Why the Hole in the Relief Valve Cover?

A Necessary Hole

Many have not noticed the hole on the relief valve cover until it starts to either slowly drip out of it or streams out like a water pistol being shot at your face. Other times you will notice the calcium that builds up around the hole indicating it has leaked, but it may not be at that time. Some think that it is a defect in the manufacturing process. It is not a defect, but it is a necessary hole to allow the relief valve to function.

If there was not a hole there the relief valve would not be able to open or perhaps close depending on the situation and where the relief valve was positioned, because there would be a vacuum inside the valve.  I am not sure if vacuum is the right word or not but the result is the same; the relief valve would not be able to move.

Why do Some Relief Valve Covers Not have a Hole?

This may cause some of you to realize that not all relief valve covers have a hole.  Why do those work?  Good question…for someone with a better understanding of hydraulics, and vacuums, etc than me!  So I asked Apollo why theirs works without a vent hole.  Their relief valve, and I would assume others without the vent hole, use an internal stem riding on a lubricated o-ring and centered by a guide outside the stem.

009M2 1 1/2″ Cover with vent hole shown in the center.

The ones with the holes in the relief valve cover are ones that the cover serves as the guide for the top portion of the stem and has an o-ring on that stem.  I can think of one exception that does not have a hole in the relief valve cover which is the Watts 1″ 009M2.  In my opinion what they did is ingenious.  Instead of venting it out of the relief valve cover they put a hole all the way through the center of the relief valve stem from top to bottom.  They have not done this in the other sizes.

So Why Does it Leak  Out the Vent Hole?

975XL/975XL2 RV Cover with vent hole in center.

In my experience there are three reasons that it leaks out that hole.  It may be one reason or a combination of them.  First, the o-ring that fits around the stem portion that goes into the cover has a nick or tear in it or is completely broken.  Second, the plastic piece the o-ring sits on is cracked or broken allowing water to go around the o-ring and out the hole. Or third, the cover where the o-ring should be sealing is pitted or corroded and not allowing the o-ring to seal or slowly cutting away at the o-ring.

Most commonly the o-ring is the issue followed by the top of the plastic stem that the o-ring sits on.  Some stems may be brass that the o-ring sits on and they can go bad but usually not as likely in my experience.

One interesting thing about that 1″ 009M2.  Sometimes you will see a stream coming out of the relief valve opening in a small steady stream, like a squirt gun being shot at you.  That is because the hole coming out of the relief valve stem is about the same size and there is enough force going through it to resemble the squirt gun being shot at you.  I suppose the downside to this type of design is that if it is just a slow dribble it may look like another issue, however do your troubleshooting and when you determine that the cause is the relief valve look closely at the top of the relief valve stem guide, o-ring and cover to see if that is the issue.

What Kit do I Need?

So what kit do you need?  Contractor’s approach this differently and I won’t say which is the better way.  You will have to determine that.  Do you take it apart and inspect it and determine what you need?  Do you take everything you might need?  Do you just take a rubber kit for the relief valve?  These are all questions you need to determine for yourself.  I think you can make some assumptions of what you will need.  How much is coming out of the vent hole and how it is coming out the vent hole?  Is it dribbling or coming out with some force behind it?  But you still will not know for sure until you open it up.

Other considerations does it look like from build up around the outside of the hole if this is an old or recurring problem or did it just begin?  Are you familiar with the water quality and what the inside of this particular backflow or others on the same site look like inside?  Meaning if they tend to be green and corroded inside or if it is an old problem you might need a cover.

I hope those of you wondering about why the hole and why leaking out the hole have a better understanding of how the relief valve functions and how to resolve the leaks out the vent hole.  If not, give me a call and we can chat about it.


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