Heating Help

check valves

what is the purpose of check valves on hw zones on steam boilers

check valves

They exist to keep the zone from gravity feeding and to help keep the water in the zone

thanks

does it matter if the check valves are above or below the water line and if they are before or after the purge valves

check valves

if the entire loop is below the water level are the check  valves necessary? also is it necessary to purge a loop that's below water level? if yes are two purge valves necessary or can the loop have a single purge valve and the boiler it self will function as the discharge valve? thanks