Toilet Flush and then loud pipe knock

[jlalum01]

I have a toilet on ground floor level that when flushed first thing in the morning causes the pipes to knock real loud just one time. After that any time the toilet is flushed the pipes do not knock. Why is this? Is it water hammer? a loose pipe or something else. If I reduce the water flow at the supply valve in the basement will this correct the problem? Our residence is on the lowest part of our street. Please advise. thanks.

[LazyPup]

I am assuming the bang is occurring after the toilet is flushed and the water tank refills. One or two loud bangs the instant the water fill valve in the toilet shuts off. If that is the case it is water hammer.

In order to understand why it only occurs in the morning we need to discuss what causes water hammer.

When all the valves and faucetts in the structure are turned off the pressure throughout the structure is in equilibrium with the supply pressure, this is called static head pressure.

The Venturi Prinical states that when the velocity of a fluid, Whether liquid or Gasseus, is increased, there is a proportional decrease in pressure.

As the toilet fill valve opens flow begins (increase in velocity) which in turn causes a decrease in pressure in the pipe feeding the toilet valve. (The working pressure under flow is technically called the Dynamic Head Pressure.) Depending upon the size of the pipes in question there can be a substantial difference between Static Head Pressure and Dynamic Head Pressure.

When the tank if full and the float closes the valve the Dynamic Head pressure immediately jumps up to the Static head pressure. The sudden increase in pressure causes a physical shock in the pipes that can cause physical damage to the piping system. In order to prevent water hammer all structures are required to have water hammer arresters on both the hot and cold water systems.

A water hammer arrestor is simply a short lenght of pipe usually 12" to 18" long installed veritcally at a high point in the system with a cap on the top of the pipe. When the piping is first installed all the pipes have air in them. As the water enters the piping system the air in the vertical risers is trapped inside and forced upwards. Air will compress while water will not, therefore as the water fills the system the trapped air is compressed upwards until the air pressure is equal to the water pressure, while still allowing a small space at the top of the pipe. When flow begins the pressure in the pipes will drop and the air in the hammer arresotor will expand. When the flow stops the water pressure immediately rises and the air will again compress to absorb the shock.

Over a long period of time the air in the water hammer arresters can be absorbed into the water, which then carries the air away a little at a time until there is no longer a sufficient amount of air to absorb the shock.

Now, why does it only occur in the morning? Because the demand for water on the municipal main is lowest at night, therefore the pressure in the municipal line is greater at night or early morning than what it would be during the day when everyone in the neighborhood is using water. Being on the low point in the neighborhood, you are also at the highest pressure area on that municipal line, therefore the difference between Static Head Pressure and Dynamic head pressure would be much greater at night or in the morning.

Fortunately the solution to you problem is very easy. You need to restore a full charge of air in the water hammer arrestors. In order to do so, begin by turning the water off at your "Main water shutoff Valve" which is located on your main water line where it enters the structure. Next, open a faucett at a low point in your water system. Since you have a basement the lavatory in the basement or a laundry sink would be at the low point. (for structures on slab construction an outside hose bibb is the low point). Now open the hot and cold water faucetts at a high point to permit air to vent into the pipes. The high point would be the bathroom lavatory on the highest floor in your structure.

After you water system has drained down, close the faucetts at the high and low points and open the main water valve to refill the system. During the filling operation the air trapped in the pipes will refill the water hammer arrestors. You will then need to open all faucetts to bleed off the excess air. Expect a bit of sputtering as the excess air vents out. As soon as you have full flow, shut that faucett off and go to the next one.

Depending upon the water quaility conditions and the size of your water hammer arrestors you may need to repeat this operation about once every 3 to 5 years.

[shepherddd]

getting hammered

i tried the sequence to refill the air -
but this did not work

1 toilet still hammering - and this still appears to happen when the
flapper snaps shut after refill.

i lifted the toilet lid - flapper attached via chain, and let it run a few seconds longer then replaced the top + flapper and there was no knocking that time.

so it appears that the quick drop of the flapper is trapping air

I have 2 other toilets exactly the same - without problems.

ideas ???

[Redwood]

if the flapper is adjustable then change where it closes or, change out the flapper.