Why Isn't My Duplex Sump Pump System Alternating?

Duplex Sump Pump Problems: Why Pumps Stop Alternating

You have two sump pumps in the pit.

So you have a backup.

Right?

Not necessarily.

In a properly operating duplex sump system, the pumps are typically controlled so they share the workload or so a second pump becomes available as demand increases.

When that sequence stops working, the problem can go unnoticed for months.

One pump runs.

The water disappears.

Everything looks fine.

Meanwhile, the second pump may barely run at all.

Why Duplex Pump Systems Alternate

Alternating pumps helps distribute operating hours between the two units.

Instead of Pump 1 accumulating thousands of hours while Pump 2 sits idle, the controls can switch the lead position from one operating cycle to another.

Depending on the system, the second pump may also provide additional pumping capacity when the water level rises beyond the normal operating point.

This gives the system two important advantages:

Shared workload.

And redundancy when demand increases or a pump is unavailable.

But only if the controls are actually working.

Sign #1: The Same Pump Starts Every Time

This is one of the easiest clues to spot.

Watch several normal pumping cycles.

If the same pump starts every time and the other never operates, the alternating controls should be investigated.

Possible causes can include:

• Alternator or relay problems

• Control panel faults

• Float switch issues

• Wiring problems

• A pump that has been manually disabled

• A control setting that has been changed

The important point is not to assume the second pump is ready simply because it is installed.

Sign #2: One Pump Has Significantly More Wear

Uneven operating hours eventually show up mechanically.

One pump may have significantly more bearing, seal or impeller wear simply because it has been carrying nearly the entire workload.

If maintenance records show repeated repairs on one pump while the second unit rarely needs attention, look at how the pumps are being sequenced.

The issue may not be the pump itself.

It may be how the system is operating.

Sign #3: The Second Pump Doesn't Start at High Water

The standby or lag pump should be tested intentionally.

Allowing water to reach the appropriate control level during a controlled test can confirm whether the second pump actually receives a start signal.

If the water continues rising while only one pump runs, the system may have a control, float, electrical or mechanical problem.

Finding that problem during a test is much better than finding it during a heavy rain event or sudden influx of groundwater.

Do Not Forget the Floats

Float switches and level controls have a simple job, but they operate in a difficult environment.

Debris can interfere with their movement.

Cables can become tangled.

Floats can hang against piping or the basin wall.

Components can wear.

Electronic level sensors can also develop faults or become affected by conditions within the basin.

Testing the pump without testing the device that tells it when to run gives you only half the answer.

Test Each Pump Individually

Each pump should be capable of operating on its own.

During inspection, verify that Pump 1 runs properly. Then test Pump 2.

Listen and observe.

Does one take longer to empty the basin?

Does one vibrate more?

Is one noticeably louder?

Does either fail to start immediately?

These differences may identify a developing problem before one of the pumps is called on during an emergency.

Two Pumps Should Mean More Reliability

A duplex sump pump system is designed to provide greater pumping capability and redundancy than a single-pump arrangement.

But redundancy needs to be verified.

If one pump has silently been doing all the work, you may effectively have a single-pump system without realizing it.

Academy Pump & Motor provides commercial pump sales, service and repair throughout Calgary and surrounding areas.

If your duplex pump system is not alternating properly, or you simply haven't tested both pumps recently, now is a good time to find out what is actually happening.

Call Academy Pump & Motor at 403-437-7888 or visit academypump.ca.