A heat exchanger apparatus has a heating stage with a product side and a hot water side and a cooling stage with a product side and a coolant side, and also regeneration stages with treated and un-treated product sides. There are valves including a cooling stage outlet valve, and pressure sensors, and pumps for pumping process liquid through the product sides. A PLC controller is programmed to operate the pumps with outlet valves closed to pressurize the product sides of the stages at a pressure dynamically maintained by control of the pumps in response to sensed pressure. The valves are controlled to firstly vent the heating and cooling sides of the heating and cooling stages with the product sides pressurized, and then in a second phase to vent the downstream (treated) product sides of regeneration stages. Also, an in-line holding time test is performed by monitoring time for step rises in temperature to reach a temperature sensor and the outlet end of a holding tube.