heater only working on one half of car?

as stated in title only gettin hot air in passenger side of car. thermostat?? matrix?? any idea's? gettin lot of pressure build up in water tank .

You don't say if it's got ECC buddy. If it's a manual system then I'd be looking for broken/kinked control cable. The ECC is electronically controlled and should show a fault code if all is not well. Pressure in the header tank is normal unless excessive in which case you would have other symptoms ie high reading on temp guage and complete loss of heat in cabin (poss h/gasket) but you would be well aware if this was the case.HTH