Air trapped in household plumbing is a relatively common problem, especially in older homes. Trapped air is one of the causes of noisy plumbing, ranging from a vibrating sound to repetitive jack-hammer noises. Once air gets trapped in the circuit, especially in higher pipes, apart from draining the system and reconnecting the water supply with open faucets, it can be difficult to eliminate.
Apart from trapped air, noisy plumbing can be caused by a phenomenon known as water hammer. If you close a faucet quickly and hear a loud bang followed by repetitive but diminishing banging noises, it’s probably water hammer or “hydrostatic shock.” The weight of water rushing through pipes generates high velocity and momentum. In addition, water is incompressible. When a faucet or appliance valve closes suddenly, water momentum causes the stream to crash against the valve. This causes cavitation and a small vacuum to occur downstream, pulling the water back momentarily and then releasing it back against the valve. This continues in diminishing bangs until the movement stops. Water hammer only occurs when a faucet or valve is closed suddenly. If you close the valve slowly without causing the banging noise, the problem is definitely water hammer.
On the other hand, if banging occurs when a faucet or valve is opened suddenly, the noise is probably caused by air trapped in the pipes. This confined air could be a long way from the faucet, collected in pockets that are highly compressed under water pressure. Suddenly opening a valve or faucet releases the pressure like an air gun going off, causing banging against closed faucets and appliance valves. If you open the faucet slowly, you may feel air coming out of the spout for a second or two before water starts to flow. If this happens without banging occurring, the problem can be attributed to trapped air.
Both air and water hammer can cause pipes to vibrate and rattle, especially in older residences. In addition, expansion and contraction by pipes heating up or cooling down can cause a weird scraping or rubbing noise at tethering points. This happens when pipes are not tightly secured to walls or ceilings. In extreme cases, constant rattling over long stretches of time can cause damage to valves, fixtures and pipes.
If you’ve diagnosed trapped air as the cause for noisy pipes, flush the system. Turn off the household water supply shutoff valve situated on the main pipe, just downstream of the water meter. Connect a hose pipe to the lowest shutoff valve or faucet in the home and lead it out into the garden or the nearest roadside rainwater drain. The lowest drainage point is often the drain cock mounted on the bottom of your water heater, but it could be a garden faucet protruding from the side of the house. Turn the lowest valve open to start draining the system, then open all faucets including garden stand pipes to prevent a vacuum forming. Disconnect the water hoses leading to your washer and other major water-fed appliances and place buckets under the outlets. Once you have drained the system, open the mains shutoff valve and work your way from the lowest point to the highest point, shutting off all water outlets systematically as soon as water flows out unimpeded and without blowing out air. Alternatively, correct water hammer by following the instructions that came with the devices and install two or three water hammer arresters in separate areas close to the outlets most affected by water hammer. Correct rattling pipes by systematically strapping or tethering the pipes to studs, beams and floor joists throughout the system.
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