If you're upgrading your Autoterm Air 4D installation to run 90mm ducting on the intake side, this adapter is what you need. The heater ships with an 80mm inlet — this adapter bridges the gap, letting you connect standard 90mm air ducting directly to the unit without any modification.
Running 90mm intake ducting reduces air resistance through longer or more complex duct runs, helping the heater pull in the airflow it needs to run efficiently at full 4kW output. It's a small part that makes a real difference on builds where the intake pipe has to travel any distance to reach fresh air.
Specifications

Inlet: 80mm (heater side)
Outlet: 90mm (ducting side)
Material: PA6 plastic
Colour: Black
Part number: ATR2-A80903D-BK (also listed as P5313)
Compatible with: Autoterm Air 4D (4kW diesel air heater)

Note: This is the intake adapter only. The Air 4D hot air outlet uses a separate 96mm to 90mm adapter — check our Diesel Heaters category if you need that too.