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How Expansion Tanks Work | HVAC Hydronics

The difference between a steel expansion tank and a bladder type expansion tank.

How Expansion Tanks Work: What is an expansion and how does it work? I get that question all the time even though I have written an extensive article on expansion tanks.  The two most common expansion tanks you will see in HVAC applications include the bladder type expansion tank and the steel type expansion tank.  The steel type expansion tank needs some basic maintenance from time to time to ensure it doesn’t become waterlogged or overfilled which would defeat the purpose of having an expansion tank. For more on the basic maintenance of a steel expansion tank and how to properly fill it check out this article.

How Expansion Tanks Work

How does an expansion tank work and why do you need one in your boiler or chilled water system? If you have a hot water boiler system (not a low-pressure steam boiler - there is a huge difference between the two) or a chilled water system (not many chilled water systems in residential but I have seen them) then you have a closed loop piping system that runs to either convectors, radiators, or a coil or two somewhere in the system where the main heat exchange process takes place. This is a closed loop where the water runs to these convectors or coils to provide heating and/or cooling. Furthermore, it is important to maintain the proper pressure inside that closed loop piping system. An expansion tank help to maintain the proper pressures in the loop.

Inside this piping is water (sometimes a water/glycol mix but that is mainly in commercial applications) and when water heats and cools down it expands and contracts. The piping system along with the parts of the boiler or chiller that have water cannot handle this expansion and contraction with breaking or busting open. Therefore in every hydronic system, there is an expansion tank to handle this expansion and contraction. Furthermore, an expansion tank is basically a shock absorber that absorbs this expansion and contraction of the volume of the water.

How Expansion Tanks Work | Maintenance

Hot Water Boiler Expansion TanksIt is critical that expansion tanks receive periodic maintenance. For bladder type expansion tanks maintenance should include:

  • Test the pressure charge on an annual basis. As the system ages, more frequently. This should be done by a professional as there are certain procedures to follow when checking the charge and recharging the pressure in the event it is not at the proper charge.
  • Check the tank for rust or corrosion.
  • Lastly, ensure tanks are properly mounted and installed the proper location. The tank should be supported to prevent stress on connecting piping.

Steel Type Expansion Tank maintenance should include:

  • Test of the level in the tank to ensure it is not waterlogged or overfilled. This can cause problems with the loop and cause the pressure relief valve to vent unnecessarily.
  • Check the tank for rust and corrosion.
  • Lastly, ensure tanks are properly mounted and installed the proper location. The tank should be supported to prevent stress on connecting piping.

How Expansion Tanks Work | Conclusion

Again, these checks should be done by a professional to ensure the proper procedure is followed. In conclusion, by performing periodic maintenance your system will offer you reliable heat throughout the heating season.

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How Expansion Tanks Work

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