Surface cutting geometry in several planes
 

Hello everyone,
I am modelling a rectangular natural circulation loop with isothermal heat source and heat sink and the rest of the surfaces adiabatic.
I am using carbon dioxide as a fluid and employing NIST real gas model in the supercritical region of CO2.

I wanted to calculate bulk mean temperature at a cross section of the isothermal heat source .For that I created a plane surface in FLUENT.It is creating a cross section in the heated section but it is also creating a cross section in the cooled section(since the geometry is rectangular and Fluent just asks the 3 points on the plane,not the bounds of the plane)
As a result my geometry is cut into 2 places ,wherever the plane intersects it

Can any1 tell me how to create a bounded plane in one of the sections only? Thnx