LARGER VIEW of a heating boiler expansion tankExpansion Tank / Compression Tank Air Valve Leak Diagnosis & Repair
Water leaks at the heating system expansion tank air valve or air vent

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Heating boiler expansion tank / compression tank air adjustment valve & air vent valve troubleshooting guide:

This article explains the difference between the air pressure adjustment valve on an internal-bladder compression/expansion tank and the a automatic air vent valve found on both bladder type and bladderless type expansion tanks on hot water heating systems. We explain how each of these valves may be leaking but the implications, cause, and repair are quite different.

This article series provides a heating system expansion tank (compression tank) troubleshooting & repair guide.

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Expansion Tank Air Valve vs Vent Leaks

Automatic Air Eliminator - Auto Vent No. 67 from  Maid O Mist (C) Daniel FriedmanReader Question: water coming out of the top brass venting/valve atop the boiler's expansion tank

I found information on the expansion tank noted in the subject line on your web page. This is the kind I have on my gas furnace. There is water coming out of the top brass venting thing above the tank. My water pressure cold is almost down to zero.

The unit runs and heats the house but wondering what is going on here something is off. Any suggestions on what to do would be greatly appreciated.

At left we illustrate a new automatic air bleed valve.

[Click to enlarge any image]

Reply: First let's distinguish leaky automatic air bleed valves from a leak at the air pressure adjustment valve on an internal-bladder heating system expansion / compression tank


Water Leaks at an Automatic Air Bleed Valve

The most likely cause of leakage at the automatic air bleeder valve is that the valve core or stem itself has failed or its seat has debris on it, preventing the valve from closing when it should, or the float inside the valve has failed and no longer opens and closes the valve properly. That leaky automatic air bleed valve should be replaced. Water leak at an air bleed purge valve on a heating system (C) Daniel Friedman

If the part that is leaking on your boiler looks somewhat like the device shown at left, that's an Automatic Air Eliminator - Auto Vent No. 67 from Maid O Mist in our example. (Your heater may use a different brand or product).

If that's the leak source the part replacement is usually a minor repair.

Temporarily you can usually stop the leak at an automatic air vent by screwing closed the valve cap atop the valve. But that will also prevent the valve from venting air and thus can lead to an air-bound heating system and thus to loss of heat in one or more areas of your home.

Therefore while you want to stop any immediate leak to avoid making a mess or dripping onto electrical components, the proper repair is to replace the automatic air vent promptly.

See AIR BLEEDER VALVES for details it's just a hop skip and a jump right over


Leaks at an air pressure adjustment valve on an internal-bladder compression / expansion tank

Heating boiler compression / expansion tank (C) Daniel FriedmanHowever if the leak is not at the automatic air bleed vent (shown in our photo above), but is at the air valve mounted on an internal-bladder type heating boiler expansion tank, then we have a different problem.

Our page top photo shows an internal-bladder type expansion / compression tank used on a hot water heating boiler. In the page top photo a small blue cap on the under-side of the tank covers its air pressure adjustment valve.

[Click to enlarge any image]

At left, that little red cap atop the heating boiler compression tank hides the air pressure adjustment valve. Manufacturers typically ship these tanks pre-charged to 12 psi cold - suitable for most circumstances. For some heating systems operating at higher pressures the service technician may use this valve to adjust the pre-charge air pressure in this little compression / expansion tank.

What does it mean if water squirts out of the air adjustment valve on an expansion tank?

If the expansion tank you are using is an internal-bladder type and water is coming out of the air charge valve as you describe, I suspect the bladder is burst - it's time for a replacement tank bladder or if that's not an option on your tank, a new expansion tank is needed.

Watch out: before even considering expansion tank replacement, if the water leak you are describing is not at an air charge vent on the expansion tank itself (a part that is indeed not present on most bladder type expansion tanks for heating systems) then you are probably talking not about a problem with the expansion tank but rather a problem with an automatic air bleeder vent - a separate part intended to bleed air from a hot water heating system.

Our photo below illustrates water squirting out of a water tank air pressure adjustment valve. This particular tank is in use on a potable water supply system pressure tank, it is not a heating system expansion tank. But this is what you might see when pressing the center pin of the air adjustment valve on an internal-bladder-type heating system pressure tank if its bladder has leaked and the tank is waterlogged.

Water squirting out of a water tank air adjustment valve (C) Daniel Friedman

Watch out: unlike the cold water squirting out of a water pressure tank with a burst internal bladder, the water that may squirt out of a heating system's expansion tank will be hot and nasty.



Continue reading at EXPANSION TANK DIAGNOSTIC FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BOILER LEAKS CORROSION STAINS a catalog of all types of leaks found at heating boilers

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EXPANSION TANK AIR VALVE LEAKS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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