There are two types of expansion tanks used on hot water boiler systems. The steel expansion tank and the bladder type expansion tank. These expansion tanks are used to provide a cushion for the expansion of water when it is heated. Without this cushion of air the pressure in the boiler system would rise and exceed the setting on the pressure relief valve causing the pressure relief valve to open and discharge water to relieve the pressure. It is necessary to for an HVAC technician to understand how these expansion tanks work in order to properly troubleshoot hot water boiler problems which may occur as a result of an issue with an expansion tank. One of the first things to check on a hot water boiler system when the pressure relief valve discharges water excessively is the expansion tank.
Bad Air and Hydronic Air Lock
As the loop is filled for the first time and subsequently by the make-up water feed little tiny bubbles of air enter the system. These bubbles leech out of the water as time goes on and build up in the system in places where the air is not wanted, needed, and may cause problems with flow. It can cause hydronic air lock and will prevent the flow of water throughout the loop or in specific zones. It becomes necessary to purge this air when hydronic air lock occurs to restore heat to the zones or part of the loop affected. With proper air management and air control hydronic air lock can be eliminated or drastically minimized. Air inside of a hydronic system is necessary but only if the air is in the expansion tank where it provides a cushion for the water when the water is heated and expansion occurs. Depending on the type of expansion tank will depend on if the air is in contact with the water or not and the type of air management controls and air elimination and minimization procedures used to purge the bad air in unwanted places out of the hydronic loop and leave the proper amount of good air in the right place in the hydronic loop.
Steel Expansion Tanks in a Hot Water Boiler System Loop
Steel expansion tanks still exist today although many newer hot water boiler systems use the bladder type expansion tank. In a steel expansion tank the air the water touch each other while in a bladder type the air and the water are separated by a diaphragm. Liquid does not compress but air does compress so when the boiler system heats the water the water needs a cushion and the air, which can be compressed in the closed loop system, is compressed by the expansion of the heated water. The air inside the steel expansion tank acts like a spring and absorbs the extra pressure produced by heating the water in the boiler system.
Ratio of Air to Water inside the Steel Expansion Tank on a Boiler System Loop
The ratio of water to air inside the steel expansion tank is optimal at 2/3rds water to 1/3 air. Tanks not equipped with a site glass need an Airtrol fitting. This fitting is located on the bottom of the expansion tank and has an air bleeder on the bottom of it. This fitting has a dip tube which goes approximately 2/3rd’s into the expansion tank. This allows you to adjust the level of water to the 2/3rd’s optimal level. The expansion tank should be isolated from the hydronic loop before opening the vent on the bottom of the Airtrol expansion tank fitting. This means there should be a gate or ball valve in the pipe leaving the boiler system and running to the expansion tank. If the expansion tank is flooded with water the Airtrol fitting (sometimes referred to as a B&G Airdrop Expansion Tank Fitting) can be used to easily restore the proper air to water ratio inside the steel expansion tank after isolating the steel expansion tank from the loop. A site glass, which some steel expansion tanks are equipped with, can also be used to set the proper ratio of air to water inside the steel expansion tank. If the steel expansion tank is neither equipped with a site glass or an Airtrol fitting filling the tank to the proper ratio is a guessing game and the proper ratio of air to water inside the steel expansion tank will probably never be achieved.
Sizing Expansion Tanks in Hot Water Boiler System Hydronic Loops
The ratio of air to water inside the steel expansion tank is as important as having the proper size expansion tank. The sizing of the steel expansion tank is based on the size of the hydronic loop and hot water boiler system. The sizing of the steel expansion tank in a hot water boiler system is done before installation and is usually done by the HVAC hot water boiler system designer based on calculations of the size of the hot water boiler system and hydronic loop. If the hot water boiler system ran for more than a year with no problems and suddenly a problem arises where the relief valve is venting on a regular basis it is unlikely that the steel expansion tank size is the problem.
Automatic Air Bleeders and Steel Expansion Tanks in Hot Water Boiler System Loops
It is not recommended that steel expansion tank hot water boiler systems have automatic air vents in the hot water boiler loop however there are many hot water boilers system with steel expansion tanks that do have automatic air bleeders in the system at the highest point in the loop. The piping for the make up water on a hot water boiler system and hydronic loop equipped with a steel expansion tank should be directly below the steel expansion tank. Ideally the make up water is fed into the boiler system and the tiny bubbles of air in the make up water will rise and end up in the tank rather then in the hot water boiler hydronic loop. This will reduce prevent hydronic air lock. Because the air and the water inside the steel expansion tank touch, the water is apt to absorb some air when the water in the system cools. This is partly the reason why some steel expansion tanks flood with water and need to be drained. Steel expansion tanks with site glasses can easily be monitored for flooding simply by observing the site glass.
Bladder Type Expansion Tanks Sizing and Pressure Ratings
Unlike steel expansion tanks bladder type expansion tanks have a diaphragm which separates the water from the air. This diaphragm inside the bladder type expansion tank prevents the problems which occur with steel expansion tanks and the absorption of air into the water in the expansion in the hydronic hot water loop. The bladder type expansion tank serves the same purpose that the steel expansion does; it absorbs the extra pressure created in the hot water boiler hydronic loop when the water in the hydronic loop is heated. It is very important in a hot water boiler loop to size the bladder tank properly based on the static pressure of hot water boiler hydronic loop and the pressure of the hot water hydronic loop at the top of the hydronic loop. If an incorrect bladder expansion tank is used or the proper air pressure in the air side of the bladder expansion tank is not maintained then the relief valve will discharge intermittently. Pressure on the air side of the bladder type expansion tank should not exceed 2 to 4 psi of the water pressure setting at the pressure reducer valve. Most residential pressure reducer valves are set for 12 psi but could be different depending on the lift or height of the riser in the boiler loop.
Bladder Expansion Tank Sizing in Hydronic Loops
Always follow the manufacturer’s recommendations for sizing the bladder expansion tank and use the following formula to fill the air side of the bladder expansion tank to the required air pressure: This will be the static pressure needed to raise the water to the highest level and then a 4 P.S.I. (Fill pressure + 4 P.S.I. = air side of bladder expansion tank fill pressure). In most residential applications this will be 12 P.S.I. Note that it is important to isolate the tank from the hydronic loop pressure before filling air side of bladder expansion tank with air pressure.