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indirect Plus 40 air vent & odd behavior

Embarrased to say we had a Weil McLain indirect Plus 40 installed 6 yrs ago & haven't done any maintenance 'til now. Manual recommends a service tech. operate temp/pressure relief valve at least once a yr., so instead I released about 1.5 gallons. Also followed instruction to remove air vent cap, push valve in briefly, replace cap & unscrew a full turn. <strong>Is water supposed to come out of the air vent?</strong> (except for the 1st time tank is filled) 'cause it did. What prompted me to check it out was recent odd performance. A week ago, the water temp became unusually hot, almost scalding for a few days. Then today, water seemed only lukewarm, can't recall it ever not being hot enough to wash dishes. An hour after releasing the T&P valve and pushing in the air vent valve, temp. is back to normally hot level. During the time the DHW wasn't hot enough, the boiler cycled normally and maintained house temps. Any thoughts on the abrupt changes from scalding to tepid and then hot?
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