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Hot Water Boiler Expansion Tanks - 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 exceed the setting on the pressure relief valve. Finally, this will cause the pressure relief valve to open and discharge water to relieve the pressure.
Hot Water Boiler Expansion Tanks - HVAC Hydronics
It is necessary for an HVAC technician to understand how water boiler expansion tanks work. This understanding is important to properly troubleshoot hot water boiler problems, which may occur as a result of an issue with an expansion tank.
Additionally, 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.
If a hot water boiler expansion tank is waterlogged then you likely have a boiler pressure problem especially if the expansion tank is the steel type. More on types of expansion tanks below. However, it is important to understand expansion tanks and how it works if you are a boiler technician.
Bad Air and Hydronic Air Lock | Hot Water Boiler Expansion Tanks Maintenance
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. Furthermore, 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 create a hydronic airlock and will prevent the flow of water throughout the loop or in specific zones. Finally, it becomes necessary to purge this air when a hydronic airlock occurs to restore heat to the zones or part of the loop affected.
A boiler with proper air management and air control, hydronic airlock will be eliminated or drastically minimized. The air inside of a hydronic system is necessary. However, only if the air is in the expansion tank where it provides a cushion. A cushion for the water when the water is heated and expansion occurs.
Additionally, the type of air management controls and air elimination and minimization procedures are used to purge the bad air in unwanted places. Air will settle in areas such as risers in the piping loop. That is to keep the bad air out of the hydronic loop. Additionally, it leaves the proper amount of good air in the right place in the hydronic loop.
Types of Water Boiler Expansion Tanks
Hot Water Boiler Expansion Tanks
There are two basic types of expansion tanks used for hydronic applications:
- Steel type expansion looks like a long steel drum
- Bladder type expansion tank
For both of these types of expansion tanks, you can see a description and technical information below. Expansion tanks are used in hot water heating applications, chilled water cooling applications, and domestic water heater applications where pressure and water hammer are a problem.
Expansion tanks are used in residential and commercial applications for Hydronics including chilled water and hot water systems.
Steel Expansion Tanks in a Hot Water Boiler System Loop | Hot Water Boiler Expansion Tanks
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 and the water touch each other. Additionally, while in a bladder type, the air and the water stays separated by a diaphragm.
Any liquid does not compress. Air does compress, so when the boiler system heats the water, it needs a cushion. Furthermore, air, which can compress 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.
Water will expand and contract in the system, but not because of compression. Furthermore, water expands and contracts when heated and cooled. Basic physics. Any thermal water system, whether it is a water heating system or a cooling system in a closed system, needs an expansion tank for expansion and contraction.
The Ratio of Air to Water inside the Steel Expansion Tank | Hot Water Boiler Expansion Tanks
Hot Water Boiler Expansion Tanks
The ratio of water to the 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. Located on the bottom of the expansion tank is a fitting. It has an air bleeder on the bottom of it. The fitting has a dip tube that goes approximately 2/3rd’s into the expansion tank. Finally, this allows you to adjust the level of water to the 2/3rd’s optimal level.
The expansion tank needs to be isolated from the hydronic loop before opening the vent on the bottom of the Airtrol expansion tank fitting. That means there should be a gate or ball valve in the pipe, leaving the boiler system and running to the expansion tank.
Therefore, an isolation valve. If the expansion tank is flooded with water the Airtrol fitting (sometimes referred to as a B&G Airdrop Expansion Tank Fitting) is used to easily restore the proper air to water ratio. Finally, proper air/water ratio levels inside the steel expansion tank are restored after using the Airtrol device.
A sight glass, which some steel expansion tanks have, is also be used to set the proper ratio of air to water. The ratio of air/water inside the steel expansion tank. If the steel expansion tank is neither equipped with a sight glass or an Airtrol fitting, filling the tank to the proper ratio is a guessing game.
Additionally, the proper ratio of air to water inside the steel expansion tank will probably never get the proper fill.
Sizing Expansion Tanks in Hot Water Boiler System Hydronic Loops | Hot Water Boiler Expansion Tanks
Expansion tank sizing happens according to an equation in the mechanical codes. However, most boiler manufacturers will give you sizing charts based on the BTU/h output of the system. Additionally, another factor includes the type of convectors you have in the loop, such as copper baseboard or cast iron radiators.
The best sizing calculator, which I use personally when sizing expansion tanks, can be found at Amtrol.
The ratio of air to water inside the steel expansion tank is important. It is as important as having the proper size expansion tank. The sizing of the steel expansion tank is comparative to the size of the hydronic loop. And hot water boiler system capacity.
The sizing of the steel expansion tank in a hot water boiler system happens before installation. The Boiler System Designer sizes the expansion tank based on calculations. Calculations of the size of the hot water boiler 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 intermittently, it is unlikely that the steel expansion tank size is the problem. Finally, thermal expansion tanks do have issues from time to time, whether it is a steel tank or a bladder tank.
The hot water heating system needs an expansion tank to work properly and maintain proper pressure. As the hydronic heating system heats the water in the loop, the water needs a cushion storage tank.
That is for the increased pressure and volume of water. The system pressurizes, and the expansion of the water needs a place to go.
Automatic Air Bleeders and Steel Expansion Tanks in Hot Water Boiler System Loops
Hot Water Boiler Expansion Tanks
It is not recommended that steel expansion tank hot water boiler systems have automatic air vents in the loop. However, there are many hot water boilers with steel expansion tanks that do have automatic air bleeders.
Air bleeders are in the system at the highest point in the loop and on risers. The piping for the makeup water on a hydronic loop equipped with a steel expansion tank should be directly below the tank.
Ideally, the makeup water feeds into the boiler system. The tiny bubbles of air in the makeup water will rise. The air will end up in the tank rather than in the hydronic loop. That will reduce or prevent hydronic airlock.
Additionally, 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.
Furthermore, this is partly the reason why some steel expansion tanks flood with water; they will need draining. Additionally, steel expansion tanks with site glasses are monitored for flooding simply by observing the site glass.
Bladder-Type Expansion Tank Sizing and Pressure Ratings | Hot Water Boiler Expansion Tanks
As opposed to steel expansion tanks, bladder-type expansion tanks have a diaphragm that 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.
It is important to follow the manufacturer’s instructions when installing a new bladder-type expansion tank. Furthermore, most manufacturers require installation as close as possible to the suction side of the circulator pump.
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. Furthermore, some well systems have expansion tanks to absorb the pressure induced by the “well pump”.
It is essential in a hot water boiler loop to size the bladder tank properly. That will be based on the static pressure of the hydronic loop and at the top of the hydronic loop. If an incorrect bladder expansion tank is in place or the proper air pressure on the airside of the bladder expansion tank is not maintained, then the relief valve will discharge intermittently.
Pressure on the airside of the bladder-type expansion tank should not exceed 2 to 4 psi of the water pressure. Furthermore, the setting of the pressure reducer valve. Most residential pressure reducer valves have a setting of ~12 psi. Finally, this pressure could be different depending on the lift or height of the riser in the boiler loop.
Fill Pressure for Expansion Tanks in Hydronic Loops | Hot Water Boiler Expansion Tanks
Always follow the manufacturer’s recommendations for sizing the bladder expansion tank. Additionally, see above for proper sizing and a link to a calculator. Use the following formula to fill the air side of the bladder expansion tank to the required air pressure.
That will be the static pressure needed to raise the water to the highest level and then add 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 crucial to isolate the tank. It should be isolated from the hydronic loop pressure before filling the airside of the bladder expansion tank with air pressure.
How Does a Boiler Expansion Tank Work?
To learn how expansion tanks work, see our article on the subject. How do Expansion Tanks work?
Bladder Expansion Tank Charging Issues in Hydronic Loops
An uncharged bladder expansion tank will allow cold water to enter the tank when the loop water is cold. An overcharged bladder expansion tank will not allow water, when heated in the loop and boiler, to expand into the expansion tank.
Furthermore, if a problem exists with the pressure relief valve venting intermittently. Finally, the pressure in the bladder expansion tank needs checking for proper pressure.
Always remember when checking this pressure to isolate the system from the bladder expansion tank. It is best to charge the tank when it is totally disconnected from the boiler loop. Hopefully, the boiler installation crew added a ball or gate valve to the line coming from the boiler loop going to the bladder expansion tank.
Air Management and Air Control in Hydronic Loops - Preventing Problems with Expansion Tanks | Hot Water Boiler Expansion Tanks
Bladder-type expansion tanks also need good air management and air control to eliminate the air. Air migrates into the hot water boiler loop and air, which leaches out of the water. An air separator is necessary along with an automatic air vent to get rid of air.
It gets rid of air that comes into the hydronic loop and hot water boiler system from the make-up water. Air also comes from separation through heating and cooling. With steel expansion tanks, the air moves into the expansion tank. However, with bladder-type expansion tanks, the air must be eliminated and purged from the hydronic loop system.
The hot water boiler hydronic loop with a bladder-type expansion tank is a completely sealed and hermetic system. Therefore, when the air is introduced into the system by whatever means, it must be eliminated. If not, a hydronic airlock will occur in a hot water hydronic loop itself or a branch hydronic loop.
The airlock will prevent heating by creating a boiler circulation problem. It is thereby essential to have good air management and air control. Especially in hydronic hot water boiler systems which utilize bladder-type expansion tanks.
Expansion Tank Placement and Installation in Hot Water Boiler System Hydronic Loop | Hot Water Boiler Expansion Tanks
The pressure must be maintained in the hot water boiler system and hydronic loop. Maintained at appropriate pressure levels. Otherwise, expansion tank problems will occur.
For better performance and for reducing the problem of air absorption into the loop system it is best to install the bladder expansion tank just before the suction side of the circulator pump in the supply side of the hot water boiler system hydronic loop.
Because the bladder expansion tank absorbs the extra pressure created in the hot water boiler system in a hydronic loop, this is the area where no pressure change in the hot water boiler system hydronic loop occurs. That is the balance point or pressure of the hot water boiler system hydronic loop.
That is where little to no pressure change happens in the hydronic loop. It is also best to introduce the make-up water for the hot water boiler system and hydronic loop at a point close to the bladder expansion tank.
Fill Pressure - Heating System Expansion Tanks
Since the air side of the expansion tank is charged according to the fill pressure plus 4 P.S.I. Then the pressure reducing valve (PRV) is set to the same pressure as the bladder expansion tank air side pressure. Following this installation recommendation is optional.
However, for best performance, pressure control, and air management in the hot water boiler system hydronic loop, this installation scenario will offer the highest performance for the hot water boiler system hydronic loop. If the fill pressure is incorrect, you will experience boiler pressure problems.
If you are having problems with your pressure valve venting, the most common causes are either the pressure-reducing valves or the expansion tank. The system is over-pressurized by either too much pressure from a faulty PRV or a bad expansion tank.
Drain the expansion tank down and recheck the system. As a homeowner, if you are not comfortable doing this, please call a professional HVAC company or a plumber.
Especially with bladder tanks. Draining any expansion tank down and checking it for problems needs some knowledge and skill. The pressure-reducing valve can be found near the backflow preventer and needs the attention of a professional if it is faulty.
If this is your home heating system, you can check the pressure gauge to see how much the system is pressurized. 12 PSI is normal, and anything over 20 is suspect for a big problem.
Installation Checklist for New Systems
Note: Always follow the manufacturer’s guidelines for new installations. Additionally, ensure code compliance with new installations. Typically, the manufacturer’s instructions will supersede code.
That is a basic list I use before filling the loop for the first time.
- The expansion tank has been installed per the manufacturer’s instructions?
- Does expansion tank piping have an isolation valve for future service and maintenance?
- All piping has been flushed before introducing the expansion tank to the loop? (meant to keep construction debris out of the tank)
- A properly piped drain valve is located near the tank?
- The pressure reducing valve, based on fill pressure has been set?
- For bladder-type tanks, the pre-charge has been done and rechecked?
- All other components in the loop that will affect the expansion tank have been checked for proper operation?
How Do I Know if My Expansion Tanks Are Working Properly? Causes of Failure | Hot Water Boiler Expansion Tanks
If you are having problems with your boiler system you want to know the specific problem. With an expansion tank problem, you’ll likely see the problem in pressure issues. Your pressure relief valve will likely be venting. If you want to know if the expansion tank is working properly check:
- The pressure for the system. The pressure gauge should never be over 20 psi.
- The pressure relief valve or safety relief valve. If this is venting hot water then you likely have a problem.
If you have a bladder-type expansion tank check the pressure on the airside of the tank. If you have the steel-type check the water level. Finally, check the pressure reducer valve to make sure it has the proper setting (see fill pressure section above). Lastly, an expansion with holes in it is not good. Replace the tank.
Hot Water Boiler Expansion Tanks