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Expansion Tank for hot water heater

Can someone tell me how to replace an expansion tank?  It doesn't look that difficult but don't want any surprises.  I know they are pre-pressurized and someone told me they have water in them. 



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Posted 2012-01-14T06:40:28+0000  by thomasdunaj thomasdunaj


Hi, hello and thank you for joining our How-to-Community.


Thomas I’m assuming you are referring to a potable expansion tank; if this is not a potable tank please disregard provided info.


Let’s start with; you are right (that is a good start right :)  ), replacing an expansion tank it is not a difficult nor labor intensive project.


However choosing the right components and sizing the tank according to your system can be a little bit confusing.


First of let me  briefly explain, how does expansion tank work so you can better understand what you dealing with and to also eliminate any unwanted surprises with your installation.


Water expansion occurs when water is heated during non-use periods. Years back this excess water would simply flow back in to the city’s main system. However nowadays plumbing codes require a closed system (back flow preventer valves) to prevent water from flowing back in to the main system. Obviously with this closed system outlet needs to be provided for the additional volume. This is where expansion tanks come in place.


These tanks are designed with a flexible rubber diaphragm inside of the tank. This flexible diaphragm divides part of the tank for air and obviously part of the tank for water. See attached image;



Air pressure inside of the tank its set based on your water heaters volume and cold inlet (water) supply pressure (PSIG). Most manufactures pre charge their potable tanks at 40PSI (pounds per square inch) and limit their tanks at 80PSI. Exceeding this pressure (80psi) or whatever the manufacturers’ limitations can result in personal and property damages.




It is crucial to correctly size your expansion tank.

I know you said you are just replacing an existing (old) one but I would still advise to double check if one installed right now it’s the rights size for your water heater and cold water supply pressure in your home.

Over charged or undersized tanks have shorter life expectancy and on a side note this could be a reason yours is right now in need of replacement.


There is couple ways of determining size of the tank needed for your system.


But to make things simple I’m just going to link you to this “sizing” tool from our vendor’s website.

To access this tool simply click on the attached picture.






Installation it’s not difficult or complicated but you still need to follow manufacturer’s instructions.


First and foremost before anything else turn off gas or electric if this is an electric water heater and cold water supply to the expansion tank.

Second drain the cold line and remove the old tank.

Finally pre charge the new one (if needed) before it’s under water pressure and install.

Do not try to pre charge while under water pressure.

Ideally you would want tank to be installed in vertical position and firmly secured.


For your reference I have attached Installation Instructions for series of Watts potable tanks.


Hope this helps.



Posted 2012-01-16T18:51:21+0000  by George_HD_CHI

My water pressure is 100.  I intend to install a pressure reducing valve and an expansion tank.  My question is, How far from the water heater can I install the expansion tank?  Since my water heater is in the attic and expansion tanks are suseptible to leakage, I would like to put it adjacent to the water softener in my garage about 50 feet from the heater.  Is this ok?  And, is there a container or similar to enclose an expansion tank in to collect the water in case it does leak?

Posted 2012-09-07T19:00:52+0000  by Ikonoblast

Hello Ikonoblast,


Manufacturers recommend for the expansion tank to be installed on the cold supply line adjacent to the water heater, and after the shut off valve.  They recommend it to be as close as possible to the water heater so that extra volume of water, created by thermal expansion, inside the water heater can promptly be diverted.


This said expansion tank, technically, will work no matter where it is installed in the system (provided there are no obstacles) but it will perform better when placed closer to the expansion point.


I’m not aware of any kind ET enclosure; typically these tanks are installed in mechanical rooms with floor drains or directly above the water heater placed over the pan drain.

 drain pan.jpg

Hope this helps,



Posted 2012-09-18T19:41:56+0000  by George_HD_CHI
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