Pipe Knocking when Hot Water Turned On Causes

The sudden loud knocking from pipes when hot water starts running can be alarming. These knocking noises seem to appear out of nowhere, leaving homeowners bewildered about what causes pipes to erupt with racket when hot water flows.

While this phenomenon may only last a few minutes, persistent pipe knocking should not be ignored. The vibrations indicate issues needing attention before causing extensive damage.

What Makes Pipes Knock with Hot Water Activation

Several culprits might be behind pipes rattling loudly when hot water begins coursing through them:

• Trapped air vibrating as it expands
• Water hammer effect from quick valve closure
• Loose pipe straps and fittings knocking about
• Mineral buildup and corrosion rattling loose

As hot water from the water heater flows into cold pipes, the temperature change causes vibrations and pressure fluctuations. Air bubbles trapped in the pipes rapidly grow, water hammer slams valves, and loose fixtures clatter.

These phenomena happen so suddenly when we use hot water that it translated into a racket of knocks and bangs emanating from the pipes.

Air Pockets Create Knocking Noises

Small air bubbles enter the plumbing system during installation or develop over time as water flows. The air remains trapped in pockets within pipes and water heater chambers.

When hot 120degF+ water begins flowing after tapping the faucet, cold 50degF pipe walls abruptly expand. That rapid temperature change simultaneously causes air pockets to heat up and violently expand.

As these air pockets swell they slam against pipe interiors creating vibrating pulses triggering loud banging noises. The louder the knocks, the larger the trapped air bubbles rattling about.

Water Hammer Slams Pipes

Water hammer causes thumping sounds resembling a hammer banging on a pipe. This hydraulic shock happens when high-velocity water flow suddenly switches on or off.

Quickly shutting off a faucet valve creates a pressure spike forcing water to instantly stop, its momentum launching a shock wave. That shock wave travels rapidly through the pipes until hitting an obstruction like a loose fitting or elbow bend.

A pressure wave up 5000 PSI slams the obstruction unleashing violent vibrations and the loud pipe knocking sound.

Rattling Pipe Straps and Fittings

Another common source of pipes knocking when hot water starts relates to loose fittings and straps securing pipes.

As hot water expands metal or plastic pipes up to 4 inches longer, the rapid temperature change physically moves the pipes. If loose straps or wall studs fail to hold them firmly, pipes will rattle loudly against frames and studs.

The same goes for drain pipes secured underneath homes and secured by loose metal straps or silicone rings too wide to keep PVC drainage pipes from vibrating.

Sediment Knocking Loose

Hard water rich in calcium, magnesium and silica deposits leave mineral sediment accumulating inside hot water pipes and heaters over time.

As higher flowing hot water slams this gritty buildup, chunks shake loose and rattle loudly through pipes. Opening the sink valve hot dislodges this mineral sludge and corrosion knocking about.

Knocking pipes when turning on the hot water will continue until pipes are cleared of this sediment buildup.

Finding Source of Noisy Pipes

Figuring out what section of pipework the knocking noise emits from helps trace its underlying cause.

Follow these tips to isolate the source:

• Determine if from hot or cold line: Turn on just cold water to see if knocking persists.
• Check supply lines last point: Trace from faucet valves back down routes towards water heater.
• Access and inspect fittings at knock points: Look for loose strap hangers, corrosion, leaks.
• Consider nearby appliances: Dishwasher, washing machine and refrigerators with water lines should be inspected.

Process of elimination by room and checking pipe routes helps zero in on the exact knock source. It also reveals if coming from drainage pipes underneath or within wall voids.

Getting to Root Causes

After identifying the section of pipework rattling, take a closer look for these likely causes:

• Bulging pipes: Can signal interior corrosion and mineral buildup ready to break free and rattle about.
• Black sediment: Mineral deposits and corrosion visible at connections is knocking loose inside.
• Pinhole leaks: Weak points created by trapped air bubbles or corrosion buildup.
• Fitting gaps: Check if compression fitting nuts or strap hangers are loose.
• Valve wear: Inspect washing machine and dishwasher water valve for leaks and cracks.

Addressing these underlying problems requires getting to the pipes in question for a visual inspection from all angles.

Fixing Loudly Knocking Pipes

Several methods help quiet pipes rattling loudly every time hot water starts running:

Install Water Hammer Arrestors

To protect pipes from high-pressure shock waves install water hammer arrestors. These mini-shock absorbers install near quick closing valves on supply lines.

Inside is an air cushion and piston which compresses to absorb hydraulic shock neutralizing loud vibrations and banging.

Secure Rattling Pipe Runs

Look for gaps allowing movement in areas where pipes knock. This might be at stud penetrations or hanging strap connections to joists.

Tighten any loose nuts on compression fittings securing pipes. Ensure hangers and straps firmly grip pipes with insulation preventing any play or vibration when hot water flows.

Address Loose Wall Studs

Where pipes penetrate wall studs inspect for any play allowing vibration transfers through walls. This requires opening drywall to access stud bays for inspection and securing.

Use rigid metal ties around pipes and torque wall plate screws into solid framing to stop transferred knocking noises.

Insulate Pipes

Applying closed cell pipe sleeve insulation stabilizes temperature fluctuations and muffles pipe vibrations.

Securely wrapping cold and hot water pipe runs prevents spare shrinkage and expansion behind knocking noises when hot water flows.

Replace Valves and Fittings

Inspect valve stems, seat washers, and rubber gaskets for wear. Cracks and pitted surfaces on valve components can trigger water hammer.

Swapping out a high use valve for a new quarter turn model with smooth operating parts will give water flows a clean start and stop.

Preventing Future Pipe Knocks

Completely eliminating pipes from knocking again when hot water activates takes some preventive measures:

Update Old Pipe Lines

If consistently knocking in the same areas, repiping with modern PEX or copper lines will remove risks of bursting from temperature fluctuations.

New efficient water heater units also help regulate hot water flows and pressures reducing strain on old pipes.

Water Treatment System

Hard water leaves mineral deposits inside pipes and heaters needing periodic drain flushes.

A whole home water softener prevents further limescale accumulation keeping water flows smooth and sediment free.

Insulate Pipes and Heaters

Ensuring all hot and cold pipe runs have a consistent layer of closed cell foam insulation balances temperatures.

This prevents expansion contractions behind vibrations and banging fits when hot water flows start.