Heating System Expansion Tank Information
This does not apply to domestic hot expansion water tanks.
Decades ago we used non-diaphragm style tanks. At this time there were no automatic air vents yet. There were devices with dip tubes to get air free water from the boiler. The chamber around the dip tube would trap the air in the boiler and send it to the expansion tank. The old steel expansion tank was known as an air control system, and a maintenance free tank.
At some point the automatic air vent was invented. This was a good and bad thing at the same time. The good thing was the introduction of diaphragm style expansion tanks requires automatic air vents. The diaphragm style tank is known as an air elimination system so the air removed from the system must be vented to the atmosphere.
The two terms I used above were air control and air elimination systems. The difference is the old-style tanks, known as air control systems, should never have automatic air vents installed anywhere in the system. Nitrogen is a big percentage of our air and a smaller amount is oxygen (20.9%). The oxygen will cause a minimal amount of corrosion of ferrous metals and leave nitrogen in the tanks. For simplicity when comparing the two systems I am still going to use air as a reference throughout the rest of the article. As water heats it expands into the expansion tank. When the water in the tank cools it will absorb some of the air in the tank. As the water in the tank continues to cool it becomes heavier and ends up down in the boiler. The circulator will send it around the system. When the air finally gets back to the boiler it is directed back to the expansion tank. If the air gets vented due to automatic air vents the tanks will become waterlogged and require draining. Whenever the tank is drained the system will add fresh water which includes oxygen and minerals. This becomes detrimental to the boiler and system. Eliminate the automatic air vents and eliminate the need to drain your expansion tank.
Bladder Type Tanks
The bladder type of expansion tank is known as the air elimination system. Any air in the system must be eliminated through an automatic type air eliminator. The downside of the bladder style tank is the diaphragm is oxygen permeable. This means the air charge in the tanks allows the oxygen to enter the water system over time. As this happens the pressure in the tank will drop and require adding a charge. Most contractors will use a bicycle pump or small air compressor to bring the pressure back to the proper pressure. Most, if not all manufacturers have moved to nitrogen charges instead of air charges for this reason.
The purpose of the expansion tank or compression tank is to absorb the expansion of the water as it is heated up. When you raise water temperature you raise water pressure. Since water is a non-compressible the air charge in the expansion tank acts like a pillow and compresses as the water is pushed into the tank. This keeps the pressure in the boiler from rising to high and blowing off the pressure relief valve. As you may have figured out the amount of air in the tank is important in controlling water pressure.
There are times when air charge gets too low and the pressure rises dramatically when the boiler runs. Too much air is not much of a problem. The old ceiling mounted tanks when drained properly by allowing air to enter while draining, will allow the tank to take on air at atmospheric pressure. The drain valve is closed and the water valve is reopened the tank fills to the appropriate pressure, controlled by the pressure reducing valve.
The bladder type tank has to have the air charge checked and adjusted periodically. The air charge must be equal to or a couple of pounds above the system fill pressure.
Let's discuss in more detail the bladder type tank as this is most commonly replaced with the new boiler.
Air elimination Systems
The bladder type tanks today have a diaphragm between the water and the air charge. They are usually mounted on the air separator with an automatic air vent on the air separator to eliminate the air thus air elimination system. They may also be installed with a micro-bubbler type air eliminator to get rid of the air in the hydronic system. The water side is open to the heating system. There is an air Schrader valve on the bottom of the tank to check the air charge. When installing this tank, the air charge must be checked before water is added to the heating system or at least before the tank is charged with water. The air charge must match the system fill pressure and up to 2 lbs. over is OK. If you need to operate above 12 psi as this is the standard air charge raise the air charge to the higher water pressure. For example, if you are heating a three-story building you will normally need 18 psi to get the water to the third floor. That means before you put water in the system raise the air charge in the tank to 18 - 20 psig.
Once the tank is installed and the system is filled the tank should have semiannual maintenance to verify the air charge is at the proper level. The tank air charge cannot be checked with water pressure on the heating system. The water pressure must be at "0". The system radiation type and water volume dictate the tank size. Manufacturers will have charts to size the bladder type tanks. When the tank is properly sized, and the air charge is correct the pressure should not change but a few pounds as the system heats up from a cold start to 180f.