Furnace side wall venting question

[Plumber69]

Whats the main reason a mid efficiency furnace can't side vent, but a high efficiency can. I understand you can get a side vent kit for the mid. But why can you just not run a pipe straight off the mid furnace outside(side vent)

 

[wwhitney]

The exhaust is too hot?

Cheers, Wayne

 

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[Fitter30]

Most furnaces are rated category 1 horizontal flue isn't approved. Category 3 is approved for horizontal. In the installation operating manual for your furnace it will have what category it is. Adding a external flue fan is not code for residential