TryBuildCalcCondensate Drain Trap Height Calculator (Draw-Through vs. Blow-Through)
Size your condensate trap height instantly.
🕒 Last updated: September 15, 2026
Inputs
ℹ️Single unit: flat 3/4in floor, any tonnage. Manifolded: IMC Table 307.2.2 sizes by combined tonnage.
ℹ️From the nameplate (1 ton = 12,000 BTU/hr). Sets condensate flow; pipe size stays the 3/4in floor regardless of tonnage.
ℹ️Sets the Sensible Heat Ratio for condensate flow — humid climates produce more than dry ones at the same tonnage.
ℹ️Max. NEGATIVE pressure AT THE DRAIN PAN, not whole-system ESP — from spec or measurement, worst-case dirty-filter.
ℹ️Draw-through = negative pressure at the pan; blow-through = positive. Each uses a different trap-height formula.
ℹ️Total horizontal distance the drain line travels from the trap to its disposal point.
ℹ️Code minimum is the legal floor; the steeper recommended slope drains more reliably where headroom allows it.
Broader than just an attic — also covers a closet over finished flooring, near electrical panels, or anywhere an undetected leak could reach a building component.
Minimum Condensate Drain Size
3/4 in minimum
IMC code floor for a single, non-manifolded unit's own drain — subject to the equipment's connection size
System & Condensate Load
Drain line type: Single Unit (Not Manifolded)
Cooling capacity: 4 tons (48,000 BTU/hr)
Load condition: Average / Moderate Climate
Estimated condensate flow: 1.19 GPH
Trap & Slope
Drain-pan static pressure / configuration: 0.7 in. w.c. / Draw-Through (Negative Pressure — coil before the blower)
Trap height (H) needed: 1.7 in
J (outlet leg, half of H): 0.85 in
Slope target: Code Minimum (1/8 in per ft)
Total drop over 20 ft: 2.5 in
Assumptions Used
Minimum drain pipe diameter depends on drain line type: a single, non-manifolded unit's own drain uses IMC's 3/4in minimum floor (the line must never decrease in diameter from the drain pan connection to the disposal point, and must never be smaller than that connection itself — a large single unit's factory connection can exceed 3/4in even though this calculator has no way to know that specific figure); a manifolded line combining more than one unit's condensate is sized by IMC Table 307.2.2 against the combined tonnage sharing that line. Manifolding combines the LINE's sizing only — each unit's own drain pan still needs its own individual trap before joining that shared line, per published manufacturer trapping guidance (a shared trap lets a unit under greater negative pressure, or one that cycles off, pull air through another unit's untrapped drain). Estimated condensate flow (GPH) is derived from your system's latent cooling load: Latent BTU/hr = Total Capacity (tons × 12,000) × (1 − Sensible Heat Ratio), then GPH = Latent BTU/hr ÷ 8,840 (the BTU absorbed per gallon of water evaporated/condensed) — the Sensible Heat Ratio preset for your selected load condition is a typical value for that climate, not a measured figure for your specific equipment. IMC 307.2.4 does not prescribe a trap-height formula — it defers entirely to the equipment manufacturer; the trap height (H) and secondary dimension shown here follow one widely-published manufacturer method (Trane), specific to your selected draw-through/blow-through configuration and to the MAXIMUM static pressure AT THE DRAIN PAN itself (not a whole-system external static pressure figure), as a planning reference for EACH unit's own trap, not a code value or a single shared trap — if units on a manifolded line have different drain-pan pressures, rerun this calculator once per unit. Total drop is your entered slope target multiplied by your entered horizontal run length. This is a planning estimate — always confirm your local plumbing/mechanical code amendments and your specific equipment's own manufacturer guidance, both of which govern over this calculator.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Condensate Drain Line Calculator.
Condensate trap height sizing
IMC 307.2.4 defers trap sizing entirely to the equipment manufacturer — there's no single universal code formula. This calculator follows a widely-published Trane method using the maximum static pressure specifically AT THE DRAIN PAN (not a whole-system external static pressure reading): draw-through (negative pressure) trap height H = drain-pan static pressure + 1 in; blow-through (positive pressure) H = drain-pan static pressure + 0.5 in.
This page defaults to a draw-through configuration at 0.7 in. w.c. drain-pan static pressure — edit the static pressure and configuration above to match your actual equipment's rating.
Condensate Drain Formula: How Is It Determined?
Several independent calculations run off your inputs — pipe size depends on whether the line is single-unit or manifolded, condensate flow from a latent-load formula, trap height from a published manufacturer method specific to your coil/blower configuration, and total drop from your chosen slope and run length.
Minimum Drain Pipe Diameter
Single unit (not manifolded): flat 3/4 in minimum, any tonnage
Manifolded (IMC Table 307.2.2), by combined tons —
Up to 20 tons: 3/4 in
Over 20 to 40 tons: 1 in
Over 40 to 90 tons: 1-1/4 in
Over 90 to 125 tons: 1-1/2 in
Over 125 to 250 tons: 2 in
IMC 307.2.2 states a condensate line shall be not less than 3/4in AND not smaller than the drain pan connection, and shall never decrease in diameter from that connection to the disposal point — that's the complete rule for a single, standalone unit's own drain, regardless of how large that one unit is. 3/4in is only the FLOOR: a large single unit's actual factory pan connection can exceed it, and code requires the line be at least that size too, even though this calculator has no way to know that equipment-specific figure. Table 307.2.2's step-up brackets are invoked specifically "where the drain pipes from more than one unit are manifolded together," sized by the combined tonnage sharing that line. Local jurisdictions can still amend either rule, so always confirm the specific code edition and any local amendments your jurisdiction has adopted, and check your equipment's own spec sheet, before finalizing pipe size.
Estimated Condensate Flow
Total Capacity (BTU/hr) = Tons × 12,000
Latent BTU/hr = Total Capacity × (1 − Sensible Heat Ratio)
Condensate (GPH) = Latent BTU/hr ÷ 8,840
This is the standard engineering conversion for latent cooling load into condensate volume — 8,840 BTU is roughly what's absorbed evaporating (or released condensing) one gallon of water at typical coil conditions. The Sensible Heat Ratio (SHR) preset for your selected load condition (0.90 dry, 0.78 average, 0.65 humid) is a typical value for that climate category, cross-verified against ASHRAE-cited reference points (a 22.43% Latent Heat Ratio is independently cited as producing 0.30 gal/ton-hr, which this formula reproduces almost exactly at the "average" preset) — it is not a measured figure for your specific equipment and duty cycle.
Trap Height (H) and Outlet Dimension
Draw-through (negative pressure): H = Max. Static Pressure AT THE DRAIN PAN + 1 in; J (outlet leg) = H ÷ 2
Blow-through (positive pressure): H = Max. Static Pressure AT THE DRAIN PAN + 0.5 in; K (min. outlet dimension) = 0.5 in
IMC 307.2.4 explicitly defers trap sizing to the equipment or appliance manufacturer — there is no single universal code formula. These two formulas follow a widely-published Trane trapping-design reference for draw-through (coil upstream of the blower, negative pressure at the pan) and blow-through (coil downstream, positive pressure) configurations. The static pressure entered above must be the magnitude AT THE DRAIN PAN itself, not a whole-system external static pressure reading — and it should be the worst-case value (under a dirty filter or dirty coil), not a clean-condition rating. This is a planning reference only — always follow YOUR specific equipment's own installation manual where it differs. On a manifolded line, this figure sizes EACH unit's own trap individually — the same Trane guidance explicitly warns against relying on a single shared trap for multiple drain pans, since a unit under greater negative pressure (or one that cycles off) can draw air through another unit's untrapped drain, bypassing a shared trap entirely. If units on a manifolded line have different drain-pan static pressures, rerun this calculator once per unit with that unit's own value — a single shared input cannot correctly size every unit's trap at once.
Total Drop
Total Drop (in) = Slope (in per ft) × Horizontal Run Length (ft)
Code requires a continuous minimum slope of 1/8 in per ft (about 1%); this calculator also offers a steeper 1/4 in per ft recommended target for more reliable drainage where headroom allows it. This figure tells you how much vertical space your routing needs to clear over its full horizontal run.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Drain line type | Single Unit (Not Manifolded) |
| Cooling capacity | 4 tons |
| Load condition | Average / Moderate Climate |
| Static pressure / configuration | 0.7 in. w.c. / Draw-Through (Negative Pressure — coil before the blower) |
| Run length / slope | 20 ft / Code Minimum (1/8 in per ft) |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| Total capacity | 4 × 12,000 | 48,000 BTU/hr |
| Latent load | 48,000 × 0.22 | 10,560 BTU/hr |
| Condensate flow | 10,560 ÷ 8,840 | 1.19 GPH |
| Minimum pipe size | Single-unit 3/4in floor | 3/4 in minimum |
| Trap height (H) | 0.7 + 1 | 1.7 in |
| J (outlet leg, half of H) | 1.7 ÷ 2 | 0.85 in |
| Total drop | 0.125 × 20 | 2.5 in |
Therefore, this 4-ton single unit (not manifolded) needs a 3/4 in minimum drain line, producing roughly 1.19 GPH, with a 1.7 in trap height and 2.5 in of total drop over the 20 ft run.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.