Common pump faults Grundfos and how to fix them
Common pump failures of Grundfos and how to eliminate them
Grundfos pumps are among the most reliable devices you can find in a household or industrial operation. Despite this – no technology is indestructible. After years of experience in servicing and replacing various types and capacities of pumps, we know that the vast majority of failures have several recurring causes. The good news is that many of them can be handled by a skilled DIY enthusiast, provided they know what to look for and where to look. This article will guide you through the most common problems – from noise and pump stoppage to electrical and sealing issues – and show you specific steps on how to resolve or at least properly diagnose the situation before the service technician arrives.
If you are new to this topic, we recommend reading the article How to choose the right Grundfos circulation pump for heating, or Maintenance and service of Grundfos pumps – what to check every year, where you will find the basics of caring for these devices. Here we will focus exclusively on failures, their causes and solutions.
Why it is important to diagnose the failure as soon as possible
Many homeowners underestimate the initial symptoms – a faint humming, slight temperature fluctuations in the distribution system, or a slight leak around the pump. These symptoms are usually not sudden – they develop over weeks, sometimes even months. The problem is that an ignored failure typically does not stop on its own: cavitation, which today causes only noise, tomorrow will destroy the impeller; a small leak around the seal can result in flooding of the technical room in a month; a blocked pump that does not start in winter can cause freezing of the distribution system and damage worth thousands of euros.
Therefore, a simple rule applies: as soon as the pump shows any non-standard behavior, it is worth paying attention to it immediately many times more than waiting for the problem to "solve itself".
Pump does not start or stops immediately
This is probably the most common reason why people call service companies. Winter comes, you turn on the heating and the pump – nothing. Or it starts for a few seconds and turns off. Here is the procedure used in practice:
1. Checking the power supply and electrical circuit
The first step is always electrical. Check whether the pump has voltage – either with a multimeter on the terminals, or check the circuit breaker in the distribution board. Grundfos circulation pumps (e.g. type UPS) operate at 230 V AC, 50 Hz. If the circuit breaker has tripped, reset it – but if it trips again immediately, it is a sign of a short circuit or a fault in the motor winding. In such a case, do not try to turn it on repeatedly, as you may cause further damage.
For pumps operating in solar systems (e.g. GRUNDFOS Solar 15-80 or GRUNDFOS Solar 25-120), it is also necessary to check whether the solar regulator is supplying the required voltage and whether the photovoltaic panels are generating sufficient power (e.g. on cloudy days or with dirty panels, the power may be insufficient).
2. Jammed (blocked) impeller
This is a classic problem especially after a summer break. The pump has not been running for several months, deposits (lime, rust, magnetite) remain in the system, the impeller "sticks" to the stator and the motor cannot rotate the rotor. The motor hums, but the rotor does not move – and after a while the thermal protection turns off the pump.
Solution: On most Grundfos circulation pumps (including the UPS series), there is a screw on the front side of the housing for the vent hole. Turn off the pump, slightly loosen this screw (carefully – water or steam may spray out!), wait until the pressure drops, and then insert a flathead screwdriver or a longer pin into the hole and gently try to rotate the shaft manually. Often it is enough to "break off" the impeller from the deposits and the pump will then start without problems.
If this does not help, it is necessary to disassemble the pump and mechanically clean the housing from deposits. A detailed disassembly procedure can be found in the article Installation of Grundfos circulation pump step by step.
3. Thermal fuse (motor protection)
Grundfos pumps have built-in thermal protection that disconnects the pump in case of motor overheating (e.g. long-term operation without liquid, blocked impeller). After cooling down, most models reset automatically, some older models require manual reset (a small button on the motor housing). If the pump does not start even after cooling down (about 20–30 minutes) and after manual reset, the motor is likely damaged and it is necessary to consider replacing the head or the entire pump.
Pump is humming, vibrating or making loud noises
Noise is the second most common reason for complaints. Grundfos pumps are exceptionally quiet in normal operation – especially electronically commutated (EC) models are almost inaudible. Any significant noise should therefore be taken seriously.
Air in the system – the most common source of noise
Air blockages in the pipes are responsible for a large percentage of noise complaints. They manifest as a clicking or grating sound that comes and goes, sometimes depending on the temperature cycle. Solution: systematic air venting – start with the highest air vent in the system and proceed downwards. Use a hose and a container, open the valve, let the air out until clean water starts to flow, then close it. If the system has automatic air vents, check whether they are calcified or not working properly.
Replenish the pressure in the system – the ideal pressure for heating systems in single-family homes is 1.0 to 1.5 bar (cold). If the pressure is lower, the system draws air through the expansion tank or leaks.
Cavitation – the silent pump destroyer
Cavitation occurs when the pressure of the liquid at the pump inlet drops below the value of the saturated vapor pressure. The liquid locally "boils" and bubbles form, which implode with enormous force. It is characterized by a distinctive crackling sound resembling gravel being crushed. Cavitation is destructive – it damages the impeller and pump casing over hours to days of intensive operation.
Causes: low system filling pressure, dirty filter before the pump (Y-filter or ball valve with filter), too high pump performance for an improperly designed network, or excessively high liquid temperature (above 90 °C without sufficient overpressure).
Solution: increase the pressure in the system, clean the filter, reduce the pump speed setting (if adjustable), and check the correct dimensioning – more in the article What Grundfos pump power do I need for my home?.
Pump is leaking – liquid escape
Leaks are another common fault. It is important to distinguish exactly where the leak originates, as this determines the repair procedure.
Leak at the flange or threaded connection
The most common type of leak. The gasket between the pump and the pipe flange ages, dries out, or is damaged by thermal cycles. Solution: drain the system, replace the gasket (O-ring or flat gasket depending on the type of connection), and tighten the flange bolts in a crisscross pattern to the specified torque (usually 20–30 Nm for standard flanges DN 25–40). For threaded connections, use a new Teflon gasket or sealing hemp rope with paste.
Leak through the shaft seal
Liquid drips along the shaft from the point where the hydraulic housing connects to the motor. This is a mechanical seal (also known as a floating seal or mechanical seal). With normal wear after years of operation, this seal starts to leak. On Grundfos UPS series pumps, the mechanical seal is part of the so-called wet-end assembly and is usually replaced together with the complete hydraulic head or as a service kit. Replacement requires disassembling the motor from the hydraulic part and is suitable for an experienced technician.
Condensation on the motor housing
Not every moisture around the pump is a real leak. During periods when the distribution water is cold (beginning of the heating season), air humidity may condense on the motor housing. This in itself is not a fault – dry the surface and observe whether the moisture returns. If it does and you determine it originates from the terminal box area, contact service – water in the terminal box is a serious safety issue.
Pump does not generate the required pressure or the flow is too low
This is evident when some radiators are cold, thermostatic heads do not open, or the temperature difference between the supply and return is too large (normally 10–20 °C). The pump is running, but it literally "cannot keep up" with transporting the liquid through the system.
Clogged filter or sediment in the system
Corrosive magnetite (black sludge-like sediment) accumulates in old steel systems and clogs the filter before the pump. Result: the pump draws air and liquid in an improper ratio, flow decreases, and pressure is low. Solution: clean the filter (usually a Y-filter or a ball valve with a screen), consider installing a magnetic separator. Grundfos recommends cleaning the filter at least once a year – more about preventive maintenance in the article Maintenance and service of Grundfos pumps – what to check every year.
Incorrect speed setting
Classic three-speed pumps such as GRUNDFOS UPS 25-30 must have the correct speed setting. For most single-family homes with underfloor heating or smaller radiator networks, speed 2 is sufficient. For larger buildings or long pipe runs, speed 3 may be necessary. On the other hand, too high a speed may cause cavitation or excessive noise. The setting is done by rotating the switch on the motor housing.
Worn impeller
After many years of operation (typically 10–15 years, depending on water quality and operation), the impeller wears out mechanically, its efficiency drops, and the pump does not achieve the designed flow even at maximum speed. In this case, the most cost-effective solution is to replace the entire pump – Grundfos unit pumps generally have a favorable price-to-performance ratio, and repairing a worn impeller is simply not cost-effective.
Failures of UNILIFT pumps – submersible and sludge types
Pumps of the UNILIFT range (e.g., GRUNDFOS UNILIFT KP 150 A1 or GRUNDFOS UNILIFT KP 250 A1) operate under much more demanding conditions than standard circulation pumps. They are submerged in liquid, work with sludge and impurities, and are exposed to significant mechanical stress. Therefore, their failures have a slightly different character.
Float switch not working
UNILIFT pumps are equipped with a float switch that automatically starts the pump when a certain liquid level is reached and turns it off when the level drops below the minimum. If the float switch does not maintain the correct position (e.g., it gets stuck on the wall of the pit, clogged with sludge, or the float is mechanically damaged), the pump will either not start at all or will run dry – which will quickly destroy it.
Inspection: Remove the pump from the pit (disconnect it from the power supply BEFORE any manipulation!) and check whether the float moves freely throughout its entire range. The power cable must not be wrapped around the float arm. If the float is damaged, order a replacement kit – these are standard for models KP 150 and KP 250.
For more information on the correct installation of these pumps, see the article How to connect a Grundfos UNILIFT pump – installation procedure.
Cable damage in submersible pumps
The cable of a submersible pump must have the correct cross-section and be resistant to water and mechanical stress. A common problem is that the cable bends at the point where it enters the pump (where it passes through the seal), or it is mechanically damaged during handling – for example, when pulling the pump up with a rope and pulling by the cable itself. Damaged cable insulation is a SAFETY RISK. If any damage to the insulation is detected, replace the cable immediately or have it repaired by a professional.
Clogging of the suction basket
Models KP 150 and KP 250 have a suction basket with openings at the bottom, through which the liquid is drawn in. Coarse impurities, stones, leaves, or plastic waste can clog the basket, and the pump will stop pumping, although the motor is still running. Solution: Remove the pump and clean the basket manually, or replace it. For regular cleaning, pits that are accessible from above without dismantling the surrounding area are ideal.
Failures of Grundfos solar pumps
Solar pumps such as GRUNDFOS Solar 15-80 or GRUNDFOS Solar 25-120 must withstand specific conditions: a glycol mixture as the heat transfer medium, high temperatures up to 130 °C, and variable power supply from the solar unit. Their failures have their own characteristics, different from standard heating pumps.
Overheating and protection at high temperatures
The glycol mixture changes its viscosity and hydraulic properties at higher temperatures. If the temperature in the collector is too high (over-temperature, above 130–135 °C), the solar controller usually shuts off the pump. The system will restart after cooling down. However, if overheating occurs repeatedly, the cause may be: insufficient volume of glycol medium, contamination of the medium (degraded glycol – pH and inhibitors should be checked at least every 2 years), or a fault in the solar controller.
Degradation of the glycol medium
This is one of the most overlooked problems. Glycol degrades over time, becomes acidic, and starts to corrode metal parts – including the impeller and the pump housing. Degraded glycol has a dark brownish tint (normally it is green or orange, depending on the type). Every two years, a sample should be taken and sent for analysis, or at least the pH should be checked using test strips (the standard recommends pH 7–8). If the pH is acidic (below 6), the medium should be replaced immediately – otherwise, accelerated corrosion of the entire system is at risk.
Compatibility with the controller and power supply
Solar pumps from Grundfos are designed to work with specific solar regulators and input voltage ranges. If the regulator provides unstable or insufficient voltage (e.g., during panel shading), the pump may operate in an inappropriate speed range, leading to increased wear. Always use a regulator recommended by the manufacturer or verified for the specific pump model. Learn more about the operation of solar pumps in the article Grundfos solar pumps – what they are and how they work.
Error codes and signaling on electronic Grundfos pumps
Modern Grundfos pumps (e.g., ALPHA, MAGNA, Comfort series) are equipped with a digital display or LED signaling that shows error codes. Here is an overview of the most common ones:
- Flashing green LED (normal state): The pump is operating correctly in automatic mode.
- Flashing red LED / alarm: The pump is in a fault state. Typically, a blocked impeller, overheated motor, or electronic fault. Check for mechanical blockage and temperature conditions.
- Alternating red and green flashing: The pump is attempting to resume operation after a fault (auto-restart). If the condition persists for more than 5 minutes, manual diagnostics are required.
- Code E: (Electronics fault): A fault in the control electronics. Usually requires replacement of the control board or the entire head. Do not attempt to repair it yourself.
- Code L: (Locked rotor): The impeller is blocked. See the solution above.
- Code T: (Temperature fault): The medium is too hot or the motor is overheated. Reduce the system temperature, check flow and pressure.
A detailed description of codes for a specific model can always be found in the supplied manual, or on the Grundfos website in the technical documentation section.
What generally cannot be repaired by yourself
Caution is important here: not every fault is suitable for home repair. We recommend entrusting the following situations to a professional:
- Motor winding fault (diagnosed with a resistance/continuity tester, the value should be symmetrical between phases and without a short to ground)
- Control board fault (electronics in the motor) – replacement of the entire module or pump
- Cracked pump housing or hydraulic part – cannot be reliably sealed, replacement is necessary
- Any intervention in electrical installation, if you are not a qualified electrician
- Fault in a system operating with steam or pressure above 6 bar
Prevention is cheaper than repair
Experienced technicians will confirm this: 80% of pump faults can be prevented with regular annual inspection. A basic annual checklist includes:
- System pressure check (1.0–1.5 bar when cold) and water top-up in case of a drop
- System venting – open air vents on all radiators and highest points
- Cleaning of filters before the pump
- Visual inspection for leaks around flanges, seals, and shaft
- Test pump startup at the beginning of the heating season (September/October)
- For UNILIFT: check float switch and suction basket, cable condition
- For solar pumps: check pH and color of glycol medium, condition of solar regulator
If you are unsure about the scope of the inspection, read the detailed instructions in the article Maintenance and service of Grundfos pumps – what to check every year.
When it is worth repairing a pump and when to replace it
This is the question customers struggle with most often. A basic guideline from practice:
- Repair is worth it if it is a mechanical fault (seal, impeller, filter) in a pump that is less than 8 years old and in overall good condition. Spare parts for Grundfos are mostly available and the repair usually costs 20–40% of the price of a new pump.
- Replacement is worth it if the pump is older than 10–12 years, if it is a motor or control electronics fault, or if repairs are starting to repeat. Modern EC pumps also have significantly lower energy consumption (up to 50–60% less) compared to old A-class pumps, so replacement pays for itself in 2–4 years.
A comparison of pump types will help in the decision-making process in the article Grundfos UPS vs. Grundfos solar pumps – which one is suitable for your system.
Most frequently asked questions (FAQ)
Why is my Grundfos pump humming even when the heating is off?
If the pump hums at night or outside of heating hours, it is likely set to continuous operation (manual mode instead of automatic), or the electrical command line from the thermostat/controller is constantly engaged. Check the pump's switch setting (AUTO vs. RUN) and verify that the thermostat is not continuously requesting heating. Another possibility: if the pump is equipped with a check valve, it may be causing a "partial closure" effect, which can produce noise.
The Grundfos pump is running, but the radiators are cold – what to do?
First step: verify that the boiler is actually turned on and reaching the set temperature. If yes, the problem is either air in the radiators (airlock), a closed ball valve on one branch, a clogged filter before the pump, or insufficient pump pressure. Bleed the radiators, check the position of all valves and ball cocks, and clean the filter. If the system still does not work, try increasing the pump speed setting.
Grundfos UNILIFT KP is not pumping water from the basement – why?
Most common causes: the float switch is stuck or damaged and the pump will not start, or the suction basket is clogged and the pump is sucking air. Removing the pump (after disconnecting from the power supply!) and a visual inspection usually immediately reveals the problem. Also check whether the pump is placed too high above the bottom of the pit – the minimum water level for starting is usually 50–80 mm from the bottom of the suction basket.
Solar Grundfos pump does not start in the morning, even though the sun is shining – why?
Solar pumps start when the solar regulator detects a sufficient temperature difference between the collector and the storage tank (typically ΔT = 5–8 °C). In the morning, when the tank is still warm from the previous day and the collector is slowly heating up, this difference may not yet be reached. This is normal. If the pump still does not start after several hours of full sunlight, check the regulator's power supply, temperature sensor connections, and the ΔT parameters of the regulator.
Is it normal for the Grundfos pump to make noise for the first few days after replacement?
Yes, after replacing a pump (or after refilling the system), it is common for air bubbles to circulate in the system for the first few hours to days – the system gradually pushes them out through the air vents. Once the system is sufficiently bled and the pressure stabilized, the noise will subside. If the noise persists for more than 2–3 days after refilling and bleeding, look for a leak or another source of air ingress.
Can I replace only the motor of a Grundfos UPS pump and not the whole pump?
Yes, this is one of the advantages of the modular Grundfos concept. On the UPS series, the hydraulic housing is separated from the motor part by a few screws. If the hydraulic part (housing, impeller, seals) is in good condition and the fault is only in the motor part, you can replace only the motor module. Just make sure that both parts are compatible (same series, same shaft diameter). In practice, however, motor replacement alone is not done very often, as the price of the motor head is often close to the price of a new pump – but when switching to an EC motor, it can be economically interesting.
Conclusion: faults can usually be resolved – if you know what to look for
Grundfos pumps are reliable devices with a long service life, but like any technology, they also require attention. The key is early diagnosis – do not ignore signs of faults. Most common problems such as bleeding, filter cleaning, setting the speed, or replacing a seal can be handled by a skilled DIY enthusiast without a service technician. More complex cases – electronic faults, winding faults, or complete shaft seal replacement – should be entrusted to a professional.
If you are wondering whether your existing system is correctly sized and whether a new pump could bring savings, also take a look at our other articles in the Knowledge Centre: What Grundfos pump capacity do I need for my home? or Common questions about Grundfos pumps, where you will find answers to other practical questions.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.