In a traditional cooling system, you have 2 types of water pumps: condenser water pumps and chilled water pumps. 

The condenser water pumps take hot water from the condenser to the cooling tower, and the chilled water pumps take cold water from the chiller to your AC unit. 

When the 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 circulating 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.

 

1. Air in The System

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.

 

2. Incorrect Pump Size

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

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 

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.

 

3. Excessive Wear of Bearings 

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. 

The life of a bearing is determined by how many hours it takes for the metal to “fatigue,” but many factors can affect this, 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 preventive maintenance and a comprehensive inspection of your system. 

 

4. Clogged System

Water with rust and other sediments can wear out the circulating 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. 

 

5. Incorrect Speed Setting

High-quality and modern pumps usually have 3 flow settings, while older pumps may have only one or two. 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. 

If you operate with a VFD and your pump is still making a humming noise, check to see if you have a correct grounding of the motor to the VFD. In many cases, incorrect grounding allows the system to act as a noise transmitter.

 

6. Lack of NPSHa or Incorrect Install Causing Lack of NPSHa

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 cooling tower 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 circulating pumps, contact us. We have 3 offices in California, and we can go wherever you are to inspect your pump if you are experiencing loud and unusual noises. 

The Vertical Systems team of sales engineers and technicians can point out the cause of pump noises, and determine the best solution to eliminate it. Experts in all types of circulating pumps, our professionals can solve any problems with installation, worn components, leaks, and issues related to water temperature, pressure, and air bubbles. They can also recommend energy-efficient upgrades that will make your system perform better and save you money.