Diagnosing Hydronic Heating System Noise

When a water-based (hydronic) heating system is installed and working correctly it should heat your home without you even knowing it’s there. Noise in a system is a sign that something is not right. However, properly diagnosing and solving a noise issue can be tricky—reasons for that banging, knocking, or whooshing range from the trivial and easily solved to the dangerous and difficult to fix.

In almost all cases, it’s best to call an Xylem RCW dealer, distributor, or installer to help diagnose and address the problem. To help you ask the right questions, here are some common sources of noise in a hydronic heating system, along with possible reasons and suggested solutions.

Description: Rushing, whistling, gurgling, or rattling noise in the pipes
Possible Cause: Entrained air in the pipes

As water heats up, dissolved air becomes separated and bubbles swoosh around the system, causing noise, loss of efficiency, and premature failure of components. Xylem RCW air separators and compression tanks can remove entrained air if venting the system (usually via a bleed valve on a top floor radiator) is not enough. Check also for system leaks because a feed valve will automatically add more air-laden water to a leaking system.

Description: Rattling noise in the circulator pump
Possible Cause: Misaligned motor, debris in motor

Circulators should operate quietly, so if they make a noise, they need to be checked. Rattling could be a sign of a misaligned motor or shaft or caused by a piece of debris caught on the impeller blades. Switch to a modern, maintenance-free circulator if you suspect yours has reached the end of its working life.

Description: Squeaking, squealing, or grinding noise from the circulator pump
Possible Cause: Bad bearings, too little lubrication.

In the worst case, that squeaking could be a sign of worn-out bearings. If true, it’s time for a new circulator. But Xylem RCW oil-lubricated circulators should last 30 years or more. The circulator might need oiling. A Series 100, for instance, needs to be oiled a couple of times a year with approximately four drops of 20-weight, non-detergent oil (“electric motor oil” or “bearing assembly oil”) in the two motor ports and approximately six to eight drops of oil in the bearing assembly. Don’t over-oil as this can damaged the motor!

Description: Hissing noise from a radiator
Possible Cause: Bad radiator valve

Bad radiator valves give rise to two symptoms. The first is a cold radiator, especially after the valve has been closed and re-opened. The second is hiss. Valves that are not fully open create a noise as water squeezes through the small gap. Contact your Xylem RCW dealer, distributor, or installer for help addressing a bad radiator valve.

Description: Groaning noise from water feeder
Possible cause: Water pressure too high

A water feeder handling high pressures can make a groaning noise. A pressure reducing valve can help—make sure it is set to 12 to 15 psi, enough to pressurize a two-story home but much lower than municipal water pressure. In the worst case, consider replacing your water feeder.

Description: Creaking or snapping from pipes
Possible cause: Thermal expansion in copper pipes

Pipes in a hydronic heating system expand and contract as they heat and cool down. Unfortunately, they are not always installed with this movement in mind, but no one wants to rip up floors to re-install. A modern control system can help by keeping system temperature more constant, eliminating dramatic temperature swings. Check also for pipes rubbing against drywall, nails, or other obstructions—something this simple sometimes be the cause of irritating noise!

Description: “Velocity” noise not explained by entrained air
Possible cause: Over-sized circulator pump

In a “closed” hydronic system—pressurized and completely filled with water—a circulator only has to overcome water friction and inertia. A circulator that generates too much pressure—called “pump head”—pushes water around too fast, creating a whooshing noise. If this is the case, consider a circulator with lower head.

Description: Hammering or banging noise in pipes
Possible cause: Zone valves that close too quickly

Fast-closing (often solenoid) zone valves are often the cause of a classic hydronic noise issue: water hammer. If you suspect zone valves of causing your hammer, switch to sophisticated valves engineered to close slowly.

Description: Knocking coming from pipes fed by a steam boiler
Possible cause: “Wet” steam

This problem can occur when new steam boilers are attached to old systems. Older boilers had large capacities producing “drier” steam. Water-laden steam hitting cold pipes causes condensation, and as this water is pushed around, knocking occurs. Have your Xylem RCW dealer, distributor, or installer check your situation, especially whether or not the boiler manufacturer’s piping instructions (designed to dry out steam) were followed.

Description: Banging, knocking, or boiling noise in a hot water heater
Possible cause: System pressure too low

Boiling in a hot water heater is a potentially dangerous situation. Your Xylem RCW dealer, distributor, or installer will ask you to check the temperature of the boiler and system pressure. A household hydronic system should be pressurized to about 12 to 15 psi, enough to fill the system plus a little extra to allow venting and to prevent boiling in topmost radiators. If pressure is too low and temperature too high, boiling in the heater will occur. Xylem RCW offers aquastats, pressure reducing valves, and other components if necessary.

Description: Low rumbling or sharp rattling from a circulator pump
Possible cause: Pump cavitation

Cavitation is created by a lack of suction in a circulator. In this situation, pressure in the pump falls below the vapor pressure of water, causing bubbles that are carried to the volute (pump exit) where they implode and damage the pump. A badly installed pump can cause this issue, although properly diagnosing cavitation is a job for your Xylem RCW dealer, distributor, or installer.

Description: Whining from pipes in one heating zone
Possible cause: Unbalanced system

A hydronic system that is not balanced can cause water to take the shortest route around the pipes, starving distant zones of heat and causing noise in the zone with excess flow. It’s essentially a “short circuit.” Xylem RCW offers many kinds of balancing valves for home heating systems.

Description: Banging in the topmost radiators
Possible cause: Fill pressure too low

If the fill pressure in a home heating system is too low, water in low-pressure topmost radiators can “flash” to steam, causing banging. A home heating system should be pressurized to 12 to 15 psi. Installing a minimum pressure valve at the tank can help, as can a vacuum breaker at the top of the piping.