Check valves: the cheapest part with the largest effect

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 the failure that produces a pump which "runs constantly but the water never goes down". It looks like a sizing problem and it is a missing three-inch component.

Swing versus spring, and what the quiet costs

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

A spring-loaded check, usually sold as a silent or quiet check valve, holds the disc closed with a spring so it seats before the column reverses. It is genuinely quieter. It also presents more restriction, which costs head.

Those figures are for a ten-foot lift with a thirty-foot run on inch-and-a-half pipe. The penalty for the quiet valve over the swing valve is 0.25 feet of head — small in absolute terms, and worth paying in a finished basement where the alternative is a bang through the floor joists several times an hour at three in the morning.

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.

The severity scales with velocity, which is why the head tool reports velocity alongside head. Above roughly eight feet per second the effect is usually audible through a floor; below about five it generally is not. The lever is pipe diameter — the same change that reduces friction also reduces velocity at the same flow, so upsizing a discharge line frequently solves the noise and the head loss in one move.

A spring check reduces the hammer directly by seating before the reversal builds. Mounting the valve lower in the vertical run helps too, because it shortens the column that has to stop.

Where to put it, and the detail everyone regrets

Fit the check valve where you can reach it. It is the component most likely to need attention — debris holds a flap open, a spring weakens, a disc wears — and a valve glued into a run behind a finished wall turns a fifteen-minute job into demolition.

Use union fittings on both sides so it can come out without cutting pipe. This costs a few dollars 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 hole the size of a pencil lead and it is the difference between a working pump and one that has quietly stopped being a pump.

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.

If you are only going to change one thing about an existing installation and there is no check valve in it, this is the one. It is a small part, it costs very little, and it removes an entire category of wasted work from every cycle for the rest of the pump's life.

Related pages

• 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.