Why is the circulator pump making noise? 

When the circulator pump fails, water is not circulated as efficiently or at all, compromising the performance of your entire system. Thankfully, pumps often signal that there is something wrong, and these signals usually come in the form of odd and loud pump noises.

Loud and unusual sounds emanating from circulator pumps are always a red flag, a sign that something is wrong either with the pump or in the water lines.

Let’s take a look at a few problems in a building system that are commonly the cause of pump noises, and how to fix them.

Air in The System

Summary: The air in the system leads to noise. Then the entire system must be inspected and bled.

If you don’t have an air separator installed, you will most likely have to deal with air in your system at some point. When this happens, it’s important to inspect the water lines and bleed the system.

Modern pumps have bleeder valves that make the process very easy. Slowly open the valve until you start hearing a hissing noise. Once the hissing ceases, you will see a slight dribble of water, indicating that the pump no longer has air. At this point, you can close the valve.

After doing this, make sure that the pump has been installed correctly. Even a few degrees of inclination or misalignment will allow air to get locked in the pump.

Incorrect Pump Size


(1)The pumps may be oversized for the system in question. To fix this, throttle the pressure-side valves until the noises are eliminated.

(2)The water in the system is boiling because the pump is undersized. This can be remedied by installing a larger pump.

Both oversized and undersized pumps can lead to noises in the system, but the solution to deal with each case is different.

Oversized Pump

Pumps can be oversized for several reasons. It could happen due to a degree of error in the planning and design stage when engineers need to “guesstimate” piping length and fittings, or it could be purposely designed this way so the system can expand in the future and the “right-size” pump today would not be able to meet the future demand of tomorrow.

Sometimes a pump replacement is needed right away, and the supplier didn’t have the perfect replacement in stock, or the engineers choose an oversized pump already considering an expected build-up of corrosion in the pipes that require more pump head.

Regardless of the reason that led to an oversized pump, having one can always cause excessive noise and vibration, loosing up connections and joints and causing piping fatigue.

To solve the issue, you can take the following actions:

  • Throttle the pressure-side valves until the noises are eliminated
  • Trim the impeller diameter
  • Reduce pump speed
  • Add a flow recirculation line
  • Install a variable frequency drive and remove control valves

Undersized Pump

Under sizing a pump presents a more serious issue. That’s because you, unfortunately, don’t have much choice other than replacing the pump and installing a larger one.

If the system is undersized, it can’t provide the duty needed, it can also lead to deadheading – when a pump’s discharge is closed because of a blockage in the line or an unintentional closed valve. When this happens, the fluid churns inside the pump until it heats up into a vapor, causing noise and damage. Deadheaded pumps can lead to motor burn out, a damaged impeller, seal leakage, cracked bushings, and compromised elastomers, ultimately killing the pump.

In systems with undersized pumps, you can verify if the existing pump can handle a larger motor to avoid dead head. Even though it may be the cheapest way to handle the problem, it is not the best and the solution would just be temporary.

Excessive Wear of Bearings

Summary: Excessive wear of bearings (rattling). Then the pump needs to be replaced.

Only some pumps have bearing assemblies, not all. However, all electric pump motors have bearings, and the excessive wear of bearings – whether on the assembly or inside the motor – can cause pump noise.

The good news is that pump models with bearing assemblies usually have the components available for purchase, and they are inexpensive and easy to replace.

The bad news is that motor bearings are not sold as components, and when the bearings wear out in the motor, you need to replace the entire part.

Many factors can affect the life of a bearing, such as static overload, corrosion, lack of excess of lubricant, overheat, misalignment, and contamination. So, the best way to avoid bearings from wearing out too fast is via comprehensive inspection of your system as the bearings of wet rotor circulator pumps are lubricated by water in system.

Clogged System

Summary: A system that has already been entered into operation has become clogged. Then the system needs to be cleaned.

Water with rust and other sediments can wear out the circulator pump and clog the impeller. When this happens, noise is a consequence. To get rid of it, there is no magic bullet: the solution is in cleaning the system.

Many HVAC systems have dedicated filtration systems and dirt separators to prevent clogging from happening. Sediments can be easily removed from the system through water blowdown.

These systems protect not only pumps but all the other HVAC units in the system.

Incorrect Speed Setting

Summary: The pump speed sets too high. Then Locate the flow switch and turn it down one level

High-quality and modern pumps usually have 3 flow settings, while older pumps may have only one. That’s why older pumps are typically noisier than others. They are less efficient, and the energy loss is usually translated into a humming noise.

If your pump is making this noise and you have more than one option of flow setting to work with, locate the flow switch and turn it down one level. Then check the radiators and tower rails to verify if they are still getting up to the temperature they should. If so, then leave it this way.

Along with the technologies develop, the modern variable frequency pumps can adapt the system requirement automatically and make no noise. If you are using such circulator pumps, you won’t have the worry of incorrect speed setting.

Lack of NPSHa or Incorrect Install Causing Lack of NPSHa

Summary: Air bubbles form at the inlet of pump and leads to noise. Then reduce the pipework friction or increase the pump head.

To understand this, we need to take a step back and take a quick look at pump operation. Bernoulli’s principle shows us that fluid flows from areas of high pressure to areas of low pressure.

HVAC pumps operate by creating low pressure at the inlet, allowing the water to be pushed into the pump. As the fluid flows through the pump, the pressure decreases. If the pressure at the inlet happens to fall below the vapor pressure of the fluid, air bubbles form at the inlet. These bubbles can cause cavitation, leading to pump noise, damage, and lower capacity.

Net Positive Suction Head or NPSH is the difference between liquid pressure at the pump suction and liquid vapor pressure and is expressed in terms of the height of the liquid column. NPSH needs to be usually 3 to 5 feet to avoid cavitation.

If detected in an inspection that there’s a problem with the NPSH, basically two things can be done: first, there’s the option to choose a more appropriate pump for the application (our recommendation if the pump has already suffered irreparable damage due to cavitation). Second, the system can be reevaluated to see if elevating the water tank can increase NPSHa (the absolute pressure at the suction port of the pump) or if fittings that rob current NPSHa can be reduced.

If you need assistance with anything related to circulator pumps, contact us.