How do you check a knock arrestor?

[SteelerEd]

I understand that a knock arrestor has a piston and O rings inside the chamber that compress the air above it to arrest the sudden spike in water pressure. If you remove the arrestor and push on the bottom of that piston, should you be able to feel it move inward and then return? Would that be a valid check to see if it is still working? I realize you could also maybe hear if it is no good anymore, but I'd like to know if this is also a test.

PS this would be in a home laundry..........about a $11 to $20 item.

 

[James Henry]

The spring most likely wouldn't fail it would be water leaking by the o-rings which would eliminate the air chamber. So no, I would say that would not be a suitable test.

 

[Sponsor]

 

[Weekend Handyman]

Not a Plumber. I would think weight would be a good indicator or if you can hear water sloshing around inside.

 

[LLigetfa]

Test it by replacing or comparing to a known good unit. I don't know how much pressure they are precharged to but suspect it would be too much to move the piston by hand. I doubt they have a mechanical spring.

 

[Reach4]

I think your plan makes sense, but I don't know if there is some pitfall, such as the bottom of the piston being too delicate, or the the path not being straight or big-enough.

Force required would be considerable I think.

 

[SteelerEd]

Well I got them yesterday and using a piece of roundstock to try to move the piston was not possible. I really pushed hard and neither one would budge. Makes me wonder how much pressure they charge them with. I'd say I pushed with more then 100 pounds. What is the spike on a typical copper tubing system on a residental system with washing machine solenoids?

 

[Reach4]

I would think maybe 60 psi. I did some looking, and I think that was the highest number I saw.

Now if we presume the cross sectional area is 0.5 square inches, that would correspond to 30 pounds of force to get started, and maybe 40 to get a noticeable displacement.

 

[LLigetfa]

I tested one I had laying around and it was too easy to depress the piston. In fact there was no precharge on it and the piston did not return all the way back.

 

[John Gayewski]

Water Hammer Arrestor Technical Information | Technical Resources | Zurn

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[SteelerEd]

Well, I have them on hand now so I'm going to mount them in a vertical position. If they fail I'll go back to the 2 foot copper tubing that I had before, only this time I'll put some valves on the supply lines. Maybe twice a year I'll just close them and drain the 2 footers to recharge them. That way I know it is working.

Would there be an advantage to using 3/4 or 1 inch pipe instead?

 

[Kreemoweet]

Would there be an advantage to using 3/4 or 1 inch pipe instead?

The bigger the pipe diameter, the greater the capacity to absorb shock, and the longer it takes to lose the air.
Most of these little plunger-type arrestors I've dealt with barely work when new, and fail completely soon after.

 

[Reach4]

Well, I have them on hand now so I'm going to mount them in a vertical position. If they fail I'll go back to the 2 foot copper tubing that I had before, only this time I'll put some valves on the supply lines. Maybe twice a year I'll just close them and drain the 2 footers to recharge them. That way I know it is working.

Would there be an advantage to using 3/4 or 1 inch pipe instead?

For your homemade water hammer arrestors, you would open the valves to drain the water, to admit air.

 

[John Gayewski]

If you look way back in the code when pipe was legal to use as Janet arrestors the sizing for them was ridiculous. They have to be huge to really be proper.

 

[SteelerEd]

For your homemade water hammer arrestors, you would open the valves to drain the water, to admit air.

OOPS .............I typed that wrong. Thanks for correcting that!!