Recently, I moved into a new home that has gas heated forced hot water w/ 3 zones and no vents at any of the radiators to purge air, because I was informed that the system is a closed system w/ a automatic vent near the end of the return. The zones are controlled by electronice valves. One zone adjusts to the temperature of the thermostat while the next zone is avg. 6 degrees cooler and the final zone is 2-4 degrees cooler than the cooler one. Example, all zones are set to 70 degrees: Zone one will be at 70 degrees, second zone will get to 64 degrees, and the third zone will be 60-62 degrees. What might be the problem? Could one or more of the valves need replacing or could there somehow be air in the system? Also, the water pressure in the house is low but sufficient, could that also affect the heating system? Any help would be appreciated.