Oil burner - Pressure Relief Valve Discharge

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riprock69

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I have periodic discharge (once a day) of water in my oil fired heating system through the pressure release valve. The system is a Weil McLain WGO oil burner (one year old) with 6 zones (1 zone used for indirect water heater). Here is what I've done to troubleshoot: replaced the pressure release valve, replaced the pressure reducer valve (set to 15psi), purged all zones, and emptied the expansion tank (its a steel, 15 gallon tank located between the joists). After the expansion tank is emptied, the system pressure does go down to less than 5psi, but after a day, the system eventually goes back to approximately 27psi and the discharge continues.

Just last night, I shut off the incoming water and drained the expansion tank to reduce the system pressure to around 14 psi. The system pressure started to climb up to 21 psi after 5 hours and the next morning, the system pressure was up to its steady state of ~28 psi (with some discharge that night). So, I think it must be the expansion tank. The tank is a 15 gallon steel expansion tank. Bladderless. Is it too small?
Thanks in advance!
 

Smooky

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If you drain the water out of the tank I would not think it should be full of water again the next day. When you drained it out did you open an air vent or something? Is there an air vent on the tank or some other way the air can leak out of the tank?
 

riprock69

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If you drain the water out of the tank I would not think it should be full of water again the next day. When you drained it out did you open an air vent or something? Is there an air vent on the tank or some other way the air can leak out of the tank?
No air vent on the tank. Its a closed, steel expansion tank without a bladder. I have a drain faucet to let the water out which is obviously closed after I drain the tank down. The water in the expansion tank and the pressure build up slowly to a steady state system pressure of ~28 psi.
 

Smooky

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Air is leaking out of the expansion tank somewhere so if there are no other valves, plugs etc that could be leaking on the tank, the tank it self must be bad.
 
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