stone rule - no auto vents in an ac exp tank?

Jim Erhardt_3

An importnat point to keep in mind is that there are 2 distinctly different ways of dealing with air in a hot-water system: air collection vs. air elimination.

The "old style" compression tanks are part of an air collection system. Two important points to keep in mind about these systems:

1. Do NOT install automatic air elimination

2. DO install a compression tank fitting in the tank

These systems are designed to collect any air that comes out of solution. When the compression tank "goes empty" from collecting air, use the tank fitting to bleed out some of the air. The tube is positioned in the tank that when water begins coming out when venting air, the tank is filled to the correct level.

Another VERY imprtant function of the tank fitting is that it provides a thermal trap to prevent circulation between the tank and the system. This cicualtion will eventually waterlog the tank by allowing the "air saturated" water in the tank to fall back into the system, be heated and then deposit the air that comes out of solution into the radiation.

Installing an automatic air eliminator in this sytem defeats its purpose and will only cause problems - usually in the form of a water logged compression tank.

Systems that use pre-pressurized bladder-type expansion tanks are air elimination systems. In these systems, an air separator with vent is mandatory as the tank itself cannot collect air, only accept water due to thermal expansion.

[Deleted User]

stone rule - no auto vents in an ac exp tank system?

or at least that's what i keep reading, bec they'll cause the exp tank to log. but at this new acct, the glycol system has 6 spiros, and we haven't heard of any history of complaints. been in operation for yrs. might it be bec the system has a b&g hydraulic sep?

paul_79

vents

is it a steel compression tank or is it a bladder expansion tank? We had nothing but touble when someone put auto vents on the boilers that had steel compression tanks the vents leaked and also made the tanks water log. the guy who put them on thought it had to have vents not knowing any better. when the boiler was down the last time, the water pressure in the system was down and the pumps where cavitating . water was pouring out of the float valves. i pulled all of the auto vents off. and tried to explain to the maint man not put them on.

[Deleted User]

steel tanks

that's what i meant by ac, air cushion, exp tank

[Deleted User]

As Doc Radiant has pointed out...

If there is an Airtrol fitting on the compression tank, it is doing its job and not allowing the air to migrate out of the compression tank. If it stops doing its job, and the auto vents see the free air, they will expel it and you will eventually end up with water logged tanks, and leaking relief valves.

Generally speaking, if there is a perceived "air problem", auto vents are sprinkled thoughout the system, when in reality, its really not an air problem, but more of one of the location of the main circulators in relationship to the expansion tanks connection to the system. Once that is addressed, the alleged "air problem" disappears magically...

But as Jim has pointed out, if it is a system using compression tanks, it shouldn't have automatic air eliminators, and if it is a diaphragm expansion tank it SHOULD have air eliminators. Not to say that it can't be done as you are seeing, but if something decides to head south, the whole system may follow... Best keep a close eye on it, or valve off the Spiro outlets and only open them on occasion to let out any air they've inadvertently accumulated.

ME

Jim Erhardt_3

I haven't been called that....

...in a long time! At one time, Mark's handle here was "Guru of BTUs." I think it still applies. ;-)

To add to what we've pointed out here, a link worth visiting:

http://www.bellgossett.com/Press/BG-proper.asp

heatboy_26

Man........

.........Doc Radiant. History, my boy, history!

Jim Erhardt_3

HB,...

...I hope all is well in your neck of the woods!

[Deleted User]

yeah doc, you're citing textbook

and that's what i've always believed; textbook and one experience. but as i said, another 'reality' is staring me in the face, so now i have to ask; how is this possible when it goes against what's known from another 'reality'?

glycol/exchanger/boiler system; 1 pump, 1 non-bladder tank with a bg tank fitting, a b&g hydraulic separator, 6-7 keyed bleeders cu tubed down from the highest piping, and 6ish spiros with ball valves but not valved off. and no reported history of logging

is it bec the glycol isn't heated to boiler temp, but instead only 'warmed' and used in heat pumps?

Mark Eatherton

BINGA

Fluid temperature has a LOT to do with air in suspension. Or the lack of air i suspension.

Hence the answer to your puzzle.

ME

Unknown

Got me...

I thought it meant an Air Conditioning system ie chilled water, I'm thinking "OH NO!! I just put one of those in!!" Haha

[Deleted User]

fluid temp - air in suspension

got it. but i'm wondering what the science is behind the temp making the difference. as in, without a bladder, how can more water get to the tank and log it just bec it's hotter water and is being auto-vented? the fill valve isn't open, so the water had to come from somewhere and create a void somewhere else, no? btw, it's 3:25am, and i woke up with this water log in me head

Jim Erhardt_3

Air letter-inners....

Air eliminators can quickly become "air letter-inners" if not placed correctly in the system. Or at least, they can be ineffective.

In my experience, one air separator properly located in the system - at the point of least gas solubility (highest temperature, lowest pressure)- is all that is needed. In all cases, this will be on the supply header upstream of the circulator, with the fill valve piped in to the bottom tapping of the air separator. In air collection systems, the top tapping of the air separator is piped through an Aitrol fitting to the compression tank. In an air elimination system, the top tapping of the air separtor is fitted with an automatic air vent and the expansion tank is piped into the bottom tapping with the PRV.


IMHO, multiple auto-air vents/eliminators (not including manual vents on radiation) are simply an invitation to more air problems.

[Deleted User]

Air letter-inners.... yes, that's the

other case that i was refering to. and i told the guys, via everything i read, to get rid of them, repipe the tank and use one manual eliminater. they did. problem solved

but on this glycol/heat pump system, each hp has a set, supply/return, of open, b-valved spiros on ts at the top of the piping. and being glycol, i'm guessing that the water supply fill valve is off. i'll have to check it tomorrow during rounds

[Deleted User]

Oxygen and water...

As, again Doc Radiant pointed out so eloquently, oxygen comes out of suspension in hotter water, under the lowest pressure. If the water is cool, and under pressure, it (oxygen) won't go in to nor out of the solution as easily.

It can be entrained (i.e. glycol charging pump coming back through screen causes oxygen to go into solution and act like sodium bicarb in suspension. Billions of microbubbles.), but for the most part, it doesn't want to go into or out of suspension under the conditions in which you are running.

In EVERY case, where I have seen HOT water (greater than 90 degrees F) in use with a conventional compression tank, AND there were auto vents in place, the expansion tank became water logged (even on systems with an airtrol fitting on the tank), and then caused the relief valve to do its thing, which then caused the make up valve to do its thing, and it just gets uglier from there.

Without fail.

ME

Unknown

Stop it,,,

you're scaring me!