Automatic Air Vent and an Old Steel Compression Tank

MrCofDG

Just read Dan Holohan's "The Problem Solver" podcast transcript, and also saw Ed's follow-on comment -
EBEBRATT-Ed Member Posts: 9,410
September 21
Amazing how many jobs you still see with auto air vents and a steel compression tank. A lot of guys in the field don't know that it's wrong

Yeesh, I'm one of those guys.....had a Spirovent installed a few months ago - and my system has an steel compression tank. I had never read/heard that this is wrong.
What specifically is the drawback of this ??

Jamie Hall

Real simple. That steel compression tank has to have air in it to function. That's where any air in your system is supposed to go, and there is a little valve on the tank which lets air out if there is too much -- which there almost never is.

By putting a Spirovent on your system, you've made another way for air to get out -- and unfortunately, the air in the tank will gradually dissolve in the water (it does) and then eventually go out the Spirovent. With time, that results in no air in the tank, and the tank can't absorb system pressure changes.

At which point, you will probably find the boiler relief valve opening pretty much every time the system heats up, and then when it cools off again the autofeeder will add more water.

End result -- system will rust out really fast from all the fresh water and you will have water all over the basement.

May I ask why the Spirovent was installed? If those compression tanks are functioning, they pretty much last forever. If they've lost their air charge, the airtrol valve may have failed, but the tank hasn't. Solution to that is to close the valve on the line from the system to the compression tank, drain the tank, close the drain valve, open the valve from the system to the tank and you're done.

If for some reason you choose to keep the Spirovent, you will need to install a properly sized bladder type expansion tank. It should connect to the system a few pipe diameters upstream of the circulating pump inlet.

MrCofDG

I had the Spirovent added just because for years I'd been reading/hearing that a properly installed boiler system should have an automatic air removal device; as per my OP, I'd never heard/read that an auto air vent shouldn't be added to a system with an old steel compression tank.

My steel compression tank (> 40 years old) which has no airtrol valve (just a drain valve), has just started to leak from bottom. Aaargh. So it looks like I'll need to remove it and add an modern bladder type expansion tank.

EBEBRATT-Ed

@MrCofDG

I mean if you od tank isn't leaking you can keep it and just replace the leaking valve underneath it. While the system is drained install a valve under the Spiorvent.

Then you can use the spirovent to help get the air out of the system when it is drained and refilled to be worked on and then shut the valve off and keep it off

or

Put in a bladder tank but you will have to install an air scoop with it and that will mean a little repiping and then put the spirovent on top of the air scoop.

Why not post a few pictures so we can take a look

MrCofDG

To be specific, it's a Spirovent Junior Air Eliminator, and the installation/operation instructions show it on top of a diaphragm expansion tank, i.e., diaphragm expansion tank would be screwed into/onto bottom of the Spirovent Junior Air Eliminator.

JUGHNE

I service a school that has a huge 80 gallon compression tank, it would water log every season.
I then learned here about not having air vents on the system, removed them and the problem went to maybe every 3 seasons.

A tank this large has a sight glass on the end of it, then realized that the top sight glass valve was leaking a small amount of air.
Fixed that and have not had a problem since doing that maybe 5 years ago.

EBEBRATT-Ed

@MrCofDG

When you said air vent I assumed you had a spirotop air vent, If you have a spirovent jr with the tapping for the bladder tank and if it's installed in the right location your good to go.

In that case what shoud be done is a ball valve on the bottom of the Spiro vent JR then a tee under the ball valve. The bottom of the tee you mount the bladder tank and the cold water make up goes in the branch of the tee. Are your pumps located on the supply or return? Pumping into or out of the spirovent jr.Makes a difference

jumper

Didn't there use to be a fitting for old style tanks that helped with this issue?
Some larger buildings used compressed nitrogen to pressurize expansion tank.
And what is so bad about draining a tank once in a while?

MrCofDG

Speaking for myself, I have drained the old compression tank over the years.
My immediate 'pickle' is the new leak in that tank, as mentioned above.

Ed, to answer your last question, my Spirovent Junior Air Elimntr is in a horizontal supply pipe in the basement near the boiler; the circ pumps are on the returns.

EBEBRATT-Ed

@MrCofDG

It's better to pump away from the expansion tank but a lot of older systems worked ok pumping towards it.

JUGHNE

Jumper, as far as draining a tank once in awhile:

First the tank gets water logged, relief valve is dripping/dribbling. HO knows nothing about it, call HVAC company.
They come out and replace relief valve, soon the same thing happens.
HVAC comes back.....must have been a bad new valve....replace it.

Eventually some HVAC determines the compression tank is bad, adds a bladder tank.

That is the maybe the best scenario.

What I have seen happen is the HVAC guy bleeds the second floor rads of air.
They were acting as the expansion tank.
The relief valve drips and is replaced. The new relief valve is now dripping again,......the helper comes out to plug the relief valve opening.
That takes care of the dripping.
Now one of the rads starts to drip water from a section nipple, because of high pressure developed,......the relief valve was not dripping anymore.
No one ever looked at the compression tank.


The HO are certain they new a new boiler, they call someone else,
they did not need a new boiler. But they did need a good chimney which was not possible with the house structure.

So after several thousand dollars they now have a modcon boiler side wall vented.

That can be what is so bad about draining a tank once in awhile.

Besides adding fresh water to closed system.

jumper

I have to agree with JUGHNE. Young techs do not know HHW.
At least not the ones sent to single family homes.
Of course if homeowner educates himself somewhat and is willing to do just a little....

>>Jumper, as far as draining a tank once in awhile:<<

WMno57

I had a rusted out 70 year old tank one year ago. Had to endure all kinds of crap from "Pros" and Supply House geniuses about how a bladder tank is better, and I'm just a stupid homeowner.
I drove to Wisconsin and bought a new steel compression tank from Quality Tanks Inc.
WWW.qualitytanksinc.com

Paid my guy to pipe it up. Would have done it myself, but my 58 year old wrist and elbow is worn out. BAM! Done. Should be good for another 70 years.

ajc72

You can pipe the air outlet of the Spirovent into the expansion tank, so removed air goes back to the tank. This has worked for us in the past.

JUGHNE

That 80 gallon compression tank in the schoolhouse was installed in 1961, so that puts it at a mere 60 years old. Although it ASME tagged, don't know if thicker steel or just expensive name plate.

Don't know what to do if it leaks someday, hanging on the ceiling.

Jamie Hall

Get a bucket?

jumper

JUGHNE said:

That 80 gallon compression tank in the schoolhouse was installed in 1961, so that puts it at a mere 60 years old. Although it ASME tagged, don't know if thicker steel or just expensive name plate.

Don't know what to do if it leaks someday, hanging on the ceiling.

Flextape? Epoxy? Lead?

jumper

And try to hide ASME bird.

JUGHNE

I said IF it leaks someday, it is still holding water.
All the radiation and boiler on the same floor. The pressure on the system is minimal.

I know of another one about that size that is, in the attic above the boiler room in a nursing home, about the same age.
No room in the boiler room for a floor model if needed.

Jamie Hall

Is there space somewhere up above the floor where everything is otherwise located? If so, @JUGHNE , you could go really old fashioned (and utterly reliable!) and pipe in an expansion reservoir up above somewhere, vented to the atmosphere. It wouldn't have to be all that large --just enough to keep it from running out when the system is cold, and big enough to absorb the expansion. The free expansion coefficient for water is only 0.00021 per degree Celsius, so in going from 0 Celsius to 100 the water will expand by 0.021 -- or to be more useful, 1 gallon of water at 0 Celsius will become 1.021 gallons at 100.

So you would need to have a volume of about 2 gallons for a 100 gallon system...

Surely you could find room for that somewhere up above?

JUGHNE

As of now both old tanks are still holding their water.
I was just making conversation, although any ideas are good to keep in mind.

I am supposed to be retired, well semi-retired anyway.

Hopefully the tanks last beyond my full retirement....if that happens.

EBEBRATT-Ed

@JUGHNE

If it ever leaks and it may not in our lifetime (how much hot water steel pipe have we all cut apart that's 80 years old that looks like new on the inside of course the tank does have oxygen in it) Put a floor mounted bladder tank and pipe it in. Where you put a bladder tank doesn't matter it can be set on the floor. And a bladder tank will not have to be as large as a compression tank.

I am not an advocate for either one. I like bladder tanks better but they don't seem to last. I have seen ASME bladder tanks fail

Jersey2

I would drain my 16 gallon expansion tank once every two years or so. Like others said, air dissolves into the water and over time air gets lost from the tank. Mine was hard to drain because it didn't have a valve that let air in, so as water ran out, air blooped in from the same garden hose. This year I noticed a small leak in the 65 year old tank, so I decommissioned it and installed a 30x bladder expansion tank. The old tank was too heavy and high for me to take out so I just left it up in the rafters for now.
Responding to the post about a water logged expansion tank possibly leading to a new boiler, If a hvac man did not understand about expansion tanks and changed the relief valve instead of checking to see if the boiler's pressure gauge showed high pressure and if the tank was water logged, they are in the wrong business.

Jersey2

JUGHNE said:

That 80 gallon compression tank in the schoolhouse was installed in 1961, so that puts it at a mere 60 years old. Although it ASME tagged, don't know if thicker steel or just expensive name plate.

Don't know what to do if it leaks someday, hanging on the ceiling.

Maybe you can disconnect it and install three 30x bladder expansion tanks in parallel, equivalent to a 90 gallon non-bladder tank? They are small and don't need to be up high.