Every time I open my windows, air seems to flow backward through my over-the-range microwave vent, bringing a nasty cooking grease smell into the house.

I’ve replaced the microwave and even had a brand new vent installed, but over time the smell has returned.

I know there’s an internal filter that can be cleaned, although it requires partially disassembling the microwave, so I’m planning to look into that.

What would cause this backdraft? Is this normal, or does it indicate a ventilation problem? And how can I fix it permanently? I’ve an HVAC tech even go into the attic to look at it but he couldn’t find any problem with the vent, I’m so confused.