i have a 2 pipe heat system that uses convectors. i turned the system on yesterday and 2 heaters didnt work. i was able to bleed the air out of one and it is working ok- the other one is not air bound- water came out the bleeder not air, it just isnt getting flow...... the supply line is warm the return line is cold- it is not flowing thru the unit. i noticed that the return line had a valve piped in-line with a flat srew in it for adjustment. i think some debri is blocking the line and not allowing the flow. i loosened the screw which did nothing except start a leak from the screw head. i will need to drain the system to repair the valve blockage. the supply line has a union on the line but the return doesnt- i think i have to saw saw the line at the valve and remove the valve and replace it with a union....... what are the concerns from not having the valve on the return line?