Many people have problems purging their hydronic heating systems. If the heater is plumbed properly purging is simple and fast. Feed water inlet should be on suction side of pump.
First close the #1 valve then open #2 valve, connect it to a hose. Then open each zone valve one at a time until the air is gone ( by sticking the hose in a bucket you can see the air exiting). Make sure the little cap on the auto vent is loosened a couple of turns. To speed this process up you can turn a lever on the feed water valve to increase the pressure, you will need to return it later. Running the circulating pump at the same time helps also.