The pump runs, and nobody is using water

You are lying awake at two in the morning and the water pump clicks on for three seconds. Then again four minutes later. Nobody is running a tap, nothing is filling, and the rig is silent in between. That sound is the cheapest leak detector you own, and most people hear it for a week before they work out what it is telling them.

A demand pump does one thing: it runs when the line pressure falls. With every tap shut, pressure can only fall for two reasons. Water is leaving the system somewhere it should not, or the pump’s own check valve is passing and letting water slide back into the fresh tank.

Those two sound identical at two in the morning and they are not remotely the same problem. One is a floor to replace. The other is a job for the next time you are under the rig anyway. Telling them apart takes one extra reading, and this article is mostly about which one.

Catching the pump

a. The shunt you probably already have

A 12 volt demand pump pulls roughly 5 to 10 amps while it runs, which is a large enough step to see on a SmartShunt. Learn your rig’s idle draw first — the converter trickle, the fridge board, an alarm — and then an example rule is discharge above 6 amps while the rig is unoccupied and the furnace is not scheduled to run.

The weakness is that a shunt reads everything on the battery and cannot name a load. A furnace blower, a fridge on DC, a slide motor, a tank heater pad coming on in the cold — all of them look like a pump. And it reads every 60 seconds while a pump chasing a small leak runs in bursts of a few seconds every few minutes, so some bursts land on a poll and some do not. Expect to see the pattern within an hour. Do not expect to catch the first burst.

b. A DC monitor on the pump’s own circuit

This is the honest best method here, and it is also the one we cannot sell you. Wired into the pump feed, a DC power monitor sees the pump and nothing else: current above 1 amp means the pump is running, and current still above it on the next poll, or back above it again and again, is your alert. It reads every 30 seconds, the fastest interval in this family, so a continuously running pump is caught on the first poll.

It also needs a fuse-tap-level wiring job in the pump feed, which rules it out for a lot of people on its own. And it tells you the pump ran. It does not tell you the water got anywhere.

c. The fresh tank, which is slow and certain

Every other method on this page tells you the pump ran. This is the only one that tells you water actually left. Set a warning just under the level you left the tank at — left at 60 per cent, warn below 45 — and any drop with nobody in the rig went through the pump and out of the system.

A SeeLevel board reads every 120 seconds and a Mopeka every 300, but the real delay is the tank: a slow weep takes hours to move a sender. Slow, and worth having anyway, because it is half of the pair that answers the question the whole article is about. Reading tank levels is its own piece.

d. A puck under the pump itself

The pump strainer and its two fittings are among the most common leak points in the whole rig, so a leak puck on the wet bay floor beside the pump very often catches the reason the pump is running. It reports every 120 seconds, or 60 on a BTHome device.

It only knows about the puddle it is sitting in. A split fitting at the far end of the rig runs the pump exactly as hard and never wets this puck. Condensation off a cold pump body in humid weather will, though. Where the pucks go is the longer argument.

e. The pressure-decay test

Most demand pumps cut out somewhere around 45 to 55 psi and cut back in when the line drops 10 to 15 psi below that. So the test is simple: switch the pump off, close the taps, and watch the line. A healthy system holds pressure for hours; a leak or a passing check valve does not. An example warning is pressure below 20 psi within a few hours of switching off.

The hub has a generic water pressure layout that reads every 120 seconds, carrying exactly the same caveat as the DC monitor above — a placeholder rather than a verified product — and it wants a tee in the line. With the pump switched on this method mostly shows a line bouncing between cut-in and cut-out, which is just a pump being a pump. It is a pump-off test or it is nothing.

f. Listed so that you do not try it

A pump starting does sag the house battery, by a tenth or two of a volt, for a few seconds. It is tempting to think a cheap voltage monitor could catch that. It cannot: the sag is smaller than ordinary variation and it is over long before a Lancol‘s next 120-second reading, let alone a 300-second one. Every other load on the battery does the same thing. Voltage alone cannot see the pump, and building this rule only teaches you to distrust the alert.

The combinations

The pair at the top of the cookbook for this family is the first two below, and it is the whole diagnosis.

• Pump cycling and the fresh tank falling — water is leaving the system. That is a leak on the pressure side and it continues until the tank is empty. Get the pump switched off.
• Pump cycling and the tank steady for hours — the pump is cycling on its own check valve, pushing water back into the tank rather than onto the floor. It wears the pump and it is not an emergency. Fix it at the next stop.
• Pump running continuously and the tank under 8 per cent — it is running dry and destroying itself. This is the alert that saves a pump; the leak that caused it has already finished.
• Pump-circuit current up and the puck at the pump wet — two different kinds of sensor agreeing. The pump is running and the water is right there. Drive.
• Pressure holding with the pump off, but the shunt still showing a load — whatever is drawing is not the pump. Look at the furnace, the fridge or a tank heater pad before you go anywhere.

What none of this tells you

No reading we have knows whether the pump switch is even on, and none of them can turn it off. The whole page is also useless on city water, where the pump simply never runs and a leak is fed straight from the post with no tank to empty — for that, the pucks are the whole story.

And the app alerts on a reading crossing a line rather than on a pattern, so “the pump has cycled forty times tonight” is something you see on the history chart in the morning, not something that wakes you. That is genuinely the shape of the thing this article is about, and it is worth knowing before you rely on it.

The thing that beats every sensor here

With that habit in place the sensors change job. They stop being the thing that saves the rig and become the thing that tells you what happened while you were out, which is a much easier job and one they are actually good at. If you own a shunt already, add the idle-draw rule and read the chart. If you own a tank sender, set the warning just under wherever you left it.

And if you can only have one alert reach your phone loudly, make it the pump one rather than the tank one. A tank level falling tells you about a bad afternoon. A pump running dry is a part destroying itself in real time, and that is the message that should sound different from the others.

Where to get one

The named products this article leans on, all bought for other reasons and all earning their place here as a side effect. The generic DC and pressure decoders are deliberately absent: we have nothing to point you at and nothing we have verified.