Hydraulics
How much head your discharge line actually costs you
Total dynamic head is the reason a pump delivers less than its box suggests. It has two parts and they behave differently. The first is static lift — the vertical distance from the water in the basin to the point where the discharge finally releases. You cannot negotiate with it; it is set by where your basement is relative to where the water has to end up. The second is friction: the head the pump uses pushing water through pipe, elbows and the check valve. That part is a function of choices somebody made, and it is the part of head you can change.
Break your discharge down
Illustrative assumption — not a market quote: every value filled in above is a worked-example starting point chosen by this site, not a measurement of your house. Replace each one with your own.
Total dynamic head at that flow
15.24 feet
7.0 ft of lift you cannot change, plus 8.24 ft of friction you probably can.
Biggest swing factor Pipe diameter. In the Hazen-Williams formula this tool uses, head loss varies with the inverse 4.87 power of the bore, so one nominal size up cuts friction by about half or more.
- Friction is 54% of your total head. That is pipe, not lift, and it is the part of head you can change: on this site's own formula, one nominal size up cuts pipe friction by about half or more at the same flow (53% from 1 1/4 to 1 1/2 inch, 70% from 1 1/2 to 2 inch).
Not included in this head figure
- Entrance and exit losses at the pit and the outlet, which this tool does not model.
- Any head added by a freeze plug, a partially blocked outlet, or ice at the termination, none of which this tool models.
- Head added by a backwater or anti-siphon device you have not told us about.
- The pressure needed to open the check valve itself, which some manufacturers publish and which this tool does not model.
Why one pipe size changes everything
At the example's 25 gallons per minute, the same flow through three different pipe sizes loses head at very different rates. This is the argument for not reducing the discharge below the size of the pump's own outlet.
| Nominal size | Actual bore | Friction per 100 ft | Relative to 1 1/4 in |
|---|---|---|---|
| 1 1/4 inch | 1.380 in | 7.96 ft | 1.00x |
| 1 1/2 inch | 1.610 in | 3.76 ft | 0.47x |
| 2 inch | 2.067 in | 1.11 ft | 0.14x |
Read the right-hand column carefully. Moving from a one-and-a-quarter inch line to a two inch line cuts friction to about 14 percent of what it was — not by a fifth, by a factor of several. On a long run, a larger pipe can recover more delivered flow than a bigger pump would.
Elbows are pipe you did not know you had
A fitting is modelled as an equivalent length of straight pipe, because that is the tractable way to account for the head it costs. On this site's assumed values (disclosed below), a 90-degree elbow counts as 30 pipe diameters of straight run; a 45 as 16; a swing check valve as 100; a spring-loaded check valve as 135, which is the cost of the quieter valve.
In the example loaded above, the 5 ninety-degree elbows, 2 forty-five-degree elbows and the spring check valve together add 36.5 feet of equivalent pipe to a 60-foot run. Put differently, the fittings are behaving like an extra 61 percent of pipe that nobody measured.
The practical consequence is not that elbows are bad — you cannot get out of a basement without turning — but that a route with two long sweeps beats a route with six tight turns. On this page's assumed equivalents, a pair of 45s (32 diameters) costs slightly more head than one 90 (30 diameters), so replacing a 90 with two 45s is not a head saving here.
The friction formula, written out
Friction here uses the Hazen-Williams equation in its United States customary form, a published empirical relation for water flowing in pipe:
hf = 0.2083 × (100 / C)1.852 × Q1.852 / d4.8655
hf is head loss in feet per 100 feet of pipe, Q is flow in gallons per minute, d is the inside diameter in inches, and C is a roughness coefficient. The exponent on diameter is where the leverage lives: 4.8655 is why a modest increase in bore produces a dramatic reduction in loss.
We use C = 150, an assumed value for new, smooth plastic pipe. Corrugated flexible discharge — the ribbed hose used to extend a sump line across a lawn — is rougher than this, and a long run of it can cost substantially more head than smooth pipe of the same size.
Related tools
- Sizing calculator — combine this head with your measured inflow to get the full specification.
- Check valves and water hammer — why the 5.4 ft/s above matters at the moment the pump stops.
- Where the discharge may end — a constraint that can decide the route before hydraulics do.