Air hammering in my pipes.

[mcg6637]

I have a mechanical timer attached to an outside faucet for watering. Whenever the timer shuts off, I get very obvious hammering in my water pipes. I've had this timer for several months but this problem just started a month ago. This is about the third timer I've had over the years and this is the first time this hammering has occurred. I'm not sure what has happened to cause this, what I should do about it or if I need to do anything.

Any suggestions?

[LazyPup]

While you post states that you have air hammering in your pipes, this is not the case. What you are experiencing is "Water Hammer" and it is in fact a result of a lack of air in your pipes.

In order to understand "Water Hammer" we must first discuss water pressure. When all the faucetts and valves in your house are closed and the system is full of water the pressure at all points in the pipes will be equal to the pressure being applied from the supply, whether that supply is a home well pump or a municipal water main. That pressure is technically defined as "Static Pressure" and will typically range between 40 and 85psi.

The Venturi principal states that as the velocity of a fluid (either liquid or gasseous) is increased, there is a proportional decrease in pressure, thus when you open a faucett and flow begins the initial "Static Pressure" drops to "Dynamic Pressure" or more commonly called "Working pressure". In addition to the losses from velocity there is also additional losses from friction as the water rubs the pipe walls or works its way through the fittings. There is a third pressure loss called "Vertical static Head" which is literally the physical weight of the water in a vertical column. By example, let us assume for the moment that you have a municipal water supply and the water main is in the basement at floor level. Now let us assume that your outside hose bibb is on the side of the house at the top of the basement wall 7' higher than your water meter. A standing column of water weighs .434lbs/ft vertical therefore the vertical static head loss is 7 x .434 = 3lbs.

The actual line pressure at your faucett would be the original static head pressure minus the Dynamic Head loss + Friction head loss + Vertical Static head loss.

In addition to the pressure drops we must also understand that the flowing water has "Inertial Force". The law of inertia states that a body at rest tends to remain at rest, or a body in motion tends to remain in motion unless acted upon by an outside force.

When we open or close a common faucett such as a kitchen, lavatory or tub faucett it is a gradual movement which tends to throttle these forces and we usually do not incure water hammer, however, whenever a "Fast Acting Valve" such as your timer or a solenoid valve on the dishwasher, washing machine or an HVAC unit the valve action is instantaneous. This instant change in flow results in an instant change in the internal pressures in the pipe, which then sends a shock wave through the pipe. This physical shock will shake, rattle or cause a loud bang which we refer to as "Water Hammer".

If left uncorrected this repeated physical shock can cause fittings to blow out.

In order to prevent water hammer all water distribution systems are equiped with "Water Hammer Arrestors". In its simplest form a water hammer arrestor is simply a short section of pipe rising vertically with a Cap on the top end. As the water distribution system is initially filled with water air is trapped in the vertical riser. Air will compress whereas water will not, so as the water fills the pipes and presses against the trapped air in the riser, the air is compressed vertically until the air pressure is equal to the water pressure.

When a faucett is opened and the static pressure drops to line pressure the trapped air in the water hammer arrestor expands and the air constantly maintains equilibrium with the water pressure. When a valve is closed the rapid increase in pressure is instantly absorbed by further compressing the trapped air in the hammer arrestor. While this method is extremely simple it is not foolproof. There is a direct contact between the water and the trapped air in the water hammer arrestor and over a fairly long period of time the air in the hammer arrestor can gradually be absorbed into the water and carried away leaving the charge of air remaining in the hammer arrestor insufficient to meet the need. (Typically this absorption of the air will take 4 or 5 years although under some water quality conditions it can happen more often.)

The solution is very simple. Turn the water off at the main water supply, open both a hot and cold water faucett at a high point in the system to permit air to vent into the pipes, then drain the house distribution piping from a low point in the system, typically a laundry sink in the basement or an outside hose bibb. Once the water has drained out, close the faucett where you drained the system, then open the water supply valve and refill the system. Leave the upper faucetts open to vent off excess air until you have a full flow of water from those faucetts, then close the faucetts. You will then need to go to each faucett in the house and bleed off any excess air in the lines. During this process a new supply of air will be trapped in the water hammer arrestors which will restore them to proper function.

In recent years there has been a movement to get away from field manufactured whole house water hammer arrestors. Instead they are now using commercially manufactured water hammer arrestors that have a rubber diaphram in them to prevent the air from being absorbed into the water. The downside here is that the commercial water hammer arrestors have a very small air capacity so the codes now require that we must install a water hammer arrestor in the near proximity of all fast acting valve.

In your case, you stated that you have used the timers in the past with no problem so I would guess that you do have whole house water hammer arrestors and the draining and refilling process should resolve your problem, but it would also be a good idea to consider adding a manufactured water hammer arrestor at your timer location.



In many cases the amount of pressure drop can be very substantial. By example, while the International Residential Plumbing Code requires a supply static pressure of 40psi the actual working pressure (Dynamic pressure required at the fixtures is much less, by example a bathtub is only required to have a flow rate of 4gal/min @ 8psi.

[mcg6637]

Advice much appreciated

Just thought I'd let you know, if you're not a plumbing instructor, you should be. Even I understood what you said. And even better, it worked! No more water hammering. Learn something new every day! Many thanks!

[Phelps]

I concur.

There are master plumbers but I haven't heard of Ph.D. plumbers, but LazyPup would qualify I think. I'm sure even very good plumbers reading these boards are learning things.

[pushkins]

Ya know what I like the most ?

That there are far smarter people out there than me, like the newly appointed Dr. LazyPup.

[froddan]

Dr. LazyPup

I concur with previous posts:

Dr. LazyPup is not only a pro plumber, but the way he instructs and approaches us, the students, truly reveals intelligence, diligence, and compassion!

Again, thank you for your valuable contributions Dr. LazyPup!!