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When turn on water it comes out at first with so much force it will knock cup out of your hand then its normal after using but let it not be used for awhile and happens all over again

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Only that one faucet? Hot, cold or both? –  Steven Apr 3 '14 at 0:19
Was it always like this, or has this started recently? Have there been any changes to the plumbing (new water heater, new water meter, etc.)? Does the house have an expansion tank (A metal container smaller than a 5 gal. propane tank, likely near the water heater)? –  Tester101 Apr 3 '14 at 10:06

1 Answer 1

This is a symptom of over pressurization from a closed system (check valve somewhere in the line) and a hot water heater. The check valve could have been introduced by the municipality as part of a water meter replacement, or it could be located near the water main shutoff. These prevent water from flowing back into the municipal supply should there ever be a drop in pressure.

When the system is closed like that, using the hot water causes the tank to fill with cold water. Then when the faucets are closed, the heating of the water causes the pressure to rise. This will happen on both the hot and cold sides since the water heater is connected to both.

The standard solution is an expansion tank. These tanks are a couple gallons in size and attach anywhere in the system to give water somewhere to go. They contain a bladder that can rupture over time, and they need to be pressurized to match the water pressure in your home. If the bladder ruptures, then there is no longer space for water to expand and you'll experience the same symptoms you're seeing. Other symptoms include leaking toilets and a leaking TPR valve on your hot water tank.

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