What makes a unit "convertible" to horizontal

[Alden_Sloe]

What is it (besides the manual) that determines if a unit can be install in a cross flow or horizontal orientation. On some models only the upflow is OK'd for this, with others it's only the downflow and sometimes either one can be mounted horizontally? Is there some component inside (like a control valve) that is affected by gravity? If so can that be changed? If it's the burner or heat exchanger configuration then I guess even if it's possible to change out it would be cost prohibitive.

[larobj63]

What is it (besides the manual) that determines if a unit can be install in a cross flow or horizontal orientation. On some models only the upflow is OK'd for this, with others it's only the downflow and sometimes either one can be mounted horizontally? Is there some component inside (like a control valve) that is affected by gravity? If so can that be changed? If it's the burner or heat exchanger configuration then I guess even if it's possible to change out it would be cost prohibitive.

It usually relates to the handling and drainage of condensation, either from combustion or from cooling.

"Convertable" air handlers have traps, etc that can be swung 90 degrees to allow either upright or horizontal operation.

Many a-coils (evaporator coils) have drian pans and traps in two positions, or you can change them manually when installed, for the same reasons. Sometimes you have to rotate the whole a-coil assembly.

But anyway, I think you get the idea...