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I have a boiler + indirect water heater system. Above the boiler on the hot water outlet side there are a water expansion tank and an auto air vent and on the return side there is another auto air vent. Both vents are now continuously dripping (please see red circled areas in the pictures). What's the right course of diagnostics I should do?

Most of the resources I found have expansion tanks and pressure relief valves installed on top of the water heater. There is a lever-type pressure relief valve on the boiler that is unfortunately stuck. It's also weird that both auto air vents failed at the same time. Based on the knocking sound, the expansion tank appears to have air on the bottom half.

[UPDATE] The boiler shows a feed pressure of 0 (with feed line open).. and the temperature was set at 195 F. I checked the expansion tank and there was no water gushing out. The tank pressure is 10.5 psi. Could it be that the pressure relief valve on the boiler is stuck closed which caused the air vents to leak? I was able to finally unstuck the relief valve, water starts to come out of the valve and the vents stopped leaking. However, if I keep the feed line open, the relief valve would keep draining. What situation am I dealing with?

[Update] Replaced the pressure relief valve and the two automatic air vents. Purged the system. The new pressure relief valve opens and closes correctly when the feed line pressure reducing valve is forced open (street pressure). The heating system seems to be working fine, but what concerns me is that the boiler pressure gauge shows a cold pressure of 30 psi and hot pressure of 46 psi after continuous heating. The boiler pressure gauge has a red mark at 40 psi, which appears to be the safety limit. However, the pressure relief valve did not trigger. Bad gauge?

[UPDATE] It is bad gauge. I bought a faucet type gauge and measured slightly less than 20 psi hot. Accordingly to a tech I talked with on the phone, the original problem was likely due to low boiler pressure (The automatic water feed valve likely was stuck) which caused the water to boil and broke both air vents.

Leak 1 Leak 2 Leak 3

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  • Does your boiler have a Temperature / Pressure gauge? - I don't spot one in the pictures. Which is highly unusual...it's normally in plain view on the front (burner) side, or on top near that side.
    – Ecnerwal
    Nov 3, 2021 at 23:25

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If it's dripping , it needs to be cleaned or replaced - when working correctly, it only vents air.

If your actual pressure relief valve is stuck, you need to fix/replace that, or have it fixed/replaced.

Most of the resources I found have expansion tanks and pressure relief valves installed on top of the water heater.

Because there's a vast swath of the country that has never heard of a hydronic boiler - they have hot air furnaces or heat pumps and gas or electric water heaters. So the water heater is the only place for an expansion tank or a pressure relief, and it doesn't need bleeders because the water flows through, rather than recirculating. Any bubbles go out the faucets.

The arrangement shown is dead standard for boilers - the casting in between the expansion tank and the air vent is called an "air scoop" and it's specifically designed to help get air out of solution and out the auto-vent. The other air vents are put on local high spots to catch any passing bubbles. There may be others out on the heating loops.

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  • see for a nice picture of them... diy.stackexchange.com/questions/211005/…
    – Ecnerwal
    Nov 3, 2021 at 23:09
  • Thanks! The explanation on the water heater setup makes a lot of sense. I did a bit more troubleshooting and was finally able to unstuck the pressure relief valve on the boiler. Water starts to come out of the pressure relief valve and the vents stopped leaking. However, if I keep the feed line open, the relief valve would keep draining. Could the air vent leaking earlier be due to the pressure relief valve on the boiler being stuck? What situation am I dealing with?
    – P. B.
    Nov 3, 2021 at 23:53
  • If both failed near the same time, I’d suspect the expansion tank. Even if the pressure gauge shows ok pressure at any given moment, it could have lost the ability to absorb thermal expansion.
    – Tim B
    Nov 4, 2021 at 20:46
  • @TimB I suspect it was because the pressure relief valve was stuck and the system pressure somehow got too high, breaking both vents. I updated with what I replaced and the new observation. Trying to learn a new part of our home..
    – P. B.
    Nov 5, 2021 at 14:11
  • The expansion tank should limit the ability of the system to exceed pressure. If it was able to do so implicates the expansion tank. If you have a sensitive enough bicycle pump, you might be able to check the pressure in it. (They’re also not particularly expensive, but can be a bear to get off.
    – Tim B
    Nov 6, 2021 at 2:10

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