I need to somehow filter my furnace input, but I don't know how

[dapper maple]

So I have an iron smelting setup that uses the same belt for coal input and iron plate output, but its using some of the iron output to make steel. How do I fix this?

[inner nacelle]

click on the inserter, set a filter

[dapper maple]

That's a feature?? 😭

Thank you!

[timber valve]

It is nowadays, this used to be for specific inserter only, it was ultimate life-changing moment

[vast flume]

You could also solve this by outputting onto where the coal belt is and having the coal where the iron plate currently goes. This would reduce the number of inserters needed by 1.