# Why does my water hammer arrestor ask for 5psi less than house pressure and not 10 psi or 20 psi?

I have Watts LF150A water hammer arrestors attached to the hot and cold supply valves for my washing machine (my LG washer is crazy about the water hammer). The LF150A instructions say to add pressure in the valve that is 5 psi less than the house water pressure.

Can someone explain - preferably like I'm a 5 year old - why 5 psi is the preferred amount? What would happen if it were 10, 15, or even 20 psi less than the house pressure? How much less effective would the arrestor be? At what psi would the valve stop arresting?

What would happen if there was more pressure than the house pressure?

My house pressure is 65psi and I'm really curious about the physics of this. Thanks!

• Well I guess you did not the kid level as requested. Mar 8 '21 at 22:50
• The tire analogy was appreciated but there were parts of the question left unanswered in your reply. The accepted answer with the comment "and since the elasticity does not depend on the water pressure (but perhaps water temperature), a membrane arrestor is set with a constant offset" was simple, succinct, and covered any questions I had about different PSIs. Thanks. Mar 9 '21 at 2:32

Today's arrestors separate water from the shock absorbing gas (air) by means of a piston or membrane.

Separation was not always the case. Arrestors were also once implemented by vertical capped copper stand pipes pre-filled with air. Unfortunately the air pocket's absorption capability would eventually fail as the pocket filled with water over time.

Let's compare the piston with the membrane.

The gas (air) chamber of a piston arrestors is set to about the water pressure during flow, and it is placed near the "culprit" valve (washing machine etc...). The hammer or shock absorption is accomplished by movement of the piston and back pressure from the trapped air that compresses behind the piston.

The required absorption depends on the kinetic energy of the flowing water, which is determined by the volume of flow. This in turn depends on the water pressure and the diameter of piping and valve.

Arrestors are therefore sized based on flow, and thus pressure and diameter. Buy the right sized arrestor for your case; pretty easy to pick for a home appliances, and requires a bit calculation to size for back to back washrooms, rows of urinals etc...

The gas (air) chamber of a membrane arrestor is set lower to account for the elasticity in the membrane and still achieve the same shock absorption.

And since the elasticity does not depend on the water pressure (but perhaps water temperature), a membrane arrestor is set with a constant offset.

• "And since the elasticity does not depend on the water pressure (but perhaps water temperature), a membrane arrestor is set with a constant offset." Thanks for this! Mar 8 '21 at 17:35

The airspace in the arrester is creating a shock absorber just like on your car but I will use a bicycle tire to bring it to the kid level.

With the pressure a few psi below the actual the piston can move and absorb the shock of the water hammer.

If the pressure is above the water pressure the piston will be locked in place and not absorb the water hammer it would be like riding a bike with 120 psi air in the tires - you feel every grain of sand or rock you ride over.

If the pressure is too low in the arrester it’s not going to respond on the opposite side of too full - kind of like riding a bike with a flat tire the tire rolling around on the rim and not absorbing the shock

With the air just right at 5 below the arrester absorbs the shock of the water hammer like a bike tire rolling over a stone - you barely notice it.

• Thanks, the bicycle tire analogy makes sense as I ride different types of bikes and have inflated or deflated my tires based on conditions. If my water pressure was 125 psi and I set the valve to 120 psi, that's a 4% decrease. If my water pressure is 35 psi and I reduce it to 30, we're looking at ~14% decrease. If 14% less pressure is effective at 35 psi, why wouldn't 14% be effective at 125 psi? I would think it would allow more room to absorb the hammer, but I'm sure there's an equation used to calculate this. Mar 8 '21 at 16:22
• The 5 psi is a general rule of thumb for pressure tanks. Using a slightly different pressure may improve your results it’s not hard to try. If a answer is helpful we get our thanks from up votes and or accepting the answer the check mark. Accepted answers help others with a similar question find the accepted answer. Mar 8 '21 at 16:28