Construction glossary · MEP
What is condensate drain in construction?
A condensate drain in construction is the piping that carries the water produced by cooling equipment (the moisture that condenses on an evaporator coil) from the unit to an approved disposal point such as a hub drain or floor sink. It is taken off in linear feet of small-bore pipe, commonly 3/4 inch or larger PVC or copper, plus a trap at each unit and an air gap at the receptor. The scope sits on the seam between HVAC and plumbing, which makes it a classic bid-day gap.
Updated June 2026 · Reviewed by the Ruh construction team
How condensate drain is routed
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Book a walkthroughA condensate drain moves the water a cooling coil wrings out of the air, from the equipment's drain pan to an approved receptor, and it is one of the few scopes that genuinely straddles two trades. The piping appears on the mechanical plans at the equipment, the receptor (a hub drain or floor sink) appears on the plumbing plans, and the mechanical code governs the details: pipe size keyed to equipment capacity, a trap at the unit, minimum slope, an air gap at the indirect connection, and secondary protection (an auxiliary pan with a float switch, or a separate secondary drain) for units above finished ceilings. Ownership varies by contract; a common split has the mechanical contractor piping from the unit to the receptor the plumber provides. New estimators miss condensate three ways: assuming the other trade carries it, forgetting the secondary pans and float switches at every fan coil above a ceiling, and skipping insulation on runs through return plenums where a cold pipe will sweat onto tile.
Condensate piping is measured in linear feet by size and material from each piece of equipment to its receptor, with 3/4 inch the minimum for smaller equipment in the commonly used mechanical code table (a size commonly cited as serving units up to 20 tons) and minimum slope commonly 1/8 inch per foot. Each unit contributes count items: one trap, an auxiliary drain pan with float switch where the unit sits above a ceiling, and a condensate pump wherever gravity cannot reach a receptor. Insulation is measured in lf on runs through plenums and concealed spaces. The receptors themselves (hub drains, floor sinks) are usually plumbing scope and counted each, which is where the trade boundary needs checking.
Worked example
Take off condensate for six 3 ton fan coil units above the ceiling of a tenant floor, illustrative pricing throughout. Pipe: each unit averages 25 lf of 3/4 inch PVC to the nearest hub drain, so 6 x 25 = 150 lf, priced at $9.00 per lf installed: 150 x $9.00 = $1,350. Traps: one per unit, 6 x $35 = $210 as a fitting and labor allowance. Because the units sit above a finished ceiling, code requires secondary protection: 6 auxiliary drain pans with float switches at $120 each = $720. The condensate scope totals $1,350 + $210 + $720 = $2,280, plus insulation where the runs cross a return air plenum and the cold pipe would sweat onto ceiling tile. Trap depth is its own check: a draw-through unit pulling 1.5 inches of negative static needs a trap deeper than that static, a detail that turns a $35 trap into a callback when it is skipped.
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How Ruh handles condensate drain
Ruh picks up condensate piping while reading both the mechanical and plumbing sheets, measuring the runs in lf, counting traps, pans, and pumps, and pricing the lot against the contractor's own price book. The estimator reviews the draft, confirms which trade carries the scope on this contract, and signs off, which is exactly the check that closes the HVAC-to-plumbing gap.
See AI construction takeoff and estimating softwareCondensate drain: frequently asked questions
Who installs the condensate drain, the HVAC contractor or the plumber?+
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.
Why does a condensate drain need a trap?+
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.
What size pipe is used for a condensate drain?+
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.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.