Generally, Portable Clean Room most new units are shorter than old ones. If yours is taller, you may need to cut your existing plenum back. A sheet metal shop may have to fabricate some duct for you.

Connect the return air duct up to the side of the unit.

You will have to cut a hole in the side of the unit to match the size of your return duct. It may require an extension piece or you many need to cut some off depending on your unit's configuration.

Then, reconnect the gas piping to your unit if it's natural gas. Connect the electrical wiring back to the unit. Consult local electrical codes to see how to make the connections.

Connect the condensate piping back to the unit. Ensure it is trapped and vented per local plumbing code so it drains correctly.

Reconnect the thermostat wires to the unit. Connect them according to how you labeled them above. And remove the labels as you connect them.

Finally, turn the gas valve and the electrical breaker back on to the unit. Your new China Clean Room HVAC unit should now be correctly installed. Turn the thermostat on to see if it is working correctly.