Vacuum Relief and Venting For Buffer Tank

[velvetfoot]

I thought it'd be a good idea to put an air vent at the top of the 120 gallon buffer tank I'm installing for filling and air removal. The manual mentions that some localities require vacuum protection so that the tank doesn't collapse. The tank has an extra 1.5" connection on top. I got a Taco air vent and some bushings, but now I'm thinking I should have a vacuum breaker as well. Would something like this do the trick: http://www.supplyhouse.com/Watts-0556030-1-2-LFN36M1-Lead-Free-Water-Service-Vacuum-Relief-Valve

 

[velvetfoot]

Or, maybe this one:
http://www.supplyhouse.com/Zurn-34-VR10XL-3-4-VR10-Vacuum-Relief-Valve-MNPT-Lead-Free

Yes, this could be the ticket:
http://s3.supplyhouse.com/product_files/Zurn - 34-VR10XL - Submittal.pdf

 

[velvetfoot]

The question is, how would I pipe the air vent and vacuum breaker?

 

[maple1]

I can't see vacuum being an issue - but maybe it could be somehow? Wouldn't autovents let air in in that circumstance? Something I hadn't thought about before I guess...

 

[velvetfoot]

I was initially thinking that the autovent would, but now, I don't think so. If it sees air, because the float goes down, then it opens. That float may not go down-maybe the $2000 tank collapses like a tin can. I wonder if a vacuum relief valve might not also protect the rest of the system too, if no isolation valves were closed-that would protection for a non-dumb-ass action, like a leak. For a dumb-ass action, far more likely, I can see it. One more thing to leak or require maintenance, along with that autovent.

 

[Clarkbug]

If the buffer will have an air vent, I'm assuming its pressurized like the rest of your system.

I'm not sure how you would pull a vacuum on it without having a number of other issues in the system first.

But I may be missing something.

 

[velvetfoot]

Oh, there'll be issues all right. If there's a big leak, or you drain the system without venting, something like that.

 

[Clarkbug]

I guess of there was a vac breaker and an air vent, I would just consider an atmospheric tank... since its almost the same idea. Or a diaphragm expansion tank.

Otherwise its fittings to let air out and to let air in....

 

[Bob Rohr]

I guess of there was a vac breaker and an air vent, I would just consider an atmospheric tank... since its almost the same idea. Or a diaphragm expansion tank.

Otherwise its fittings to let air out and to let air in....

No need for that valve, how could a vacuum be pulled on the tank?

Also if you have a float type air vent, and the fluid drops, the float drops and the valve will open and allow air in.

Unless someone has screwed the cap down

 

[Clarkbug]

No need for that valve, how could a vacuum be pulled on the tank?

That was sorta what I was thinking Bob. You just have a better way of putting it!

 

[velvetfoot]

No need for that valve, how could a vacuum be pulled on the tank?

Also if you have a float type air vent, and the fluid drops, the float drops and the valve will open and allow air in.

Unless someone has screwed the cap down

I've done a literature search, and I haven't found an air vent that claims double duty as a vacuum breaker.
Would a drain opened on an unvented tank cause vacuum?
I guess if atmospheric pressure won't collapse the tank, then no problem.

Edit: I'm confused. The Froling manual says:
"Vacuum breaker (if required)
Install a vacuum breaker (or vacuum relief valve) for water heater
protection. A vacuum breaker prevents siphoning of the water from the
system and collapse of the water heater."

This tank has a coil inside the tank for dhw-it's heated by the buffer water. I wonder if that wasn't just some generic language.

 

[Clarkbug]

If you have a float valve for air relief, when there isn't water on the float, it will open, so technically that's a vented tank.

Most DHW heaters don't have air vents since you would just flush it out of the system when you open the tap.

But really the vacuum breaker is a backflow preventer so if you turn on two taps at once, you don't suck hot water back into the cold water lines.

Its not to prevent the thing from imploding.


EDIT: the Internet's may prove me wrong. If connected to a municipal system with no pressure reducing valve or main check, a drop in street pressure could put the water heater under vacuum, risking failure. This according to the terry love forums.

 

[velvetfoot]

I should explain more. This tank is 119 gallons: 108 buffer and 11 gallons in a 1.5" corrugated stainless pipe inside the tank that is connected to a well system for dhw. The well is dug to 450'. I wonder if the check valve would fail down in the well what would happen? I have no idea, but if anything, I'd think the stainless pipe would survive, but the electric water heater could be at risk. Maybe nothing would happen?

Anyway, thing that I'm thinking would cause tank failure here is unvented draining of the tank water, for whatever reason.

 

[velvetfoot]

Oh sure, just a little interweb reading has me thinking the well tank will collapse upon failure of that check valve, apparently called a "foot" valve.

 

[jebatty]

Interesting question. I have the standard float-type air vents. When I first set up my system for heating, the pressure dropped to 0 or below in the summer when the 1000 gal tank cooled down to about 70F. When I fired the system back up in fall, I noticed air being discharged by the vents. Something, somewhere, allowed air to enter the system to balance, that is, no vacuum. Since, added more water to the system and added expansion capacity to maintain pressure with a cooled down tank and never an issue again.

 

[velvetfoot]

Apparently, it does happen,

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[maple1]

The bit more I think about this, the more I am thinking that more damage could be done by loosing a whole bunch of water into the basement quickly from a freely running leak that was facilitated or helped along by a gadget that would let air in the top as the water went out, than would be if it was sealed and the vacuum created stopped the water from running out or at least slowed it way down.

If I explained that right.

I would think if the vacuum built up enough, it would eventually suck air in where the water was leaking out. Like sometimes happens when you realize you forgot to open the vent cap on a 5 gallon gas jug after you get it tipped up where you want it and need a third hand to open it.

 

[velvetfoot]

I see what you mean, but eventually might not be fast enough.
But, why introduce something that will eventually fail if it's not necessary?
Being able to vent the tank automatically might be a good thing when draining the tank (hopefully that shouldn't occur too often).
They're relatively cheap; I might put on on the cold water entrance as well. The tank is expensive.

 

[maple1]

I guess I would likely rely on what the tank manufacturer recommends or deems necessary.

I have a ball valve at the top of my storage tanks - I burp a little bit of air out of it once in a while, and if I ever need to drain my tanks I'll open it to let the air in for draining. And I also opened it when I filled them.

 

[velvetfoot]

No air vent?

 

[velvetfoot]

"Vacuum breaker (if required)
Install a vacuum breaker (or vacuum relief valve) for water heater
protection. A vacuum breaker prevents siphoning of the water from the
system and collapse of the water heater."

I'm thinking that's a generic water heater comment, more related to what's going on with the potable water supply.

 

[maple1]

I have 3 air vents, but not at the top of storage. They are at points a bit higher, the main one that does most of the work is in my supply manifold just before the zone valves. Have another one at a high point on my electric boiler loop just above it, and a third at a high point on my DHW loop.

I do still want to make a bit of a change at the top of storage where that ball valve is. It might be to add a fourth vent there - or I might add another small expansion tank to it, mounted up high in the joists. Haven't decided yet. I have gotten caught a couple of times with a bit of air gathering there - which is also just above where my supply header pulls from. Ended up with an airlocked upstairs zone. Don't get much air there anymore, but sometimes it doesn't take much air to make an airlock in a zone either. (Not sure how that air made it past my main vent & scoop, but it did...)

Another one of those hindsight is 20/20 things...

 

[velvetfoot]

My tank is only 108 gallons. I was initially only going to put a ball valve and and air vent up there, but then this collapsing thing got me spooked. I'm sure it's a lot less substantial than a propane tank and also big buck. I'll try calling the distributor.

 

[velvetfoot]

Contacted distributor (Tarm). Very helpful. No vacuum breaker needed on boiler water side. Maybe on domestic water. May as well put one in now while installing. No ball valve needed on top of tank, just auto vent.

Well, I guess I can return some more stuff (at least it's within the 30 day no-restocking fee time limit.

 

[Willman]

Vacuum breakers are required if one is on "city water" versus well. reason being if fire pumper trucks hook up to hydrants they can and will pull a vacuum thru the piping and implode hot water tanks. could possibly affect boiler buffer tanks as well. Thats the way it was explained to me by a code official on a "city" job I did.