The water level is likely low, is the pump running, what is the water
pressure when cold. Wait for it to cool and open up the feed, I have an
auto feed but dont trust it and keep the line valves to it closed, many
people keep them closed or lift up on the little bar across the top of
the feed valve to see if you can get it to feed water. With only air
comming out upstairs you are low on water, start by adding only 2lb note
pressure cold and hot. For 3 floors I run 12-14lb.