Object-based CAD programming is used to take advantage of standardisation to handle the schematic design, sizing, layout and (potentially) pipe-routing for LPHW (Low Pressure Hot Water) plant rooms in buildings. From a simple specification of the plant room geometry, and the heating load in kw, our software proceeds through a number of steps. First the standard number and size of modular boilers, pumps etc. is determined from the heat load. Then a compatible optimising 3D variational solution is generated, using Constraint Logic Programming. Our approach is highly interactive. Modifying the topology of the solution is done directly through the graphic interface, e.g. modifying a boiler position is done by dragging; the system automatically updates the 3D model including the pipe-routing while maintaining all the constraints, and hence the validity of the design.