BasementSoil

Sump pump sizing: the calculation contractors skip

Quotes specify pumps by horsepower. Pumps are specified by manufacturers as flow at a given head. Those are not the same thing, and the gap is where basements flood.

Nearly every basement drainage quote names a pump by horsepower. Horsepower is close to useless as a specification. Two one-third horsepower pumps from different manufacturers can differ by forty percent in delivered flow at ten feet of head, because what you are buying is a pump curve, not a motor rating.

Here is the calculation, so you can do it before the visit and check what you are offered.

Step 1: how much water arrives

The measured method, which is far better. During heavy rain, unplug the pump, and time how far the water rises in the pit. Plug it back in immediately.

Gallons = π × (pit radius in inches)² × rise in inches ÷ 231. Multiply by 3600 and divide by your elapsed seconds to get gallons per hour. An 18-inch pit filling 4 inches in 60 seconds is about 4.4 gallons a minute, or 265 gallons an hour.

The modelled method, if you cannot wait for rain: estimate from footprint, nearby roof area, design rainfall intensity and a runoff coefficient from the hydrologic soil group. Less reliable, and it is what the calculator does when you tell it you have not measured. Both are built into the tool.

Step 2: what the pump works against

Total dynamic head is vertical lift plus friction loss.

Nine feet of lift, a 25-foot run and three elbows is not nine feet of head. It is more like eleven or twelve, and pump output at twelve feet is materially lower than the headline figure on the box, which is usually quoted at zero or ten feet.

Step 3: apply a safety factor

Take measured inflow and multiply by about 1.5. Pumps cycle rather than running continuously, storms peak above their average, and a pit has finite volume. A pump matched exactly to average inflow is a pump that is losing ground during the worst hour.

Step 4: what to ask for

“Show me the manufacturer’s performance curve for the pump you are installing, and point to the flow at my total dynamic head.”

Every reputable pump has a published curve. A contractor who cannot produce it for the model they are installing is telling you something useful about the rest of the specification.

The three things that actually decide whether you flood

  1. Backup power. The storm that produces the inflow is the storm that takes out the power. One pump with no backup fails at precisely the moment it is needed, and no amount of capacity fixes that. Buy the backup in the same contract.
  2. Where the discharge ends. Ten feet from the wall minimum, further on clay, downhill, not into a neighbour’s yard, and never into a sanitary sewer — illegal in most municipalities and capable of backing sewage into your own basement. If buried, rigid smooth-wall pipe with a cleanout.
  3. The freeze path. In almost every state we cover, a frozen discharge line makes a working pump useless. Ask specifically how the installer handles winter discharge, and be suspicious of a shrug.

The check valve, briefly

Without one, the water in the vertical pipe falls back into the pit each time the pump stops, and the pump pumps the same water again. With a badly chosen one, you get water hammer that eventually cracks fittings. A quiet check valve mounted where it can be replaced without cutting the whole line is a small detail that separates a careful installation from a fast one.

Run the numbers for your pit and pipe run, then take the result into the quote conversation.

Questions people actually ask

How long does a sump pump last?

Seven to twelve years is a reasonable planning assumption for a decent unit in moderate duty, less if it cycles constantly. It is a wear item with a motor and a switch, and the switch usually fails first. Treat replacement as scheduled maintenance rather than an emergency, and note the install date on the pipe with a marker.

Two pumps or one bigger one?

At high inflow, two pumps at staggered float heights beat one large pump. You get redundancy, the lead pump handles ordinary flow without short-cycling, and the second only runs when it is needed. One large pump has a single failure point and runs inefficiently most of the year.

Battery backup or water-powered?

Battery is self-contained but the battery is itself a consumable and needs testing; capacity is finite, so a long outage exhausts it. Water-powered runs indefinitely and needs no charging, but requires municipal water pressure — useless on a private well — and consumes treated water while running. Where mains pressure exists, water-powered is the more robust of the two.

Does the discharge line matter that much?

It is the most commonly botched part of the installation. Corrugated flexible pipe silts up and cannot be cleared. A line ending two metres from the wall on clay recirculates the same water. And in the northern counties we cover, a line that freezes turns a working pump into a sealed system.

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