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Gentlemen, can't thank you all enough for the insights and push, even academically, to resolve this. OK, my take. Agree with Delcrossv, height of 2nd floor, 20ft close enough, yields 8.65 psi as a couple pressure. But agree that once you swing the Taco valve handle to 'fast fill' you're by passing the regulated function of the valve and introduce full city flow and pressure. Have no gauge on incoming city water source but Suffolk County NY advises roughly 45-50 psi. So, once city pressure is introduced the new pressure introduced to the 2nd floor, or 1st floor, loop is the 50psi. OK, with all that said, so by introducing the 50psi to the system, why does the 2nd floor loop pop the relief valve and the 1st floor does not? Something ain't right here. Since city pressure is NOT introduced to the loop until the exit drain valve is already open, there should be no build up of pressure in the loop to the 50psi level unless the drain valve/or a restriction exist in the loop. Relief valve is functioning properly activating close to 30 psi. Might be weakened by now from repeated popping. OK, my plan of attack here. Gonna take and send pics of near boiler piping for all to see. BTW, my boiler has the same configuration of the one posted by 'Fix it with Zim' air purging tutorial on Youtube. Next, goona re-purge 1st floor loop and not the reading on the boiler pressure gauge so we better understand what pressure is being applied to the boiler once full city pressure has been introduced. Then will close up 1st floor zone and move to 2nd floor zone. Will remove and inspect exit gate drain valve for defects as a defective partially opening gate valve may just be the root cause of all this annoyance. Is it too early to hit the Bourbon yet??? Keep you amigos posted on finding. 1MM Thanks