Components

Check valves: a small part with a large effect on cycling

A check valve is a flap or a spring-loaded disc in the discharge line that lets water out and refuses to let it back. It is one of the smallest components in the system and it has an outsized effect on two things this site's cycling arithmetic tracks: how many gallons the pump has to lift twice, and how many times the switch is asked to work.

What happens without one

When the pump stops, everything standing in the vertical section of pipe is above the basin, and gravity has an opinion about that. Without a check valve, all of it drains straight back into the pit. The float rises again, the pump starts again, and it lifts the very same water it just lifted.

The volume is easy to underestimate. A ten-foot vertical run of one-and-a-half-inch pipe holds a little over a gallon — which, in a basin whose float drawdown is only a few gallons, can be a substantial fraction of a whole cycle. At worst the returning water alone is enough to re-trigger the float, and the pump enters a loop where it does nothing but pump the same gallon up and down until the switch fails.

That is one way to get a pump which runs constantly while the water never goes down. It looks like a sizing problem and it is a missing check valve.

Swing versus spring, and what the quiet costs

A swing check has a hinged flap that gravity and backflow push shut. It is simple and has little restriction when open — and it closes only once the water has started coming back, which means it closes on a moving column.

A spring-loaded check, sometimes sold as a silent or quiet check valve, holds the disc closed with a spring so that it seats before the column reverses. On this site's assumed equivalents it presents more restriction, which costs head.

Check valve	Equivalent length	Total head at 30 gpm	Friction penalty
None fitted	0 diameters	12.74 ft	baseline
Swing check	100 diameters	13.45 ft	+0.71 ft
Spring-loaded check	135 diameters	13.70 ft	+0.95 ft

Those figures are for a 10-foot lift with a 30-foot run on inch-and-a-half pipe with 3 90° elbows, at 30 gpm. The penalty for the spring valve over the swing valve is 0.25 feet of head — small in absolute terms; whether a quieter closing is worth it in a finished basement is your call.

Why the bang happens

Water in the discharge line has mass and it is moving. When the pump stops, that column decelerates, reverses, and then meets a valve slamming shut against it. The energy has to go somewhere, and it goes into the pipe as a pressure spike and into the structure as noise.

Higher velocity means a harder slam, which is why the head tool reports velocity alongside head. Assumed, no direct source: the head tool flags velocities above 8 ft/s. The lever is pipe diameter — the same change that reduces friction also reduces velocity at the same flow, so upsizing a discharge line lowers both at once.

A spring check is designed to seat before the reversal builds, which is how it aims to reduce the bang.

Where to put it, and the detail everyone regrets

Fit the check valve where you can reach it. It is a component that needs attention — debris can hold a flap open, a spring can weaken, a disc can wear — and a valve glued into a run behind a finished wall turns a service job into demolition.

Use union fittings on both sides so it can come out without cutting pipe. This adds two fittings at installation and it is the difference between servicing the valve and replacing a section of the discharge line.

Fit it above the basin lid rather than below it where possible. A valve inside the basin is in the wettest, dirtiest part of the system and cannot be inspected without opening everything up.

And drill the weep hole if the pump instructions call for one. A small hole in the discharge pipe below the check valve lets trapped air escape so the pump can prime; without it a pump can spin against an air lock and move nothing at all while sounding completely normal. It is a small hole, and it can be the difference between a working pump and one that has quietly stopped pumping.

The connection back to switch life

Everything on this page is really about the same thing as the cycling section on the sizing tool: how many times the switch is asked to operate. Backflow adds starts. Short drawdown adds starts. An undersized basin adds starts.

Without a check valve Every cycle re-lifts the standing column Extra starts, extra run time, no benefit.

Swing check +0.71 ft of head Stops backflow; least head of the two valve types.

Spring check +0.95 ft of head Quieter, slightly more restrictive.

If there is no check valve in an existing installation, it is a strong candidate for the first change: it is a small part, and it removes an entire category of wasted work from every cycle.

Head and discharge — the tool that produced the numbers above.
Install scope — a missing check valve appears there as a required line.
Pump style — the other half of the cycle-life question.