What is condensate drainin construction?

It depends on the contract, which is exactly why condensate causes bid-day gaps. A common split has the mechanical contractor running pipe from the unit's drain pan to a receptor, with the plumber furnishing the hub drain or floor sink and the sanitary connection behind it. Some jobs give the plumber everything past a point of connection. Read both Division 22 and Division 23 scopes, and name the boundary on the scope sheet.

Most air handlers pull air across the coil, putting the drain pan under negative pressure. Without a trap deep enough to beat that static pressure, the fan sucks air backward up the drain, the pan cannot empty, and it overflows into the ceiling. Trap depth is sized to the fan's static pressure, which is why the same detail does not fit every unit, and why blow-through units trap differently than draw-through units.

In linear feet by pipe size and material from each piece of equipment to its receptor, plus count items per unit: one trap, an auxiliary drain pan with float switch where the unit sits above a finished ceiling, and a condensate pump wherever gravity cannot reach a receptor. Insulation is measured in lf on runs through plenums and concealed spaces, and the receptors themselves usually count as plumbing scope — the trade boundary worth naming on the scope sheet.

The mechanical code sizes condensate by equipment capacity, and the commonly used table starts at 3/4 inch, a size that covers most packaged units and fan coils (commonly cited as serving up to 20 tons of cooling). Two other rules ride along: the drain cannot be smaller than the equipment's drain pan connection, and horizontal runs need continuous slope, commonly a minimum of 1/8 inch per foot toward the receptor.