Closed hydronic system with standard expansion tank and auto vents

[BronYrAur]

Just a sanity check here. Having auto vents on a system with a standard expansion tank is just asking for trouble, correct? Eventually the air will all purge out and you will flood the tank and pop reliefs, right?

With bladder/diaphragm tanks, on the other hand, you do want auto vents.

I'm looking at a remodel job where they are calling for the re-use of standard expansion tanks, but they are also calling for auto vents all over the place. I am going to bring it to the engineer's attention, but I want to double-check first.

 

[317069]

What do you mean by Standard tank?

 

[imok2]

When automatic air vents are installed on this system, the air enters the water during the cooling of the system water. When the next call for heat activates the boiler and circulating pump the air and water circulates through the system until it encounters an automatic air vent. The air comes out of the water and enters the air vent and gets vented out. Since the air is no longer in the system it cannot get back into the tank so the air charge becomes less and less as this continues to happen. At some point the air charge will not be enough to support the expansion of the water and the pressure gets high enough to open the pressure relief valve and you get water on the floor. Eliminate the automatic air vents and you will maintain the air in the tank and not have problems.

 

[TBP]

With a plain steel expansion tank - no bladder - you MUST use an airtrol fitting in the bottom of the tank. This separates the water & air with a copper tube, and the installer simply cuts it to the length required. The tube extends up, through the water, into the air space in the tank. If you don't install one of these, the water will gradually absorb the air over a year or two. At that point, the relief valves will lift every time the system cycles-on.

Auto vents shouldn't have any effect on the tank, as they primarily handle the air elimination on initial fill. The air separator & expansion tank handle the disolved air that comes in with the water.

The the air separator & expansion tank should be located between the boiler outlet and the circ pump suction. This puts the air separator at the point of highest water temp, and lowest pressure - which allows any disolved air to pop out of solution, and be directed to the expansion tank, where it will stay - pretty much forever. The make-up water connection should be located between the air separator and the expansion tank.

Bell & Gossett has some excellent literature on this.

 

[BronYrAur]

So would it be wrong for me to go back to the client and tell them that I think having auto vents on a system with a standard tank is a bad idea?

 

[TBP]

I don't think you'll have any problems as long as there's one of these installed in the bottom of the expansion tank.

http://completewatersystems.com/?product=airtrol-tank-fittings

 

[ChrisConley]

I'd recommend against auto-air vents on any system. We use one auto-air vent, in the mechanical room, working with the air separator. We use manual vents all over the place.

Auto-air vents spit. Depending on location, a spitting auto-air vent could cause a world of problems.