The round foam and round flat filters work great on my NV501, but the HEPA filter WILL NOT FIT, it's too big. I've already replaced a Foam and a Flat before trying HEPA and I need them way more than HEPA so I eat up the cost. But BEWARE this is advertised inaccurately and I don't know how to check if it fits so check HEPA compliance first so you can return it if it doesn't suit you. Even if it was the right part, I would happily give it 5 stars.