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Heating System Expansion Tank Information
Expansion tanks for indirect water heaters use this link
There are two type of tanks. The bladder type which hangs on the heating pipes and the older steel tank which hangs between the floor joists. They work completely different. One is called an air control system and the other is an air elimination or air removal system.
The purpose of the expansion tank or compression tank as it may also be called, 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 time when the 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 each type of expansion tank starting off with the bladder type tank as this is most commonly replaced with the new boiler.
Bladder Type Tanks
Air elimination Systems
Extrol tank by Amtrol..... Extrol Tank website
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 microbubbler 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" psig. 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 180ºf or where ever the high limit is set. If the pressure changes more than a few pounds during this heat up period either the tank is too small or the air charge is low.
Here are charts from two different manufacturers. These charts are OK to use with standard residential systems but not for commercial systems, steam to water conversions or old gravity hot water systems. Use the worst case scenario when sizing a tank. If the whole house is finned tube baseboard and the garage has one cast iron radiator use the cast iron radiator column.
Watts Company Help in sizing a Watts expansion tank
Extrol by Amtrol Home page Help sizing an Extrol tank
Plain Steel Tanks
Air control systems
These type of tanks have an air charge and work an the same basic principle as the bladder type tanks. The air acts as a pillow for the expanding water giving the water something to press against that will compress. The major differences are no bladder between the water and the air charge and no system automatic air vents. This type of system controls the air. Since there is no bladder the air will mix with the water and come out of the tank during a cool down period. The air will move around the system and end up in the air separator and back into the tank. The bubbles are mico-bubbles so you will not hear them in the system. Keep in mind even though we always say air charge in this tank it is really, after a short period of time almost all nitrogen which does not cause any problems in the system like oxygen would do. Directions on how to drain the tank.
As long as the air charge is good there is no problem with these tanks. Contrary to belief these are maintenance free tanks. As long as there are no automatic air vents in the system this tank never needs to be drained. The problem when the tank needs drained is the air charge is not sufficient enough to support the amount of expansion from the water as it is heated. Where did the air go? It worked OK for years. 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. I would suggest the air vents be eliminated and drain the tank so you know it has the proper air charge. You will not need to drain the tank anymore.
I would also like to address the proper piping of the steel tank. There will be a valve to isolate the tank from the boiler/system piping. The valve should be a full port ball valve or equivalent not a stop and waste valve. This valve should only be installed on a vertical pipe not a horizontal pipe. Any horizontal pipes should slope uphill at least 1/8" per foot. The piping between the boiler/system and the tank should be 3/4" even if the tapping's in the tank and air separator are 1/2".
I am a firm believer that an air separator should be used with either style tank. There are special air separators used with he plain steel tanks. They will have 1/2" or 3/4" holes for the piping to the tanks. Watch the manufacturers recommended distance of horizontal piping from an elbow. Below are a couple of these air separators.
B&G IAS 1" - 3" Watts AS-B Air Separator 1" & 1-1/4"
Proper piping of steel tank
When piped properly and no automatic air vents this tank should require no maintenance at all. For even better air control in the tank I would suggest the installation if a B&G airtrol fitting.
This works kinda like a trap to keep the air in the tank. It will improve the tank application especially in low water temp applications. The air control system steel tank makes for a maintenance free operation and does not require annual air charge maintenance or tank draining.
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