We noticed our boiler leaking today. It seems it is coming from the overflow of our expansion tank. Can anyone offer any assistance as to why this could be happening?

To our knowledge, this has not happened with this boiler before.

EDIT: I managed to locate our invoice and it turns out the expansion tank and relief valve were also replaced along with the boiler 2.5 years ago.

UPDATE: We put a small container under the overflow pipe last night as a precaution, and this morning we found that the boiler leaked an additional 1/2 liter of water. In the meantime, we attached some scrap downspout pieces to route the leaking water into the floor drain instead of having to constantly empty the little container.

Boiler Model: Super Hot MG100-N-E-PS
Install Date: Sept 20, 2019

Overview of installation
Overview of installation
Click all images to embiggen

Water trail across the floor
Water trail across the floor

Pressure/Temp Gauge:
Pressure/Temp Gauge

Close-Up of Pressure Valve:
Close-Up of Pressure Valve

Expansion Tank (Side View 1):
Expansion Tank (Side View 1)

Expansion Tank (Side View 2):
Expansion Tank (Side View 2)

Expansion Tank (Overhead View):
Expansion Tank (Overhead View)

Temporary Fix to Route Leaking Water to Drain:
Temporary Fix to Route Leaking Water to Drain

  • That's a temperature/pressure relief valve at the tag, right? It would be helpful to get pictures from the far side, standing by the ornament containers looking back at the expansion tank side of the boiler. At 8 years, the expansion tank's internal membrane could certainly have failed.
    – Armand
    Apr 20, 2022 at 9:06
  • 1
    @crip659 If the valve is at fault, would I expect to see constant leaking or dripping? The reason I'm asking is because aside from single leak, we've never seen it leak or drip before, nor does it appear to still be leaking or dripping.
    – nnamerz
    Apr 20, 2022 at 10:14
  • 2
    If just a single one time leak that stops, then it is working as it should. It opens to release pressure if it gets above a certain amount. The expansion tank should have an air valve near the top, if water comes out of that valve, then it should be replaced, can also use the knock test, the bottom(water) should sound different than the top(air).
    – crip659
    Apr 20, 2022 at 10:27
  • 3
    @crip659 but also check there is no excessive pressure or temperature, since the valve could be doing its job and the problem could be somewhere else. If a fuse blows, it doesn't mean the fuse had a problem, it means there's a problem somewhere and the fuse caught it. Apr 20, 2022 at 10:41
  • 1
    @crip659 Just to clarify terminology here, he's got the tank below the piping, so its air valve should be on the bottom as installed, the end with the air (hopefully).
    – Armand
    Apr 20, 2022 at 16:25

2 Answers 2


Expansions tanks don't have an "overflow."

Apparently your pressure or (pressure/temperature) relief valve (which is an entirely separate item from the expansion tank) is operating. Can't see from the picture if it's pressure only or not. Looked up the model number, it's pressure-only.

That either means you have too much pressure, or it's operating when it should not (or you HAD too much pressure and it did not reseal correctly when pressure got lower, which is common.)

To check if the expansion tank bladder or membrane has failed, see if you get water or air when briefly depressing the air fill valve (on the bottom of the tank, the way yours is installed.) If you get water, it's failed.

  • With the updated information and images I added to the original post, are you able to help me identify the air valve. I don't happen to see anything at the bottom of the expansion tank. Also, with the expansion tank and relief valve being only 2.5 years old, are they still very likely to be at fault?
    – nnamerz
    Apr 20, 2022 at 23:19
  • 1
    Try re-taking "side view 1" from floor level. It should be right on the bottom of the tank, normally. Alternatively, take a readable picture of the labels in "side view 2" and we can look up the catalog specs for it. The way it's installed here, you'd have to be practically lying on the floor to see it by eye.
    – Ecnerwal
    Apr 20, 2022 at 23:27
  • Thank you everyone for all the help/advice. We had the original installer come back today to take a look and it turns out the expansion tank had failed. I asked him why it failed so soon and he said it was just our bad luck and that he had seen some only last 6 months and some that had lasted over 12 years. He replaced it with a new one he had in his van and then bled the system from the boiler for any trapped air. Everything seems to be working now but I am just curious if I need to individually need to bleed each baseboard heater/radiator (especially the ones on the floor above the boiler)?
    – nnamerz
    Apr 23, 2022 at 4:17

Just posting a screengrab from the Calefactio YouTube video showing air valve location on tank bottom, once you unscrew the round black plastic cover there to reveal it: (pictured tank has a cut out section to show internal bladder that holds the water)

enter image description here

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