Grundfos pump does not pump or makes noise - causes and solutions

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Grundfos pump does not pump or makes noise – causes and solutions

When a Grundfos circulation pump stops working properly – either it does not pump at all or it makes unusual noise – most people assume the pump is broken and needs to be replaced. From experience, however, we know that in most cases it is a much more trivial issue that an experienced DIYer can resolve without the help of a service technician. At the same time, there are situations where the fault is indeed serious and replacement is the only reasonable option. This article will help you distinguish what is happening in your case and find a solution – step by step, systematically, without unnecessarily spending money on new parts.

We discuss the topic of faults in more detail in the article Common Grundfos pump faults and how to eliminate them, but here we focus specifically on the two most common symptoms of problems: complete loss of flow (the pump does not pump) and mechanical or electrical noise (humming, whining, knocking). Both symptoms may have common causes, but they can also have completely different roots.

How a Grundfos pump actually works – a short basics

Before we start troubleshooting, it is good to understand what is happening inside the pump. Grundfos circulation pumps (e.g., the UPS, Magna, Alpha series) are so-called wet-rotor pumps – the motor's rotor and the impeller are in direct contact with the pumped liquid. The liquid also serves as a lubricant and coolant for the bearings. From this directly follows the most common cause of all problems: if there is not enough liquid, if the liquid is contaminated, or if it contains air, the pump operates under poor conditions and quickly wears out or stops working immediately.

The electromagnetic drive works by the alternating magnetic field of the stator rotating the permanently magnetized rotor. This simple mechanism is very reliable, but sensitive to any mechanical resistance (dirt, air, corrosion deposits) and power supply problems.

Pump does not pump – most common causes

1. Air in the system (air lock)

This is by far the most common reason why a Grundfos circulation pump suddenly stops circulating water in the system. Air gets into the system during filling, after repairs, due to a leak, or simply due to a drop in pressure in the expansion tank. An air lock can literally "blind" the pump – the impeller is rotating, but instead of water it is moving air, which is not enough to overcome the hydraulic resistance of the system.

How to recognize it: The pump is running (the motor is warm, you can feel vibrations), but the radiators are cold or heat does not reach the distant rooms. When the pump is filled, irregular gurgling or noise is usually audible.

Solution: Bleed the pump through the bleed screw on the pump body (most UPS and Magna models have it on the front of the motor housing). Use a flathead screwdriver or a special key, unscrew it by about half a turn and wait until liquid starts to flow without bubbles. In systems with multiple circuits and distributors, each circuit must be bled separately. A detailed procedure can be found in the article Installation of a Grundfos circulation pump step by step.

2. Blocked impeller – deposits and corrosion

In older heating systems (15 years or more), sludge, rust, and magnetite deposits accumulate in the liquid. These particles settle in the grooves of the impeller and on the rotor. After a long period of inactivity (summer break), the deposits can "stick" so that the pump does not have enough torque to start rotating – the motor hums, but the rotor is stationary.

How to recognize it: Characteristic noise – the motor hums evenly, but the pump is cold (no heat is generated by the friction of the rotating parts). In the case of the UPS series, you can look through the bleed hole and try to move the shaft with a long, thin object (e.g., a screwdriver).

Solution step by step:

Turn off the pump and close the valves on both sides.
Unscrew the bleed plug and try to manually turn the shaft through the hole – use a long flathead screwdriver or the hex key provided (usually 8 mm). Turn it several times to the left and to the right.
Re-tighten the plug, open the valves and turn on the pump.
If the rotor starts rotating, we recommend letting the pump run for 15–20 minutes and then performing bleeding.
If it still does not start, the pump must be disassembled, the impeller removed and cleaned mechanically.

For the future, the installation of a magnetic filter (magnetic sludge trap) before the pump will help. This will not only protect the pump, but also the boiler and radiators.

3. Wrong direction of rotation (in new installation or after replacement)

In three-phase pumps (larger types for commercial applications), it can happen that the direction of rotation is reversed – the phases are connected in the wrong order. The pump then "pushes" the water in the opposite direction, which is manifested by almost zero flow. This problem does not occur in single-phase circulation pumps for households (230 V, UPS series).

4. Capacitor failure (in older models)

Some older single-speed UPS models and similar ones use a capacitor for the starting torque. The capacitor can degrade or burn out. The motor then "whines" and does not start, or starts but has significantly reduced performance. Replacing the capacitor is cheap (2–8 €) and can be done by anyone with basic electrical knowledge.

5. Closed shut-off valve or clogged filter

A subtle cause, but surprisingly common during service interventions – someone forgot to open the ball valve on the inlet or outlet of the pump. Similarly, the problem can be a clogged screen filter (Y-filter) before the pump, which is full of dirt. The pump then operates, but has no source of liquid to draw from.

Pump is humming, buzzing or knocking – causes and diagnostics

Noise from the pump can have several different characteristics, and each of them points to a different cause. An experienced technician can estimate quite accurately where the problem is located at first hearing. Let's go through the different types of noise systematically.

Humming and buzzing – electromagnetic sound

Low, even humming at a frequency of 50 Hz or multiples (100 Hz, 150 Hz) is a typical manifestation of electromagnetic noise from the motor. This sound can be normal at certain speed settings, but if it appears suddenly or is more pronounced than usual, it indicates:

Unstable supply voltage – voltage fluctuations below 210 V or above 250 V cause the motor to operate outside the optimal point. Check the voltage with a multimeter directly at the pump terminals while it is running.
Damage to the stator winding – a partial short in the winding changes the electromagnetic field and produces a characteristic buzzing tone. This is a condition where the pump needs to be replaced or sent for professional repair.
Installation resonance – the pump itself may be in good condition, but its vibrations are transferred to the piping or supporting structures and amplified. The solution is flexible suspended piping, anti-vibration pads under the pump, or flexible hoses for the connection.

High-frequency whistling or hissing

This sound is most often caused by cavitation – a phenomenon in which the pressure of the liquid at the suction inlet drops so much that the liquid begins to boil locally and steam bubbles form. These bubbles implode rapidly when entering an area of higher pressure and cause micro-shock waves. Cavitation is destructive to the pump – it damages the surface of the impeller and seals.

Causes of cavitation:

Too low inlet pressure (underpressure at the suction inlet) – for example, when the pump is placed too high above the water level, or the inlet pipe is too long and narrow.
Too high liquid temperature at low operating pressures – leads to premature boiling at lower pressure.
Clogged filter on the suction side – increases underpressure.
The pump is operating outside its hydraulic point – for example, with fully closed thermostatic heads and a running pump without a differential pressure valve.

Solving cavitation requires a hydraulic analysis of the system and is not always a simple DIY fix. If you have a pressure gauge in the system, check whether the operating pressure is at least 1.0–1.5 bar. If you don't have a pressure gauge, it is worth adding one – it is an investment of around 5–15 € and significantly simplifies diagnostics.

Knocking and scraping – mechanical sounds

Knocking inside the pump usually means that something physically hits the impeller or rotor – for example, a piece of rusted pipe, a loose gasket, or a larger piece of dirt. In such a case, the pump should be turned off immediately, as further operation can cause serious mechanical damage to the impeller or grooves.

Scraping of bearings indicates dry running (lack of liquid) or corrosion. In wet-running pumps, dry bearings are usually the result of longer operation without liquid or air entering the bearing area. After refilling the liquid and bleeding the system, the sound may subside, but the bearing is likely prematurely worn.

Special cases: Grundfos submersible and solar pumps

UNILIFT KP – submersible pumps for water removal

UNILIFT series submersible pumps are structurally different from circulation pumps – they are centrifugal pumps designed for pumping clean or slightly contaminated water from pits, basements, wells, and retention tanks. Their faults therefore have a slightly different character.

GRUNDFOS UNILIFT KP 150 A1 is suitable for water levels from 0 cm (i.e., it can pump almost dry) and has a maximum head of 7 m. GRUNDFOS UNILIFT KP 250 A1 with a 10-meter cable has higher performance and a greater maximum head, covering more demanding applications.

Why UNILIFT is not pumping or humming:

Float (level switch) is stuck in the "off" position – the pump will not start even when the water level rises. The float must be checked and, if necessary, cleaned of dirt.
Float stuck in the "on" position – the pump runs even when the water is drained, causing dry running and damage to the seals. Result: humming, overheating, or possibly burning out the motor.
Clogged pump with dirt – when pumping contaminated water, larger solid particles can get trapped inside. The pump must be removed, disassembled, and cleaned.
Worn shaft seal – after several years of intensive use, the mechanical seal wears out, water enters the motor part and causes its failure.

The installation and wiring procedure for these models is described in detail in the article How to connect the Grundfos UNILIFT pump – installation procedure.

Grundfos Solar – specific faults of solar pumps

Solar pumps are designed for solar-thermal collectors, where a non-freezing mixture (glycol mixture) circulates between the collectors and the storage tank. GRUNDFOS Solar 25-120 and GRUNDFOS Solar 15-80 are typical representatives of this category – they differ in hydraulic performance and the distance of the connecting nozzles (130 mm vs. 180 mm).

Typical faults of solar pumps:

Steam block – during solar system stagnation (collectors are in the sun, but the pump is off), the temperature in the collector can exceed 150–180 °C. The glycol mixture evaporates and vaporizes into the expansion tank. When the pump is turned on again, it "sees" steam instead of liquid. Result: humming, zero flow, pump overheating. Solution: let the system cool down, check the volume and pressure in the expansion tank, and possibly refill the glycol mixture.
Degradation of glycol mixture – after 3–5 years of operation, the glycol mixture chemically degrades, pH decreases, and the mixture becomes corrosive. Corrosion deposits then clog the pump. The mixture must be regularly replaced and pH checked (optimum is 7–8).
Air after refilling the system – the same problem as with a circulation pump, but a bit more complicated, as solar systems are usually pressurized and bleeding requires a special procedure via a bleed valve on the manifold.

Further information about solar pumps can be found in the article Grundfos solar pumps – what they are used for and how they work and in the comparison Grundfos UPS vs. Grundfos solar pumps – which one is suitable for your system.

Grundfos UPS 25-30 pump – practical diagnostics of a common model

GRUNDFOS UPS 25-30 is one of the most commonly used three-speed circulation pumps in households in Slovakia. You will find it in boiler rooms of family homes, in floor heating systems, and in combined radiator + floor heating circuits. That is why it deserves special attention during diagnostics.

The model UPS 25-30 has three fixed speed levels (manually switched by a sliding switch on the motor housing), a maximum head of 3.0 m w.s., and a connection diameter of DN 25 (1 inch). It is not equipped with electronics for automatic control, which on one hand simplifies diagnostics, but on the other hand does not allow for automatic performance adjustment.

Diagnostics procedure for UPS 25-30:

Check the position of the sliding switch – during movement (e.g., during cleaning), it may accidentally shift to neutral (between the levels). In this position, the pump will not start.
Check the voltage at the terminals: it should be 220–240 V AC. If it is lower, the problem is on the electrical supply side.
Try manual rotor release through the air vent opening.
Check whether the capacitor (located in the motor housing) is undamaged – visually, a swollen or burned capacitor can be identified without measurement.
If the pump still does not work after these steps, it is likely that replacement is necessary – for units older than 10 years, it is not economically viable to spend more on repairs than the cost of a new unit.

Systematic diagnostics – proceed from simple to complex causes

From practical experience, we recommend always proceeding from the simplest and cheapest causes to the more complex ones. This order minimizes time and money loss:

Electrical: Is the pump powered? Does the indicator light work? Check the fuse (automatic circuit breaker for the pump), voltage at the terminals.
Valves: Are the ball valves on both sides of the pump open? Is the filter before the pump clean?
Air: Bleed the pump and the entire system. This is free and takes 5 minutes.
Mechanical blockage: Try manually turning the shaft through the air vent opening.
System pressure: Check the pressure gauges. Operating pressure should be 1.0–2.0 bar (depends on the building height).
Speed setting: Is the speed switch in the correct position? Try a higher level.
Capacitor (in older models): Visually check the condition of the capacitor and have it measured if necessary.
Bearings and mechanics: If previous steps yield no results, it is likely mechanical wear – bearings, impeller, shaft seal.
Motor (winding): Measuring the resistance of the winding with a multimeter – this requires a professional or an experienced DIYer.

When to replace the pump and when to repair it

This is a question where customers most often hesitate. From an economic perspective, a simple rule applies: if the repair costs exceed 40–50 % of the price of a new pump of equivalent performance, replacement is a more sensible choice. For standard circulation pumps in a family home (e.g., UPS 25-30 or equivalent), the replacement cost is relatively low, so a real repair is only justified in the case of capacitor replacement or bleeding.

Another reason for replacement instead of repair: an older model without electronic consumption control (fixed speed) may have a power consumption of 60–90 W, whereas a modern ECM motor (e.g., Grundfos Magna or Alpha series) with the same hydraulic performance consumes 5–20 W. Annual electricity savings can reach 30–60 € per year, which over a 10-year horizon easily pays for a new pump. How to choose the right Grundfos circulation pump for heating will be discussed in the article How to choose the right Grundfos circulation pump for heating.

Repairs make sense in the case of:

Air lock – free of charge, 5 minutes of work
Blocked impeller without damage – 30 minutes, possibly cleaning agent
Capacitor replacement – spare part 3–10 €, 1 hour of work
Cleaning the filter before the pump – free of charge, preventive maintenance

Replacement is more sensible in the case of:

Burned motor winding (burnt smell, visible charring)
Damaged impeller (cavitation damage, material wear)
Worn bearings in a model older than 8–10 years
Corrosion of the pump body with water leakage
Models for which spare parts are not available

Preventive maintenance – how to avoid failures

The vast majority of faults described in this article can be prevented by regular preventive maintenance. From experience, we know that customers who spend 15 minutes of attention on the pump once a year have far fewer problems and their equipment lasts significantly longer.

Once a year (ideally before the heating season, i.e. in September):

Visual inspection of the pump – signs of water leakage, corrosion, condition of cable insulation
Depressurization of the pump and main radiators
Check operating pressure (1.0–2.0 bar)
Check that the pump can be manually rotated via the air vent (preventive rotation after a long summer break)
Cleaning of the Y-filter on the suction side
Check the speed setting (for the UPS series) or the setting of the automatic curve (for the Magna/Alpha series)

Every 3–5 years:

Analysis of boiler water quality (pH, total hardness, chloride content) – the customer can send a sample to a laboratory for approx. 20–30 €
Replacement of glycol mixture in solar systems
Check of the membrane expansion tank (gas side overpressure: typically 0.5–0.8 bar in a cold system)

A detailed annual service schedule can be found in the article Maintenance and service of Grundfos pumps – what to check every year.

Most frequently asked questions (FAQ)

The pump is humming, but no water is flowing – what should I do first?

The first step is always to bleed the system. Unscrew the air vent on the motor housing (flat screwdriver, turn half a turn to the left) and wait until the liquid starts to flow without bubbles. If air keeps coming out and then returns – there is a lot of air in the system and you need to bleed the radiators as well. If no water is flowing at all, try gently turning the shaft through the same hole – it may be mechanically blocked by deposits.

The pump does not start after summer – is this normal?

Yes, very common. During the summer break, magnetite and carbonate particles settle on the rotor and impeller. The rotor is "stuck". The solution is simple: manually turn the shaft 3–5 times in both directions through the air vent, then screw the vent back and try to start the pump. In 8 out of 10 cases, this is sufficient.

The pump is making a whining noise – is it the bearings or cavitation?

The difference can be recognized as follows: cavitation is an irregular, buzzing or hissing sound that fluctuates with pressure changes. Bearings produce a regular, high-frequency whine that does not change with pressure, but may change with temperature (it gets worse when heated). Cavitation is solved hydraulically (increasing pressure, cleaning the filter, reducing speed), while bearings require either service intervention or pump replacement.

Can I repair a Grundfos pump myself?

It depends on the fault. Bleeding, filter cleaning, manual rotor release and capacitor replacement can be done by any skilled DIYer. Bearing or shaft seal replacement requires disassembling the pump and using precision tools – we recommend leaving this to a professional. Electrical diagnostics of the winding (measuring resistance, insulation resistance) is also not recommended for amateurs. Never disassemble the pump while it is under voltage.

How long should a Grundfos pump last?

With proper installation, regular maintenance and good boiler water quality, a Grundfos circulation pump has a lifespan of 15–20 years. From practice, we see models that are 12–18 years old and still working reliably. Factors that shorten the lifespan are: hard water without treatment (deposits), acidic or aggressive boiler water, long dry-running episodes, cavitation and strong system vibrations. On the contrary, modern ECM motors (Magna, Alpha series) should last even longer due to lower operating temperatures.

When is the right time to replace the pump preventively, even if it is still working?

Technically – when it reaches 15–18 years for fixed-speed models and 20+ years for ECM models. Practically – when your service technician measures the power consumption during boiler inspection and finds that the pump consumes significantly more than during the initial measurement, or when its power consumption is 3–5 times higher than that of a modern equivalent. In such a case, the replacement pays for itself in electricity savings within 3–5 years.

Conclusion: systematically, patiently, from simple to complex

A fault in a Grundfos pump – whether it is no pumping, humming, cavitation or mechanical noise – usually has a clearly identifiable and solvable cause. The key is a systematic approach: first electricity, then valves, then air, then mechanics. Do not immediately reach for a new pump, but also do not unnecessarily delay the replacement when the old unit is at the end of its life – modern variable-speed pumps save electricity so significantly that the replacement is definitely worth it.

If you are unsure during diagnosis or need help choosing a replacement pump, check out the full range at atria.sk – Grundfos, where you will find circulation and submersible pumps for home and commercial applications. And if you want to know more about performance and sizing, we recommend the article What Grundfos pump power do I need for my home? and Common questions about Grundfos pumps.

Do you have a question about this topic?

Are you unsure or dealing with a specific situation in your home? Write to us – we are happy to help.