can i fit a 2 port valve to a gravity feed heating system?

Hot water is gravity fed through the boiler and through the coil in the hot water tank to heat the water. The central heating is also run off this line, there is a tee peace in the line which goes to the central heating pump and then around the rads.The problem i have is when we want just hot water the rads upstairs get hot as well, is there any reason why i can`t fit a 2 port valve on the suction side of the pump to stop this.
The vent for the gravity fed hot water comes off the inlet to the heating coil and up to the header tank,so the vent would be on the suction side of the 2 port valve if i were to fit it and not the discharge side
Regards
Ian...