Grundfos CR Pump Troubleshooting: 8 Common Problems and How to Fix Them

Most Grundfos CR problems trace back to four things: no power reaching the motor, air in the pump, the wrong direction of rotation, or not enough pressure at the inlet. The pump itself is rarely the first thing to fail. Check those four before you pull anything apart, and you will solve the majority of calls without a new part.

This guide walks through the eight symptoms we hear about most from owners of Grundfos CR, CRI and CRN vertical multistage pumps, what causes each one, and what to check first. It is built on the fault finding table in the Grundfos CR installation and operating instructions, so the fixes match what the manufacturer expects. If you already know you need a replacement, the full Grundfos pumps collection at ePumps covers CR, CRI, CRN and CM models.

Before You Start: Safety and the Nameplate

Lock out power at the disconnect before you touch the coupling, the terminal box or any plug on the pump. Close the isolating valves on both sides if you will open the pump. Then write down two things from the nameplate: the model designation (for example CR5-10) and the eight digit product number. Together they identify the exact pump, seal and motor. Our guide to Grundfos model numbers explains how to read both.

Quick Symptom Checker

1. The Pump Will Not Start

Start at the power supply and work toward the pump. Grundfos lists these causes in order, and it is the fastest way to isolate the fault:

• No voltage at the motor. Check the terminal box with a meter. If there is no voltage, the problem is upstream in the disconnect, starter or wiring.
• Blown fuse or tripped breaker. Replace or reset it, but find out why it tripped (see problem 6).
• Overload protection tripped. Check voltage on both sides of the overload and reset or replace it.
• Starter or control not calling. A pressure switch, float or controller that never closes will look like a dead pump. Our guide to pump float switches covers how those controls fail.
• Bad capacitor (single phase). Test it with a meter and replace it if it fails.
• Seized pump. With power locked out, try to turn the shaft by hand at the coupling. If it will not turn, the pump is blocked or the seal faces have bonded after sitting idle.

2. The Pump Runs but Delivers Little or No Water

This is the most common call, and it is usually air. A CR is a non self-priming centrifugal pump, so it needs to be full of liquid before it can build pressure. If you need a pump that can lift water on its own, see our explainer on self-priming pumps.

1. Vent or reprime it. On a flooded suction, close the pump isolating valves and open the priming plug on the pump head until liquid runs out steadily. On a suction lift, close the discharge valve, remove the priming plug, fill the pump and suction pipe completely, and refit the plug.
2. Check rotation. Viewed from above, a CR should turn counterclockwise. On a three phase motor, swap any two supply leads to reverse it. A pump running backward still makes some pressure, which is why this one gets missed.
3. Look for restrictions. A closed valve, a clogged strainer or a stuck check valve will starve the pump.
4. Check suction lift and leaks. Too much lift, or an air leak on the suction side, lets the pump lose prime. Lower the pump, enlarge the suction line, or tighten the fittings.

3. Pressure Has Dropped Over Time

If the pump used to hit its set point and now falls short, either the pump has worn or the system has changed. To tell which, compare the pump against its own curve. Read the discharge pressure with the discharge valve closed briefly (shutoff), subtract the inlet pressure, and multiply the difference in psi by 2.31 to get feet of head. If that number is well below the shutoff head on the published curve for your model and stage count, the impellers or guide vanes are worn or clogged.

Keep that test short. A CR running against a closed valve heats the liquid inside it fast. Grundfos publishes how long it takes for the liquid to rise 18°F with zero flow:

If shutoff head matches the curve but the system still falls short, the pump is fine and the demand has grown. That is a sizing question, not a repair.

4. A Gravel or Rattling Noise

A sound like gravel passing through the pump is almost always cavitation. Pressure at the first impeller has dropped low enough for the liquid to boil, and the vapor bubbles collapse against the metal. Grundfos sets the minimum inlet pressure for CR, CRI and CRN pumps at the NPSH required plus 2 feet of margin.

• Clean the suction strainer and open every suction valve fully.
• Check the inlet pressure with a gauge while the pump runs at its highest flow.
• Watch the temperature. Hot water raises the vapor pressure, which is why cavitation shows up so often on boiler feed and condensate duty.
• Check where the pump is running on its curve. Far to the right (high flow, low head) the NPSH required climbs quickly.

If you run a CR on a boiler, our overview of what a boiler feed pump does explains why the receiver height and water temperature matter so much here.

5. The Shaft Seal Is Leaking

CR pumps use a cartridge shaft seal. A healthy seal should not show visible leakage, so a drip at the seal means it is time to replace the cartridge. What matters more is why it failed, or the new one will go the same way:

• Dry running. Even a short run without water can crack or scorch the seal faces. Fix the cause (lost prime, low tank level) before you install a new seal.
• Running against a closed valve. See the temperature table above.
• Abrasives. Sand or scale in the liquid scores the faces.
• The wrong seal for the liquid. Most water duty uses an EPDM seal (HQQE code). Oils need FKM (HQQV). The seal code is the last group of letters in the full model designation.
• Too much inlet pressure or heat. Exceeding the pump's pressure or temperature limits shortens seal and motor bearing life.

6. The Breaker Trips or the Motor Runs Hot

Grundfos lists these causes for blown fuses and tripped relays. Most are electrical, not mechanical:

• Supply voltage varying more than plus or minus 10% from the nameplate
• Current imbalance between the three phases
• Overload protection set too low, or mounted somewhere hotter than the motor
• Loose connections or faulty wiring
• A shorted or grounded motor
• A pump that is binding or seized

Motor enclosure matters in hot, dusty or wet rooms. Our comparison of TEFC and ODP motors explains which holds up where.

7. The Pump Short Cycles

Rapid on and off cycling usually points to the controls or the tank: a misadjusted pressure switch, a waterlogged tank with no air charge, a tank that is too small, or a pump that is oversized for the demand. Every start heats the motor, and Grundfos sets hourly limits by motor:

If your system needs frequent starts no matter what you do with the tank, a variable speed setup is often the cleaner fix. See VFD vs fixed speed pump systems and our control panels and VFDs.

8. Vibration

Check that the base is bolted down on a level foundation and that the piping is supported on its own rather than hanging off the pump flanges. Pipe strain pulls the pump out of alignment and loads the bearings. If the motor was removed at any point, confirm the coupling was reset to the correct height. Persistent vibration after that usually means worn motor bearings.

Simple Checks That Prevent Most of These Problems

• Inspect the motor about every 500 hours of operation or every three months, whichever comes first.
• Follow the greasing interval on motors that have grease fittings, and do not over-grease. Excess grease raises bearing temperature.
• Look for leaks at the shaft seal at every inspection.
• Drain the pump and piping if the room can freeze, and refit the original drain plug, not a standard plug.
• Check the tank pre-charge at least once a year.

Repair or Replace?

A leaking seal or a bad capacitor is a repair. Worn impellers, a cracked pump head, or a seized pump often are not. On a CR there is a middle option: if the motor is still sound, a flanged wet end replaces the complete pump section and bolts to the motor you already have. For the general decision, our repair vs replace guide walks through the math.

If corrosion caused the failure (rust inside a cast iron pump head, pitting on the base), replace it with a stainless version rather than a like for like CR. Our guide to Grundfos CR vs CRI vs CRN explains which material fits which water. If you are also weighing a different pump type, Grundfos CR vs CM compares the vertical and horizontal ranges.

Final Thoughts

Troubleshoot a Grundfos CR in order: power, prime, rotation, inlet pressure. Those four checks clear most problems without a part. When the pump really is worn out, match the model and product number from the nameplate, and decide whether you need a whole pump or just the wet end. Browse vertical multi-stage pumps or the full Grundfos range when you are ready.

Frequently Asked Questions

Why is my Grundfos pump running but not pumping water?

It has usually lost its prime or is air-bound. A CR is not self-priming, so vent it at the priming plug on the pump head until liquid runs steadily. If that does not fix it, check that the pump turns counterclockwise viewed from above, and look for a closed valve or clogged strainer.

How do I prime a Grundfos CR pump?

With power off, close the discharge isolating valve and remove the priming plug on the pump head. Fill the pump (and the suction pipe on a suction lift) with liquid until it is completely full, then refit and tighten the plug. On a flooded suction, simply open the priming plug to vent air until liquid flows.

Which way should a Grundfos CR pump rotate?

Counterclockwise when viewed from above the motor. On a three phase motor, swap any two supply leads to reverse the direction. A pump running backward still makes some pressure, so always confirm rotation on a new install.

Why is my Grundfos pump shaft seal leaking?

The most common causes are dry running, running against a closed valve, abrasive particles, the wrong seal material for the liquid, or exceeding the pressure or temperature rating. Replace the cartridge seal and fix the cause at the same time, or the new seal will fail the same way.