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Hot Water Pipes Knocking? How to Diagnose and Fix the Problem

Is your plumbing making an annoying knocking or banging noise whenever you turn on the hot water? This disruptive hammering and clanging in the pipes can keep you up at night and drive you crazy. But what causes hot water pipes to suddenly start knocking, and what can you do to stop the racket?

Loud knocking or drumming noises coming from hot water pipes is usually a sign of water hammer. This common plumbing problem happens when fast moving water is suddenly forced to stop or change direction. The momentum generates shock waves that reverberate through the pipes, causing them to rattle and bang against joists and studs.

What Causes Knocking Noises in Hot Water Pipes

There are several potential culprits behind the annoying knocking or banging noises coming from your hot water supply lines:

hot water pipes knocking

Water Hammer

The main cause of loud knocking and banging noises in hot water pipes is water hammer. It happens when faucets or appliances using hot water are suddenly turned off, and the fast moving water slams into the closed valve, creating a shock wave through the pipes. This can make the pipes shake violently against framing and lead to a loud knocking or banging noise.

Mineral Buildup and Corrosion

Over time, minerals like calcium and magnesium can build up in hot water pipes and cause corrosion. This buildup narrows the inside of pipes, increasing water velocity. Corroded pipes are also weaker and more likely to knock around. Opening faucets causes pressure changes that make old corroded pipes knock and bang.

Malfunctioning Water Heater

Issues with your water heater like sediment buildup, a faulty thermostat, or failing heating elements can also lead to knocking pipes. As the heater struggles to heat water properly, pressure fluctuations and water surges can cause pipes to hammer and bang.

Loose Mounting Brackets or Supports

If water pipes are not properly secured to framing, the momentum of moving water can make them rattle around. Loose mounting brackets, hangers, and supports are common causes of banging pipes whenever the water is turned on.

Trapped Air Bubbles

Pockets of air trapped in the hot water system can also lead to water hammer. Air bubbles get compressed by the moving water, building pressure that gets released in loud knocking noises when faucets are closed.

When to Worry About Knocking Pipes

Occasional knocking when turning on a hot water faucet may not be a big concern. But in certain cases, you should pay attention to the noises coming from your pipes:

Loud banging noises that continue even after closing faucets likely indicate a larger underlying issue. Knocking combined with other problems points to something more serious than typical water hammer.

DIY Diagnosis and Troubleshooting

Before calling in a plumber, try these DIY steps to diagnose the cause of knocking pipes:

Pinpoint Source of the Noise

Pay attention to where exactly the knocking is coming from. Trace the sound when pipes are banging to locate the problem area. Knocking near water heater likely indicates issues there. Noise from supply lines points to loose pipes or water hammer.

Check Supply Lines, Shutoff Valves and Mounting

Inspect where supply pipes connect to fixtures, looking for loose fittings and supports. Try tightening any loose pipe clamps or brackets. Also check shutoff valves near appliances and under sinks for worn washers causing water hammer.

Inspect for Corrosion and Mineral Deposits

Remove small sections of supply pipe insulation to check for corrosion and mineral buildup. Significant rust, pitting, or scale narrowing the pipe indicates corroded plumbing. Knocking noises happen as pressure builds up trying to push through mineral deposits and weakened pipes.

Test Water Pressure and Temperature

Use pressure gauges on hose bibs and inspect your home's water pressure regulator. Pressure over 80 psi can cause pipes to knock. Check hot water temperatures at faucets - a faulty water heater thermostat leads to fluctuating temps and water hammer.

Turn Water On and Off to Isolate Cause

Turn various water sources on and off systematically to pinpoint what causes knocking. If it only happens when turning off a certain faucet, the issue is likely worn washers or bad valves near that fixture. Noises from multiple sources point to a larger system problem.

Fixing Water Hammer

To stop pipe banging caused by water hammer, try these handyman fixes:

Install Water Hammer Arrestors

Arrestors absorb shock waves and prevent pipes from knocking. They contain air pockets that compress to cushion hydraulic shock. Install arrestors near fixtures prone to water hammer like washing machines and dishwashers.

Adjust Water Pressure Regulator

If water pressure to your home is too high, lower it to 60-70 psi using the regulator. Excessive pressure and force intensify water hammer pipe knocking.

Replace Worn Washers and Gaskets

Replace old rubber washers and gaskets in shutoff valves at sinks and behind appliances. Worn washers allow faucets to close too quickly, creating water hammer forces.

Upgrade to Flexible Supply Lines

Replace old rigid water supply pipes with new flexible braided stainless steel lines. Flexible pipes absorb shock and prevent knocking noises.

Getting Rid of Trapped Air

To eliminate knocking noises from air trapped in pipes, try these plumbing fixes:

Bleed Radiators and Hot Water Heaters

Open bleeder valves to release built-up air from radiators and hot water heaters. This prevents air pockets from causing water hammer.

Install Automatic Air Vents

Auto air vents automatically release air from the system so it can't cause water hammer knocking. Install them near hot water heaters and on top of pipe high points.

Flush Debris from Pipes

Debris like sand and mineral sediment in pipes can lead to air pockets. Flush water lines to remove built-up deposits and trapped air.

Treating Corroded Pipes

To stop knocking noises from corroded pipes, consider these remedies:

Use Chemical Corrosion Inhibitors

Adding anti-corrosion chemicals to your water heater can prevent further rust and scale formation. This reduces pressure buildup and knocking in pipes.

Replace Steel Pipes with PEX

For badly corroded steel plumbing, replace with PEX plastic piping. PEX is corrosion resistant and can withstand pressure changes without water hammer.

Insulate Pipes to Prevent Condensation

Insulating hot water pipes maintains temperature and prevents condensation corrosion. Choose closed cell foam sleeves that limit corrosion and water hammer issues.

Stopping Movement and Vibration

To prevent pipes from knocking around, try securing them properly:

Secure Loose Pipes with Clamps and Anchors

Use vibration damping pipe clamps or anchors to firmly fasten loose pipes to framing. This stops them from vibrating and knocking when water turns on.

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