Condensate drain size calculator

How much condensate does an AC actually produce?

A residential AC removes water from indoor air as it cools. The amount of water depends on how humid the air is and how hard the system is running. In a dry climate like Phoenix or Las Vegas, expect about 0.1 gallons per hour per ton of cooling. In a moderate climate like Kansas City or Denver, plan for 0.2 GPH per ton. Humid climates like Atlanta or Chicago run 0.3 GPH per ton during peak summer. Very humid climates along the Gulf Coast and Florida hit 0.45 GPH per ton or higher on the most humid days.

A 3-ton AC in Houston produces roughly 1.35 gallons per hour, or 32 gallons over a full 24-hour run cycle. That water has to go somewhere safely. Undersized drains, clogged lines, and dry traps are the most common cause of water damage from HVAC equipment, and the cost of a flooded ceiling is far higher than the cost of getting the drain right at install.

IMC 307.2.2 and IRC M1411.3: the code minimums you must hit

The International Mechanical Code and International Residential Code set the floor for drain pipe size. IMC 307.2.2 and IRC M1411.3 both require:

• Minimum 3/4-inch nominal PVC from the drain pan to the point of discharge
• Never smaller than the drain pan outlet, even if the pan outlet is bigger than 3/4 inch
• Minimum 1/8 inch per foot slope (1 percent grade) toward the discharge point
• Drain to an approved location outside the building or to a plumbing fixture (never directly into a sanitary sewer without an air gap)

For larger commercial equipment, the minimum size scales up: 1 inch for 21 to 40 tons, 1-1/4 inch for 41 to 60 tons, 1-1/2 inch for 61 to 100 tons, and 2 inch for 101 to 250 tons. Most residential systems are well under 5 tons, so 3/4 inch is the legal minimum, but it is not always the best choice.

Why best practice is to oversize residential drains to 1 inch

Code allows 3/4 inch PVC for residential AC. Every experienced HVAC installer will tell you to put in 1 inch anyway. The reasoning is simple: 3/4 inch lines clog quickly from algae buildup, dust, and biological growth. A clog at the drain pan is what causes the float switch to trip and shut down cooling, or worse, water to overflow into the ceiling.

Upsizing to 1 inch costs an extra $5 to $15 in fittings and pipe over the typical 25-foot residential run. The clogging interval roughly doubles because the cross section is 78 percent larger. Service techs see the difference clearly: 3/4 inch drains often need to be blown out every 2 to 3 years on humid-climate installs, while 1 inch drains routinely go 5 to 8 years between service calls. The upsize is one of the cheapest reliability improvements on a residential install.

Slope: the difference between 1/8 inch and 1/4 inch per foot

Code minimum slope is 1/8 inch per foot of horizontal run, which works out to a 1 percent grade. Most installers use 1/4 inch per foot instead, doubling the code minimum. The reason is the same as oversizing the pipe: real-world installs are rarely perfectly straight, level, or kink-free. A 1/4 inch slope gives roughly 100 percent margin against settling, sagging, or unintended low points where water can pool and grow biofilm.

The total vertical drop required is slope times length. A 25-foot horizontal run at 1/4 inch per foot needs 6.25 inches of total drop from the drain pan outlet to the discharge point. At 1/8 inch per foot, the same run only needs 3.125 inches. If you are routing a drain through a finished basement with limited headroom, the slope choice can dictate whether a gravity drain is even possible or whether a condensate pump is required.

Trap depth on draw-through systems

Almost every residential AC and heat pump uses a draw-through air handler, meaning the blower sits downstream of the evaporator coil. The blower creates negative pressure at the drain pan, which would suck air backwards through the drain line if not interrupted. A P-trap on the drain solves this problem, but only if the trap is deep enough to hold against the suction.

The industry-standard formula for trap depth on draw-through systems is:

Minimum trap depth = blower total external static pressure (in WC) + 1 inch

For a typical residential air handler at 0.5 inches WC TESP, the trap needs to be at least 1.5 inches deep. For high-static installs (variable speed blowers running 0.8 inches WC or more), the trap needs 1.8 inches or deeper. Standard pre-made PVC traps are about 2 to 3 inches deep, which covers most residential cases but can be marginal on high-static systems. Field-fabricated traps from PVC fittings give you flexibility to go deeper.

Blow-through systems (less common in residential, more common on rooftop commercial) have positive pressure at the drain pan, so no trap is needed. An anti-siphon vent is still recommended to prevent water lock during off cycles. Check our static pressure calculator if you don't know your blower TESP.

Secondary drain pans and the auxiliary float switch

IRC M1411.3.1.1 requires a secondary drain or auxiliary drain pan when the air handler sits above living space (attic, second story, or above a finished ceiling). The most common configurations:

• Secondary drain pan under the air handler, with its own drain routed to a conspicuous location (often above a window or door so a homeowner notices water dripping)
• Auxiliary drain port on the air handler itself, plumbed separately from the primary, also routed to a conspicuous location
• Float switch in the secondary pan that cuts power to the system when water is detected, preventing further condensate production until the primary is cleared

Skipping the secondary pan or float switch on an attic install is one of the most common code violations and one of the costliest. A clogged primary drain on a 3-ton system can dump 32 gallons into a ceiling over a single hot afternoon. The float switch costs $25 and saves five-figure water damage claims.

When to use a condensate pump instead of gravity drain

Gravity drain to outside the building is the simplest and most reliable option, but it is not always possible. Basement air handlers below grade, attic units with no exterior wall nearby, and crawlspace installs often require a condensate pump to lift the water to a usable discharge point. Pump sizing follows these rules:

• Capacity: at least 1.5 times peak condensate output, in gallons per hour. A 3-ton system in Houston at 1.35 GPH needs a 2 to 2.5 GPH pump.
• Lift: pumps are rated for maximum vertical lift. Most residential pumps handle 15 to 20 feet, more than enough for any residential install.
• Reservoir size: at least 1 gallon for residential to handle pump cycling without short-cycling the float switch.
• Safety switch: every condensate pump should have a built-in float switch that cuts AC power if the reservoir overflows. Most modern Little Giant, Aspen, and Beckett pumps include this.

PVC support spacing and cleanout access

IMC 305 requires PVC drain piping to be supported every 4 feet horizontally and every 10 feet vertically. Sagging unsupported PVC develops low points that hold water and grow biofilm, which is the #1 reason older condensate drains clog. Use proper PVC clamp hangers, not just zip ties or wire wrapped around joists.

Always install a tee with a removable cap at the highest accessible point of the drain. This is the cleanout port for blowing out the drain with shop vac or nitrogen when (not if) it clogs. Service techs charge $150 to $250 per clog clear on an unaccessible drain. A $2 PVC tee at install time saves thousands over the equipment's lifetime.