I have a oil (converted to natural gas) hot water boiler running three zones of baseboard heat. Lately I have been hearing water rushing through one of my zones (I don't hear it on the other zones). Additionally the boiler pressure is 20 psi. From research it should be about 12psi. I opened the spigot on the affected zone and drained about 4 gallons of water until it flowed freely without spitting. The pressure went down to 10-12psi. However as soon as I shut the spigot, the pressure climbed again back to 20psi. I can still hear washing rushing through the pipes but could that be just refilling?
What should my next step be? Should I drain more water on the zone or is there another valve that needs opening? Here are pictures of my system. You can see the hose on the spigot I opened.
UPDATE: Following the comments below, I reduced the pressure on the fill valve which reduced the boiler to 15 psi. I will wait to see if I hear the water rushing through the pipes.
UPDATE: Water still rushing through pipes.