mechanism is as follows: when the pump sits for a long time (for example during the summer, when heating is not used), deposits from the water – limescale and corrosive – settle on the rotor and "glue" it. When the heating season starts, the motor does not have enough torque to unlock the rotor, and the overheat protection disconnects it.

The solution is to mechanically unlock it via the unlocking screw, which you will find on the motor housing. It is usually a plastic or brass screw with a slot for a flathead screwdriver, recessed in the center of the motor cover. Procedure:

Turn off the pump from the power supply
Place a flathead screwdriver into the slot and turn left, then right – you will feel the rotor moving
Repeat several times until the rotor rotates freely
Replace the screw, restore the power

Warning: when opening the unlocking screw, some water may spill out – have a cloth ready. Also, never unlock the pump while the heat transfer fluid is hot – risk of burns.

Burnt electronics

If the rotor rotates freely, but the pump still does not start, the problem is likely in the control electronics (PCB). This is a typical consequence of a voltage surge during a storm, or long-term power supply from an unstable grid. In the case of modern EVOSTA2 or EVOTRON pumps, the entire PCB module is usually not repairable on site – the pump must be taken to an authorized service or replaced.

Failure No. 2 – The pump is running, but it is not pumping or has low pressure

Air in the system

Air is the number one enemy of hydraulic systems. When there is an air bubble in the pump, the impeller is spinning in vain – the impeller cannot transfer energy to air as efficiently as to liquid. This is manifested by characteristic noise or whining, cold radiators, and when touching the pump, you feel more vibration than usual.

Deairing the pump: similar to unlocking the rotor, DAB pumps have a deairing hole (sometimes identical to the unlocking screw). When deairing, let the pump run at maximum speed, slightly loosen the deairing screw – when water starts to flow without bubbles, tighten it. Do not forget to deair the radiators and check the expansion tank for correct pressure (0.8–1.2 bar in a cold state for most homes).

Clogged filters and dirt in the impeller

An older system, where corrosion inhibitors were never added, may contain rust deposits, pieces of sealing material, or even scale. These impurities settle in the pump's impeller and reduce its flow. A characteristic symptom is a gradual performance drop during the season, not a sudden failure.

Products such as KIT with strainer DAB.EVOSTA2 40-70/180 – FILTERBALL were created precisely for this type of problem, where the pump is delivered together with a strainer and a filter directly at the inlet. If the customer, whose clogged pump I once replaced, had had such a kit from the beginning, he would have saved several years of unnecessary problems.

If you do not have a filter, when disassembling the pump, check the condition of the impeller – with heavy clogging, a narrowing of the flow cross-section is clearly visible. Flush the impeller with pressurized water or use a cleaning agent (citric acid, never hydrochloric acid). We will discuss the topic of filters in more detail in the article "DAB Pump with Filter or Without – Is the Filterball Kit Worth It?"

Failure No. 3 – The pump is too noisy

Types of noise and what they indicate

Noise from the pump is not one and the same – the type of sound says a lot about the cause. From practice, I distinguish these categories:

Bubbling, whirring sound – almost always air in the system or cavitation (see below)
High-frequency squeaking – problem with the impeller (damage, foreign object) or with the bearings
Low-frequency rumbling, vibrations – the pump is not properly mounted, or it is operating at an inappropriate performance setting
Irregular clicking – an air bubble passing through the impeller, or a foreign object in the hydraulics
Rumbling from the piping, not from the pump itself – the pump is running at too high a speed, the piping is undersized, or the thermostatic heads are too closed

Cavitation – the silent destroyer of pumps

Cavitation occurs when the pressure of the liquid at the pump's inlet drops below the saturated vapor pressure at the given temperature. Microbubbles form, which implode with enormous force directly on the impeller and the casing. In practice, they are caused by:

Too low a pressure in the system (below 0.8 bar)
Too high an inlet water temperature in a low-pressure system
A clogged filter or a partially closed ball valve before the pump
The pump installed in the wrong place in the system (wrong installation position)

Cavitation is indicated by a characteristic crackling sound (as if you were boiling rice in the pump) and leads to long-term erosive damage to the impeller and casing. If ignored, the pump will be destroyed within a few months.

Solution: increase the filling pressure of the system, check and open all closing valves at the inlet, clean the filter, and verify that the pump is not set to too high a speed in relation to the pipe dimensions.

Noise from improper mounting

Another common source of noise: the pump transfers vibrations to the piping if rubber seals in the flange are not used correctly and if the piping is rigid without compensators. Check whether the seals in the flange are complete and not cracked, and whether the piping is under mechanical stress (this can happen after improper welding or after freezing).

Failure No. 4 – The pump is running, but heating is uneven

Uneven radiator heating

The customer complains that radiators in some rooms are hot, while others are barely warm. The pump is running, and the pressure appears to be in order. This is a classic case of an unbalanced hydraulic system – but the pump itself can also be the cause.

Try the following step by step:

Check the pump speed setting – at minimum speed, it may not have sufficient differential pressure for distant radiators
Verify that the pressure difference regulator (if installed) is not set too tightly
Check the closing valves on the manifold – one partially closed valve will absorb most of the flow
The customer's problem may also be technical – air trapped in a specific branch of the system

Incorrect setting of the operating mode

Electronic DAB EVOSTA2 and EVOTRON pumps offer several operating modes: constant pressure (ΔP-c), proportional pressure (ΔP-v), and constant speed (n). Many installations are set to the wrong mode, which causes problems precisely under partial load (when some thermostatic heads are closed).

For most apartment buildings and single-family homes with thermostatic heads, the optimal mode is ΔP-v (proportional pressure) – the pump automatically reduces the generated pressure as the flow decreases, thus preventing noise problems and overheating. We discuss this topic in more detail in the article "DAB EVOTRON vs EVOSTA2 – Which Pump to Choose."

Failure No. 5 – Leaking pump or flange

Where water leaks most commonly occur

A leak is a problem that should not be delayed. Even a small water leak can damage pipe insulation, wood, plaster, and may trigger corrosion processes. From experience, I know that most leaks on DAB pumps occur at these locations:

Flange between the pump and the piping – most commonly during installation without proper sealing of the threaded connection (hemp fiber with paste, or PTFE tape on a suitable thread)
Sealing ring (O-ring) between the motor and the hydraulics – the O-ring wears out after repeated pump disassemblies
Relief / venting screws – due to insufficient tightening after service
Crack in the pump body – during freezing, if the system was empty and water in the pump froze

How to fix a leak in the gland

If the gland is leaking from the threaded connection, you need to depressurize the system (turn off the pump, close the valves), disassemble the gland, and reseal the thread. For R 1½" threads (a common size for DN32 pumps), I recommend a combination of hemp fiber (50–70 cm) and sealing compound. PTFE tape alone on the external threads of pipe fittings is not ideal – at higher pressures, it may "slip".

That is precisely why complete sets with a gland were created. For example, the KIT with gland DAB.EVOSTA2 40-70/180 includes a gland compatible with the given pump, so you don't have to look for the correct dimension – everything fits together and the risk of leakage is minimized.

Failure No. 6 – Error codes on the pump display

Modern DAB pumps from the EVOSTA2 and EVOTRON series have built-in diagnostics that display error codes. This is a major advantage over older models without displays – you don't have to guess what is happening. Here are the most common codes and their interpretation:

Code / symbol	Meaning	Solution
E1 / blinking motor	Blocked rotor	Unlocking screw, check for dirt
E2 / thermometer	Motor temperature too high	Check flow, temperature of incoming water
E3 / wave	Power failure or poor power quality	Check the power supply, consider a backup power source
E4 / sensor	Internal sensor failure	Service intervention, replacement of electronics
Blinking display without code	Unstable power supply / power line noise	UPS / backup power source, check grounding

If code E3 or similar appears repeatedly, especially during storms or when other appliances are switched on simultaneously, the solution is a backup power source. We have covered this topic in a separate article: Backup Power Supply for DAB Pump – When and Why to Use It. A practical solution for such cases is also KIT with gland DAB.EVOSTA2 40-70/180 + backup power source, where the backup power source is part of the package.

Failure No. 7 – Pump overheating

Wet-rotor pumps are warm during operation – the body temperature during normal operation can be 40–60 °C, which is normal. The problem arises when the temperature significantly exceeds these values, the motor reaches the protection limit, or code E2 appears repeatedly.

Causes of overheating:

No or insufficient flow – the pump is running dry, the liquid does not cool it. Check whether all thermostatic heads are closed (typical in summer when heating is not operating, but the pump is running).
Too high temperature of the incoming liquid – at temperatures above 90 °C (boiler without regulation), thermal protection can be triggered.
Clogged impeller – dirt increases friction and thus temperature.
Long operation without load – some older installations run the pump all summer long on empty.

Solution: always ensure that there is at least a minimal flow when the pump is running – for example, leave one radiator open or install a bypass (recirculation loop). The temperature of the liquid entering the pump should not exceed 85 °C for a long time.

Failure No. 8 – Pump is running, but the boiler is switching off due to a fault

This is a tricky situation: the pump appears to be functioning, but the boiler throws an error related to flow or overheating of the liquid. The problem may be in the incorrect orientation of the pump, in the pump's power setting being too high (generating excessive differential pressure, which activates the safety valve on the boiler), or in hydraulic separation (the pump is "fighting" with another pump in the system).

A specific case from practice: a customer had a boiler with an internal bypass and an external DAB VA35/130 pump installed. Both pumps were "pulling" the liquid in different directions, causing unstable pressure and the boiler was shutting down. The solution was hydraulic separation using a hydraulic separator, or deactivating the boiler's internal pump.

For simple single-circuit systems, DAB VA35/130 is a solid choice with manual speed adjustment, where there is minimal electronics that could "interfere" with communication with the boiler.

Preventive maintenance – how to avoid 80% of faults

From years of experience, I know that most DAB pump faults can be prevented by regular simple maintenance. It is nothing demanding – it takes about 30 minutes once a year, ideally before the start of the heating season.

Annual check before the heating season

Check the system pressure – in a cold state it should be 1.0–1.5 bar (depends on the building height; add 0.1 bar for each meter above the pump)
Deair the system – start at the pump, continue with the pipes, finish with the radiators
Check and clean the filter (if installed) – a dirty filter can reduce performance by up to 30%
Manual rotor unlocking – a short turn of the unlocking screw once a year prevents jamming after the summer
Check the tightness of the flange – visually inspect the flange, or wipe it with a suitable cloth
Check the expansion vessel – the membrane pre-tension should be 0.5 bar below the system filling pressure
Corrosion inhibitor and pH – the system should have a pH of 7.5–9.5; use test strips

A detailed procedure can be found in the article Cleaning and Maintenance of DAB Pump – How to Extend Its Lifespan.

When is the pump really due for replacement and when is it repairable?

This question comes up with every service visit to a customer. A general rule is: if the pump is older than 10–12 years and the fault affects the mechanical or electronic part, replacement is more economical than repair. Spare parts are expensive, and with an old pump in a poor system condition, another breakdown will appear relatively soon.

On the other hand, if the pump is young (under 5 years) and the fault is clearly caused by an external factor (voltage surge, rotor jammed after summer, dirty filter), it is worth solving it through service or by using the warranty.

When replacing, do not forget about hydraulic compatibility: mounting length dimensions (130 mm, 180 mm...), connection diameter (DN 25, DN 32...) and thread type (R 1", R 1¼", R 1½"). More on how to choose the right DAB pump for heating can be found in the article How to Choose a DAB Circulation Pump for Heating.

Backup power supply as protection against failure during power outages

A special category of "failure" is the situation when the pump is working perfectly, but the power goes out – during a storm, due to a line failure, or in areas with an unstable grid. In such a case, the boiler stops circulating the fluid, overheats, triggers an emergency shutdown, and the house is left without heat. In freezing weather, this can lead to pipe freezing.

A backup power supply (UPS) for a DAB pump solves this problem: in case of power failure, it automatically powers the pump from the battery. Most backup power supplies for pumps can provide 4–8 hours of operation, which is sufficient in practice for most outages. A comprehensive "all-in-one" solution is offered by SET DAB.EVOSTA2 40-70/180 + backup power supply.

Most frequently asked questions (FAQ)

Why does the DAB pump make a bubbling sound right after heating is turned on?

This is almost always air in the system accumulated during the summer break. Right after starting the heating system, it contains air that has entered the pipes during the period of non-use. The solution is thorough deairing: let the pump run at maximum speed and gradually deair all the bleed points in the system – starting at the pump and moving toward the radiators. During the first hours of operation, deairing may need to be repeated.

The pump starts and then stops after a while – what does this mean?

This behavior (so-called cycling) usually indicates that the pump is reacting to thermal protection or an error code (most commonly a jammed rotor or overheating). Check the unlocking screw – if the rotor does not easily turn freely, it is jammed. Also check whether there is enough fluid in the system to cool the motor. If the pump cycles even after unlocking, the problem may be faulty electronics.

Is it normal that the DAB pump is warm to the touch?

Yes, this is normal. A wet-rotor pump transfers heat from the system through the motor body, so a temperature of 40–65 °C on the pump surface is standard. If the pump is uncomfortably hot to the touch (over 70–75 °C), check the flow in the system – there may be a dirty filter or all thermostatic heads are closed. High surface temperature without flow can damage the electronics.

The pump is not leaking, but I see moisture around the flange – do I need to replace it entirely?

Not necessarily. Most minor leaks at the flange can be repaired without replacing the pump. Bleed the system, remove the flange and replace the gasket or reseal the thread. If the leak is directly from the pump body (crack, damaged O-ring between the motor and the hydraulic part), the situation is more serious – in modern pumps, the O-ring is replaceable, a crack in the pump body requires replacement. Never ignore a leak – even a small amount of moisture can cause corrosion damage to the pipes within a few months.

The DAB EVOSTA2 pump is displaying a code – can I reset it myself?

Most codes (E1, E2) can be reset by pressing the MODE button for a long time (see the specific procedure in the manual for the given model – usually 5 seconds). It is important, however, to remove the cause of the fault before resetting: unlock the rotor, find out what caused the overheating, etc. Resetting without removing the cause will lead to repeated faults and can damage the electronics. If the code returns even after removing the obvious causes, contact an authorized DAB service.

Is it worth buying a DAB filterball set right at installation?

Yes, and very much so. In practice, I have seen many pumps that could have lasted another 5 years, but impurities in the system destroyed them prematurely. The filter captures rust particles, pieces of gaskets, and scale before they reach the coil and impeller. This is especially important in older installations with steel pipes. The price difference between a set with a filter and one without is minimal compared to the cost of replacing the pump after an early failure.

Conclusion – key recommendations in short

Most faults of DAB circulation pumps have a common denominator: neglect of water quality in the system and lack of regular preventive maintenance. Air, impurities, inappropriate pH and poor pH are enemies that work quietly for years until the pump stops working – usually at the worst possible weather.

If you take only three things from this article, let them be these: once a year deair the system and check the pressure, install a filter before the pump, and if you live in an area with power outages, consider a backup power supply. These three steps eliminate the vast majority of faults I encounter during service visits.

For those looking for a comprehensive solution when replacing an old pump – take a look at our sets that include the pump, flange, possibly a filter and a backup power supply in one package. You will save time on ordering, installation and in case of service, since all parts are designed for mutual compatibility.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.