A gas turbine engine comprising: a fan section delivering air into a main compressor section where the air is compressed and communicated to a combustion section where the air is mixed with fuel and ignited to generate a high energy flow that is expanded through a turbine section that drives the fan and main compressor section (22,24); a turbocompressor: and an environmental control system comprising: a higher pressure tap associated with a higher compression location in the main compressor section, and a lower pressure tap associated with a lower pressure location in the main compressor section, said lower pressure location being at a lower pressure than said higher pressure location, the lower pressure tap communicating to a first passage leading to a downstream outlet, and having a second passage leading into a compressor section of the turbocompressor, the higher pressure tap leading into a turbine section of the turbocompressor such that air in the higher pressure tap drives the turbine section of the turbocompressor to in turn drive the compressor section of the turbocompressor; a turbine outlet receiving airflow exhausted from the turbine section of the turbocompressor; a compressor outlet receiving airflow exhausted from the compressor section of the turbocompressor; a combined outlet receiving airflow from the turbine outlet and the compressor outlet intermixing airflow and passing the mixed airflow downstream to be delivered to an aircraft; and a diverter valve controlling airflow from the turbine outlet into the combined outlet for controlling a temperature of airflow in the combined outlet,wherein the diverter valve controls exhaust airflow from the turbine outlet to an exhaust outlet, characterised in that the engine includes a check valve controlling airflow from the lower pressure tap through a bypass passage between the lower pressure tap and the combined outlet. a fan section delivering air into a main compressor section where the air is compressed and communicated to a combustion section where the air is mixed with fuel and ignited to generate a high energy flow that is expanded through a turbine section that drives the fan and main compressor section (22,24); a turbocompressor: and an environmental control system comprising: a higher pressure tap associated with a higher compression location in the main compressor section, and a lower pressure tap associated with a lower pressure location in the main compressor section, said lower pressure location being at a lower pressure than said higher pressure location, the lower pressure tap communicating to a first passage leading to a downstream outlet, and having a second passage leading into a compressor section of the turbocompressor, the higher pressure tap leading into a turbine section of the turbocompressor such that air in the higher pressure tap drives the turbine section of the turbocompressor to in turn drive the compressor section of the turbocompressor; a turbine outlet receiving airflow exhausted from the turbine section of the turbocompressor; a compressor outlet receiving airflow exhausted from the compressor section of the turbocompressor; a combined outlet receiving airflow from the turbine outlet and the compressor outlet intermixing airflow and passing the mixed airflow downstream to be delivered to an aircraft; and a diverter valve controlling airflow from the turbine outlet into the combined outlet for controlling a temperature of airflow in the combined outlet,wherein the diverter valve controls exhaust airflow from the turbine outlet to an exhaust outlet, characterised in that the engine includes a check valve controlling airflow from the lower pressure tap through a bypass passage between the lower pressure tap and the combined outlet.