Vacuum Relief Valve... Revisited

[Clog]

VRVs. That's how I found TerryLove online.

Vigorous debate and discussion between Moderator HJ and a member with a longer multi-syllable name that began with C. (forgot full user name as I type this, sorry).

First post here, and I first want to take a moment to thank Terry Love for creating and hosting this bulletin board space to discuss best plumbing practices, and for permitting the comingling of professional and diy'er alike.

Now on to the topic: Vacuum Relief Valves. (VRV) Not required in my jurisdiction. Not indicated for my single story installations where the top fill type hot water heater tank (HWHT) is on the same floor level as all the fixtures it serves. But in the spirit of following best practices in everything I do, I am motivated to build beyond minimal requirements, with installs that are beyond reproach, and that would pass inspection in any community.

There are a number of questions that I was not able to distill answers for from in my reading thus far:

1. If a HWHT would implode from vacuum, why wouldn't a thermal expansion tank? (TET)

2. If a TET would implode from vacuum, does preventing that implosion, and the HWHT implosion, dictate the inline location of, and the relative height of, the VRV vis a vis the TET? It is very clear that the VRV needs to be higher than the HWHT, but does the VRV also need to be higher than the TET, especially if the TET is higher than the HWHT, if the TET is also expected to be protected from implosion?

3. Does it matter if the TET is plumbed inlet down or inlet up, for purposes of preventing implosion, if implosion is indeed even a concern? It is clear that TET plumbing orientation is not a factor in the function of the TET, so this question is solely focused on the protection of the TET from implosion, if that is a risk.

4. In my planned installation, the TET would be plumbed with inlet pointing downward, to promote more circulation of the water that heated expansion pushes into the tank. As the water cools down (relatively speaking) the water in the tank, pushed by the air bladder set to the same psi as the previously measured supply pressure, might be more readily circulated into the system, rather than being encumbered by gravity "trapping" the same stagnant water in the tank if the inlet were pointing upward.

5. However, when considering trapped air, vacuum, and tank implosion, my mind gets clogged. Hence the user name. Further more, it is unclear to me how far the bladder in the TET will distend toward the water inlet in the absence of any water, if the TET were pressurized to 55 psi (the last measured supply pressure). Will the air bladder balloon all the way to the mouth of the inlet, which would overwhelmingly overcome any partial vacuum in the TET from a sudden drainage of water? Or is the bladder limited in excursion, leaving the empty half of the TET where the water was suddenly evacuated from vulnerable to partial collapse?

I think that is enough questions for an inaugural post. Thank you in advance for your help in understanding these issues a little better.

 

[Terry]

I've mainly been installing the expansion tanks with the inlet down. Mainly because it works well that way with my brass fittings. I've done it both ways.
A vacuum relief valve is required here in some local areas for electric water heaters on a second floor or above. The worry there is more about burning the top element on a building drain down. They also prevent the collapsing of a tank water heater when the pressure becomes a negative. Some areas require the vacuum relief on all water heater installs.

I have never seen an expansion tank collapse. They are a much different shape. I have seen the rubber bladder go bad and the outer shell develop rust and then a leak.

Vacuum Breaker

 

[Sponsor]

 

[Reach4]

A thermal expansion tank cannot collapse with a vacuum because it is pressurized with air.

I don't think implosion would be the right word for a water heater or water softener inward collapse. To implode is to collapse inward violently. Now if you are getting more metaphorical ("the scheme imploded"), the implode works.

 

[Clog]

I'm trying to think of another verb besides implode or collapse... because I agree with you... implosion is far too dramatic to describe what little evidence exists of the risk.

Photos and verbal descriptions point to the angular orientation of the inlet and outlet nipples/heat risers as evidence of tank distortion. Yet I've seen the photo of the nipples canted inward, toward the flue, and to my simple mind I would think that means the tank bulged under pressure, rather than collapsed under vacuum. I would think that the nipples would have to be pointed outward, like a V shape, as an indicator of tank collapse, rather than the nipples being displaced into an A shape, which to me suggests tank bulging.

Notwithstanding, I don't need to understand how to interpret the evidence to be convinced to just go ahead and spend that little bit of extra money to incorporate a VRV in all my installs. I do, however, need to understand the principles of VRV function in the entire system, in order to install the VRVs most effectively in variable plumbing circumstances.

In the most immediate circumstance, from the cold water stub out of the wall, I am elbowing up first for clearance, then installing the ball shut off valve vertically. Above this ball valve, I have a brass tee mounted vertically, and above that tee will be the TET, inlet down, mounted vertically. The tank strap will help mitigate the leveraged weight on the stub (which is strapped in the wall, when I removed the sheet rock to install 5' feet of insulation, per local requirements).

The stainless corrugated HWH supply pipe will screw into the horizontal leg of the brass tee under the TET, and function as the heat riser loop down to the HWHT cold water inlet. And THIS is where the installation of the VRV question arises. (pun intended).

I can install a threaded tank tee directly to the tank nipple, and mount the VRV directly on top of that tank tee, and the corrugated inlet on the horizontal leg of that tank tee... which is a typical method I've seen in photos (since in 40 years of residential ramblings I've never seen a VRV installed on a HWHT in person in my area)

OR...

I can install a 2' tall riser on top of the tank tee, so that the VRV ends up at the absolute highest point in the entire plumbing system, including rising above the top of the vertically mounted TET, which itself is above the HWHT. I've seen photos of 2' tall risers under VRV's too, and am guessing there must have been a good reason for the plumber to go through the trouble.

At this point, since I've seen photos of VRV's installed both ways, I need to understand that reason before proceeding.

 

[Reach4]

Collapse, or collapse inward, works for me, and cave-in could work too.

I would say to follow the manufacturers directions.
https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/11945-source

Putting it where a leak will be noticed would be important too. Other than that, I don't think it would be very critical.

You might find this of interest: https://terrylove.com/forums/index....for-softener-filter-on-input-or-output.63102/ It is about softeners and wells. I have a softener and well and no vacuum breaker. Taking the risk.

 

[Clog]

I have seen the diagram on those instructions numerous times, but like many generic diagrams, not all situations are can be covered.

My limited understanding of air venting assumes that the air admittance valve should generally be at the highest point in the system it serves. That understanding is informed by the common examples I see with dishwashers, sprinkling system valve risers, pressure vacuum breakers, etc.

Therefore, it didn't seem as if the Watts diagram fully addressed the highest and best use of the valve where plumbing elements exist above the tank, and when I called Watts technical support for further guidance, their response was very nonchalant... any 'ol place above the water heater will do.

 

[Phog]

This isn't rocket science. The 2' risers in your above example are probably a result of some local code being very strictly interpreted -- code intended to prevent siphoning of contaminated water back into the supply plumbing system through the vacuum breaker. The air admittance function will work fine anywhere above the water heater just as Watts has said. Don't over think it.

 

[Reach4]

Pretend they gave that same advice in a thoughtful and solemn way.

What is the purpose?
1. to satisfy a rule?
2. to keep the WH from collapsing due to a vacuum?
3. prevent being sued?

The vacuum breaker right above the water heater would do all of these.

 

[hj]

1. The bladder ALWAYS keeps the expansion tank under pressure
2. Pressure is constant throughout the system so the expansion tank can be located anywhere.
3. I prefer to install the tank inverted so the weight is hanging from the piping rather than "sitting on it".
4. The direction of the tank has absolutely NOTHING to do with how the water will circulate in or out of it.
5. Depending on the resiliency of the diapraghm and air pressure, the bladder could fill the entire tank in the absence of external water pressure.
6. IMPLOSION is the correct term. If the imbalance between external and internal pressures is great enough the tank WILL "suddenly" deform. (there are many examples of storage water tanks, tanker trucks, etc. which have been destroyed when the contents were removed without opening the air intake vents).

 

[Clog]

hj... #5 was my exact question... how far can the dividing bladder distend... enough to fill the entire expansion tank?

 

[hj]

It depends on the bladder and the initial pressure, but usually, yes, it can fill the tank if the pressure increases, but also fill it the other way if all the air leaks out.

 

[Jadnashua]

Regarding a vacuum relief helping out, I think about it this way...take an empty water bottle, blow into it, it won't change shape much. Then, suck in, it will fairly easily distort. Your lungs aren't capable of much pressure or vacuum.

The tank is designed for pressure. The T&P is designed to open at 150psi, and the tank itself it tested for double that. It won't handle much of any vacuum. Even slight vacuum could stress the glass lining as the tank flexes.

 

[Reach4]

hj... #5 was my exact question... how far can the dividing bladder distend... enough to fill the entire expansion tank?

More air does not cause the diaphragm to distend. Too little air with high water pressure could distend the diaphragm.

 

[Clog]

Well it appears there is no consensus on the thermal expansion tank.

On the one hand, I interpret hj's comment to suggest that the air bladder diaphragm can, with a sufficient pre-charge of air pressure, and in the absence of water, potentially distend far enough into the space in the tank intended for water so as to keep the expansion tank pressurized enough to prevent any type of vacuum induced collapse.

On the other hand, I interpret Reach4's comment to suggest that the bladder diaphragm is too stiff to be persuaded to distend by air pressure alone... but the incompressible nature of water can mechanically distend the diaphragm. And THIS leads back to my original, and now as yet unanswered, question:

If water is suddenly evacuated from the expansion tank, and if the bladder in the tank is too stiff to distend enough to fill the space in the tank that could potentially now be under a slight vacuum from the rapid water evacuation, then can that part of the tank that is unsupported by the air pressure contained behind the stiff bladder, be subject to partial collapse under the vacuum?

In other words, due to the dividing bladder diaphragm, can half of the same tank be under pressure, while the other half be under vacuum?

 

[Jadnashua]

In proper operation, the pressure on both sides of the bladder in the tank are at an identical value. Since the bladder won't extend beyond the tank, inside of the expansion tank, there should never be a vacuum, at least on the dry bladder side. I don't think you'd be able to draw the bladder out through the inlet port.

 

[Reach4]

On the other hand, I interpret Reach4's comment to suggest that the bladder diaphragm is too stiff to be persuaded to distend by air pressure alone... but the incompressible nature of water can mechanically distend the diaphragm. And THIS leads back to my original, and now as yet unanswered, question:

If water is suddenly evacuated from the expansion tank, and if the bladder in the tank is too stiff to distend enough to fill the space in the tank that could potentially now be under a slight vacuum from the rapid water evacuation, then can that part of the tank that is unsupported by the air pressure contained behind the stiff bladder, be subject to partial collapse under the vacuum?

I never implied that the diaphragm was stiff.

Check this picture out... It is not detailed, but shows how the tank that is empty of water will have the diaphragm up against the inlet, whether the air precharge is 12 psi or 80 psi.

I don't know where you are coming up with this vacuum stuff.

The diaphragm is not a tight sheet. It has some bellows characteristics, and it is not stressed out when empty of water.

 

[Clog]

I get how a captive air tank works. I've installed them on wells, and with hot water heaters. But I don't know how far the bladder extends to the opening of any given tank in the absence of water and the presence of precharged air pressure pushing against the diaphragm.

In the past, I had never considered installing a Vacuum Relief Valve, as they were never required in my jurisdiction, and I've never found, in any home I've owned, inspected, or worked on, a Vacuum Relief Valve installed on a water heater tank. None of the local plumbing suppliers even stock vacuum relief valves. I've specifically asked each supplier if they carry the Watts LFN36-M1, or the Cash Acme VR-801, and all have said they can get them, but they are non stocking. They tell me no one asks for them.

So, where am I coming from with this vacuum stuff? From a place of ignorance. Being dumb about some thing (well, ok most things) I have to ask dumb questions. Without any one locally to talk to who installs them, and with a manufacturer installation diagram that only depicts the hot water heater tank, and no other aspect of the plumbing leading into the water tank, I have to ask questions when I see photos like this:

In the image above, the installer put the VRV on a riser, seemingly in an effort to position the VRV at the highest point in the system, or at least as higher than the drain point of the thermal expansion tank, which in the horizontally oriented position above, would be half way up the tank, where the opening is. Below that opening, water will remain.

 

[Reach4]

I get how a captive air tank works. I've installed them on wells, and with hot water heaters. But I don't know how far the bladder extends to the opening of any given tank in the absence of water and the presence of precharged air pressure pushing against the diaphragm.

All of the way.

 

[Jadnashua]

Water pressure changes at slightly over 0.43#/foot. If you setup the right conditions, the tank is higher than the outlet, it will create a vacuum, the value based on the height difference. In some places, it's fairly common to install a WH in the attic (I think that's kind of silly, but it happens). Open a drain down below, and you can create a vacuum in the tank. It's not as big a deal if the tank is on the lowest floor of the building since the elevation change usually won't be very large. Every once in awhile, you see a picture of a tanker car where the inlet was plugged when they tried to drain it...crumples like a tin can. They tend to be much stronger than a typical water heater tank.

Where I live they want a vacuum breaker. Keep in mind, should there be a problem in the utility water supply, those lines generally are below ground. You live on a hill, and the elevation change could be significant. A vacuum breaker will prevent damage to your tank under those unusual circumstances.

Mistakes and accidents happen. Safety devices get developed to help prevent problems. You may never experience that problem. You might, and it could get much more costly than if you'd taken those precautions. Think of it as insurance. If you can afford the loss or inconvenience, you may decide to gamble. Sometimes you win, sometimes you lose. LIve in the flatlands where everyone has their WH on the ground (or lowest) floor, they may not be common. Move to hilly or mountainous country, and you may find more of them, or where they install them on the upper level or attic.

Mine has a vacuum breaker. I live on a hill. Hopefully, I never have an issue, but I've done the best I can to prepare.

 

[Clog]

As a follow up, I ended up ordering a half dozen Vacuum Relief Valves (not Vacuum Breakers) to have on hand. I went with Cash Acme over Watts, simply because the Watt's 1/2" HG opening vacuum seemed more likely to open up (and possibly leak) in a transient phase (pulse, hammer, air pocket, whatever) sooner than the Cash Acme valves, which operate at 1" HG, and therefore seemed more likely to stay closed unless really needed. Based on the surface area of a typical 50 gallon water heater tank exposed to 1 atmosphere of pressure, 1" of HG vacuum is not enough to crack the glass liner, which is what I was concerned about.