Boilers Circulator Pumps

Learn about Circulator Pump in hydronic systems.

Common Boiler Circulator Pump Symptoms (Quick Diagnosis)

Use this quick reference to identify common circulator pump problems, what they usually mean, and whether the issue is safe for homeowners to check.

Symptom Likely Cause DIY Safe? Next Action
Pump humming but no heat Air locked system, stuck motor, failed capacitor ⚠️ Limited Check air in system; call pro if motor does not spin
Cold radiators or uneven heat Air in piping, closed valves, weak circulation ✅ Yes Bleed system and verify valves are open
Very hot pump housing Blocked flow, seized impeller, electrical issue ❌ No Shut system off and contact a professional
Loud rattling or grinding noise Worn bearings, cavitation, debris in impeller ❌ No Professional inspection recommended
Water leaking at pump flanges Worn gasket or loose bolts ⚠️ Limited Gently snug bolts; gasket replacement is not DIY
Pump not running at all No power, failed motor, control issue ❌ No Electrical diagnostics required

These symptoms are explained in more detail below, including step-by-step troubleshooting and when it’s safest to call a licensed HVAC professional.

Circulation Pumps in Boiler Systems

Understanding Boiler Circulator Pumps

Circulation pumps, often called centrifugal pumps, are a critical component in hot water boiler systems. They move water from the heat source through piping to radiators or baseboards and back to the boiler. These pumps rely on centrifugal force to circulate water, with the impeller spinning to push water through the system. Unlike pressure pumps, circulators are designed primarily for flow and distribution rather than boosting pressure.

Popular circulation pump brands commonly encountered during boiler repairs include Bell & Gossett 100 Series, Taco 007, and Grundfos UPS26–99. These pumps are valued for reliability, simplicity, and long service life, which averages around 10–15 years depending on usage, water quality, and maintenance.

When planning installation or replacement, several key considerations help ensure optimal performance:

In this guide, we will explore common problems with circulation pumps, step-by-step troubleshooting, and when it’s appropriate for a homeowner to perform DIY checks versus calling a professional. For more advanced boiler repairs, see our boiler troubleshooting guide.

Many homes still use legacy pump models due to their simplicity and reliability. Modern circulator pumps offer advanced features such as electronically commutated motors (ECMs), digital displays, and variable flow capabilities, which can improve efficiency and comfort. These newer features will be explored in future articles.

Fun fact: In 2011, Xylem Inc. acquired Bell and Gossett, adding their products to a larger portfolio of hydronic system solutions.

Common boiler circulation pumps including Bell & Gossett, Taco, and Grundfos

Troubleshooting Circulation Pump Issues

Circulation pumps move heated water from the boiler to radiators or baseboards. When a pump begins to fail, homeowners may notice uneven heating, gurgling sounds, cold zones, or unusual noises near the boiler.

Safety First: Always turn power OFF and allow the system to cool before inspecting anything. Some symptoms overlap with air trapped in the system — see our Air Removal Guide. Electrical testing or internal disassembly should be handled by a licensed professional.

What Happens: Air pockets prevent proper water circulation.
Common Signs: Gurgling sounds, cold radiators, uneven heat.
Homeowner Action: DIY-safe if you know how to bleed radiators or purge air properly. Refer to the air removal guide.

What Happens: A blockage in the system, often caused by a closed valve before or after the pump, can restrict water flow. However, restrictions could be elsewhere in the system.
Common Signs: Pump runs but no heat circulates.
Homeowner Action: DIY-safe visual check to confirm all isolation valves are open.

What Happens: Bearings seize, preventing the motor shaft from turning.
Common Signs: Humming sound with no circulation.
Homeowner Action: Limited DIY — only with power OFF. Some pumps (e.g., Grundfos) have access port to visually allow checking shaft movement. This is a temporary measure; replacement is usually required.

What Happens: Impeller may be jammed by foreign objects, such as a rubber o-ring or gasketing material.
Common Signs: Motor humming but no water moves.
Homeowner Action: Not DIY. Requires system disassembly and resealing.

What Happens: Gaskets or O-rings wear out over time.
Common Signs: Drips or corrosion near pump flanges.
Homeowner Action: DIY-safe to gently tighten bolts. Gasket replacement is not DIY.

What Happens: Pump does not receive power during a heat call.
Possible Causes: Aquastat, zone relay, or safety control.
Homeowner Action: Not DIY. Electrical diagnostics require live testing.

What Happens: Internal motor windings fail (O.L.) or short to ground.
Common Signs: Breaker trips or motor will not start.
Homeowner Action: Not DIY. Motor replacement required.

What Happens: Motor cannot start properly.
Common Signs: Humming motor with no rotation or Breaker Trips.
Homeowner Action: Not DIY. Capacitors store energy and can be dangerous.

Circulation Pump Basics & Lifespan

Older heating systems relied on gravity circulation, which often caused uneven heating and slow response times. Circulation pumps are essential for moving hot water from your boiler to radiators or baseboards efficiently. Modern pumps ensure rapid, consistent heat distribution, even to upper floors, improving comfort and energy efficiency.

Average Life Expectancy

Most circulation pumps last 10–30 years, depending on system usage, water quality, and maintenance. Regular inspection, bleeding air, and keeping the system clean can extend pump life and prevent boiler problems.

Popular Brands & Considerations

Many homeowners and professionals rely on trusted boiler and pump brands for durability and serviceability. When choosing a pump or boiler, consider brand reliability, compatibility with system zoning, and ease of maintenance.

Installation & Notes

Proper pump installation and system zoning are critical. Multi-zone systems often require separate circulators for each zone to ensure balanced flow. Incorrect sizing, poor placement, or skipping zoning considerations can reduce efficiency, shorten pump life, and cause uneven heating.

For a deeper dive into pump troubleshooting, proper installation, and circulation optimization, see our Circulation Pump Troubleshooting Guide and Boiler Air Removal Guide.

Circulation Pump Brands Comparison: Bell and Gossett, Taco, and Grundfos

The following is a comparison of popular circulation pump brands based on my personal experience and observations. I am not affiliated with any of these manufacturers, and this is not a sponsored endorsement. The information is intended to help homeowners and HVAC professionals understand key features, strengths, and practical considerations for each brand.

Bell and Gossett 100 Series

  • Known for durability and robust performance.
  • Features traditional designs that are easy to maintain.
  • Ideal for larger systems requiring consistent operation.
  • Bell and Gossett 100 Series Boiler Circulation Pump.
Bell and Gossett Circulator Pump

TACO 007

Taco Circulator
  • Compact form factor, making it suitable for tight spaces.
  • Energy-efficient designs, including ECM (Electronically Commutated Motor) options.
  • Focused on user-friendly installation and maintenance.
  • The price of the capacitor (~$60) is about half the cost of a new motor. Replacing the capacitor is often easier than replacing the whole pump, though it’s worth investigating why the capacitor failed.

Grundfos UPS26–99

  • Advanced technology with quieter operation.
  • Offers both single-speed and variable-speed models.
  • Excellent for residential and small commercial applications.
  • Easy access port to check if the motor shaft is rotating freely.
Grundfos Circulation Pump

Circulation Pump Installation Tips

Proper installation of circulation pumps can save time, reduce maintenance headaches, and prolong equipment life. Here are key considerations every homeowner and installer should keep in mind.

Shut-Off Valves: Simplify Future Maintenance

Always incorporate shut-off valves both upstream and downstream of the circulator. These valves allow you to isolate the pump for repairs or maintenance without draining the entire system, minimizing downtime and avoiding unnecessary disruptions.

Supply Side vs. Return Side Installation

Industry opinions vary, but following the manufacturer’s recommendations is always critical. Based on experience, installing the pump on the supply side offers several advantages:

Zoning with Circulation Pumps

While a single pump may suffice for simple systems, multiple pumps can be used for zoning or targeted heating. Alternatively, a single pump with Zone Valves can provide a cost-effective solution for multi-zone homes.