Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Expansion Tank Replacement (?)

TorontoJasonV Member Posts: 4
Hi All,

My first post here!

I have a Navien tankless boiler and have noticed a small leak originating from the Pressure Release Valve.

When I tap on the expansion tank it sounds fairly full.

With the exception of tasks that require a certified professional (such as gas installation), I like to perform as much maintenance on my own in order to get better understanding of how things function (metallurgical engineer by training).

I'd like to tackle this project on my own but wanted to confirm the procedure:

1. Shut off power to the boiler
2. Close the water inlet to the boiler
3. Close the water outlet from the boiler to the radiators
4. Drain the water remaining in the system -- essentially what remains in the pipes between the inflow and outflow shut-off
5. Remove the failed expansion tank (I understand it may be quite heavy)
6. Install a new expansion tank (using teflon thread tape)

And now here's where I'm unsure -- should I be:
1. Turning on the boiler
2. Opening the inflow to the boiler and then
3. Opening the outflow

Will this introduce air into the system that I need to bleed out from radiators?

Also, I believe the pressure relief valve looks okay, but I'm thinking since I'm going to all this trouble I may as well just replace it as well.

What are everyone's thoughts on all of the above?

I've attached photos of my system.


  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    An absolutely critical missing step: charge the new tank with air to the desired system operating pressure (probably around 15 psi) before you connect it to anything.

    Unhappily, there's no way you're not going to get some air in there, which you will have to bleed out. It is possible that you can get It out without purging -- which is pushing a lot of water through with higher pressure -- and just bleeding. Possibly not.

    However -- do not turn the boiler on to heat before you do that. Tankless heaters really truly don't like to try to heat air...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 11,060
    Is there an air Schrader valve on the bottom of the tank?
    If you depress the core does water or air come out?

    Are there valves to isolate the house radiators from the boiler?
    If not you will have to drain the entire system, (that is above the boiler) refill and purge air from each.
    Is there any auto air removal device on the system.
    What type of radiators do you have...cast iron or BB.

    How long has this worked previously to this issue?
    Lets see more of the system.
  • TorontoJasonV
    TorontoJasonV Member Posts: 4
    Here's a video:


    The current expansion tank does not have a valve at the bottom (Watts ET-30). This model is also discontinued.

    This just started happening last week.

    Interestingly, if I keep the temperature lower at home (60 Celsius) the leak goes away. It's when I wanted warm it up that it starts to leak (slowly).

    I knocked on the tank and it does feel 'full' (no hollow sound).

    I have cast-iron radiators and baseboard fins both installed.

    The current tank appears to have been charged to 12 PSI and I'm thinking of replacing it with an Amtrol EX-30 which is also charged from the factory at 12 PSI.

    JUGHNE Member Posts: 11,060
    How long has it worked OK without problems...when was the new boiler installed?
  • TorontoJasonV
    TorontoJasonV Member Posts: 4
    The boiler was installed in 2016 and I bought the house in January of this year.

    The previous owners left me their receipts for the boiler but an expansion tank is not listed anywhere in the bill of materials.

    I'm wondering if they elected to use the previous tank?

    So I'm actually not sure how long it was working without problems.

    The plot thickens...

    I'm inclined to believe it's the expansion tank because when I keep the thermostat set lower (60 Fahrenheit) the pressure relief valve does not leak. But when I bump it up to 63-64 it starts to leak. So higher temperature = higher pressure and water likes to expand when heat is applied.

    P.S. I'm okay to just go ahead and replace it. The unit alone is $100 which is not too bad in the grand scheme of things.
    JUGHNE Member Posts: 11,060
    Expansion tanks are sized based upon the volume of water in the entire system. If you had only BB with small (1-3/4") piping then the 30 would probably work.
    But if you have large CI rads and larger pipes for them the 30 may not be large enough.

    Remember all the water in the system will come up to temp and all the water will expand.
    2 years is not a very long life for a tank. It could have been over worked to die that soon.

    The Amtrol web site would have sizing guidelines.
  • TorontoJasonV
    TorontoJasonV Member Posts: 4
    I'm not exactly sure how long the current tank was functioning - the system is 2yrs old but the expansion tank may not have been replaced.

    I'd guess it was not replaced given I can no longer order a replacement. The Watts ET-30 currently installed is discontinued.

    I did notice the sizing requirements on the Amtrol site and it suggests their 4.4 Gallon expansion tank.

    Since the boiler is tankless, is there a special procedure once I shut off the inflow and outflow, drain whatever remains, and install a new Expansion tank?

  • Larry Weingarten
    Larry Weingarten Member Posts: 3,289
    Hello, When you replace the tank, it might be useful to add a ball valve just above the "T" that has that drain valve in it. That lets you isolate the expansion tank from the system for service and also lets you charge the tank to system pressure without removing it. ;)
    Yours, Larry
    JUGHNE Member Posts: 11,060
    What pressure shows on the gauge with the system off/cool?
    Should be 12-15....that is what to charge the new tank to.

    Power and gas off. Water supply fill off.
    Hopefully you have isolation valves to keep all the water in the system above the boiler. You do not want to lose any of that water and avoid any air burps that might get in the system.
    Check the charge of your new tank before installing. Let it climatize to room temp for checking charge.
    Install tank, install new 30 PSI relief valve. Turn on water fill valve.
    Look at boiler instructions....maybe an air bleed valve inside??
    Bleed air from every where on the boiler...relief valve....hose bib with hose in bucket of water avoids splashes.
    You want every thing full before opening the isolation valves to the system. Your fill valve should fill to 12-15 psi. wait that it does not continue to rise. Screwdriver tip on pipe and handle in your ear will tell you when it stops feeding.

    Then open isolation valves.....hopefully you hear no air burping up. The fill valve may add water to maintain your desired pressure.
    Fire it up.

    Just one of those "10 minute" jobs that may take an hour or two. ;)
  • delta T
    delta T Member Posts: 884
    The LAST step is to turn on the boiler. Fill and purge out as much air before you turn anything on.
  • hot_rod
    hot_rod Member Posts: 22,121
    Better support for the tank also, looks like it just hangs from the boiler piping? If nothing else a piece of PVC pipe for a stand.

    The is a schrader valve, it may be under a plastic cap on the bottom, someone put an air charge in the tank :)

    A valve like this is a nice option, you have plenty of room to add one with the tank.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream