Automatic Pump Pressure Controllers: How Flow Controllers Work and Fail

If you have ever opened a tap connected to a tank-fed pump and wondered what tells the pump to switch on, the answer is almost certainly either a pressure switch with a small pressure vessel, or — increasingly common in South African homes — an automatic flow controller. These two components do the same job by different means, and understanding the difference matters when yours misbehaves.

This article covers how flow-type pressure pump controllers work, what the adjustment settings actually do, and the failure modes you are most likely to encounter.

What a Pressure Pump Controller Actually Does

A conventional pressure switch watches the pressure inside a small sealed vessel. When pressure drops below a set threshold — because someone opened a tap — it closes a circuit and the pump starts. When the vessel re-pressurises, it opens the circuit and the pump stops. That system is explained in detail on the borehole pressure switch and tank page if you want to compare.

A flow controller takes a different approach. It sits directly on the pump outlet and monitors two things simultaneously: water pressure at the outlet port and whether water is actually moving. The moment it detects a pressure drop combined with flow, it energises the pump. When flow stops — all taps closed — pressure at the outlet rises sharply, and the controller cuts power, usually within a few seconds.

The practical advantage is that you do not need a pressure vessel at all. The controller is self-contained, typically smaller than a fist, and can be retrofitted to most centrifugal pump outlets. For homeowners building a backup water system from scratch, this simplicity is attractive.

The Main Settings and What They Mean

Most flow controllers sold in South Africa carry two or three adjustment points. The labelling varies by brand, but the functions are consistent.

Cut-in pressure (start pressure): The pressure below which the controller allows the pump to start. Set this too high and the pump short-cycles, flickering on and off. Set it too low and the pump is slow to respond when you open a tap.

Cut-out pressure (stop pressure / maximum pressure): The upper pressure at which the controller shuts the pump off when no flow is detected. This is often fixed by design on budget units but adjustable on mid-range ones. It must be below the pump’s maximum rated head or the controller will never reach cut-out.

Flow sensitivity (sometimes called dry-run delay): A sensitivity trim that determines how little flow is enough to keep the pump running. Turn it up too far and the pump stays on even with a dripping tap. Turn it too low and the pump shuts off mid-flow when pressure momentarily spikes.

A useful rule of thumb: if your pump hunts (starts and stops rapidly while a tap is open), the flow sensitivity is set too low or the cut-in/cut-out differential is too narrow. If the pump runs on after all taps are closed, the sensitivity is set too high.

How to Adjust an Automatic Water Pump Pressure Controller

Before touching any setting, isolate power and note the current positions — a marker pen line across the adjuster face is sufficient.

1. Identify the adjusters. Remove the controller cover. Most units have clearly labelled screws or dials; a few have a single combined adjuster. Download the manufacturer’s datasheet if you are unsure — generic guides are frequently wrong about which adjuster does what.
2. Adjust cut-in first. Turn the cut-in screw clockwise to raise start pressure, anticlockwise to lower it. Make quarter-turn increments, restore power, and test by opening a tap.
3. Adjust flow sensitivity second. If the pump hunts, increase sensitivity slightly. If it overshoots and runs dry, reduce it.
4. Test for dry-run cut-out. Close all taps and let the pump reach cut-out. It should stop cleanly within about five seconds. If it does not, the cut-out pressure may be set above the pump’s capacity.

If you are uncomfortable working around live electrical components near water — which is a reasonable concern — this is a job to hand to an installer. Incorrectly adjusted controllers void most pump warranties.

Common Failure Modes

Flow controllers are simple devices, but they fail in predictable ways.

Symptom	Likely cause	First check
Pump won’t start when tap opens	Blocked flow sensor port, seized micro-switch	Remove and inspect inlet port for debris
Pump starts, then shuts off mid-flow	Flow sensitivity too low, or partial blockage	Clean strainer; adjust sensitivity
Pump runs continuously, never shuts off	Cut-out pressure above pump’s head, or faulty pressure sensor	Verify pump specs against controller range
Pump short-cycles (rapid on/off)	Cut-in/cut-out differential too narrow	Widen differential or fit small pressure vessel
Controller trips on overload	Dry-running or motor overloading	Check tank level; inspect impeller

Dry-running is the most destructive. When a tank empties and the pump pulls air, a flow controller may not detect the absence of flow correctly — particularly on cheaper units without a dedicated dry-run sensor. The pump then runs hot until the thermal overload trips, or until the impeller or mechanical seal fails. Some mid-range controllers include a low-pressure cut-out specifically to prevent this; it is worth paying for if your tank supply is unreliable.

Sand or fine sediment is the other common killer. South African boreholes and even municipal mains carry particulates that lodge in the controller’s narrow sensing ports. A sediment filter or strainer upstream of the controller extends its life considerably. For a broader look at how the pump control electronics tie into the rest of the system, the borehole pump control box explained article covers the electrical side in more depth.

Not sure which controller type suits your setup? Compare 3 free installer quotes — vetted suppliers, no obligation.

Flow Controller vs Pressure Switch: Which Should You Choose?

Neither is universally better. The right choice depends on your pump type, water source and how consistent your supply is.

Flow controllers suit: smaller centrifugal and peripheral pumps, installations where space is tight, situations where a pressure vessel would be awkward to fit.

Pressure switches with a vessel suit: larger pumps, applications needing very stable pressure, or where the water source is intermittent enough that short dry-run events are likely.

If you are weighing up whether you need a pump at all for a gravity-fed tank, the do you need a pump for a water tank guide walks through the decision.

Water Quality and Controller Longevity

South Africa’s drinking-water quality standard, SANS 241 (2024 edition), sets limits for turbidity, dissolved solids and microbial content in potable water. Municipal water that meets SANS 241 is generally clean enough for a flow controller’s sensing ports. Borehole water is more variable — high iron content causes deposits, and even moderate sediment loads block the fine internal passages over time.

If your water source is a borehole or rainwater tank, fit a 100-micron inline strainer before the controller as a minimum. Replace or clean it every three to six months, or more often if the water is visibly turbid.

If your controller keeps failing and you suspect the wider pump setup is at fault, it may be time for a professional assessment. Get quotes from up to three vetted installers and find out what needs fixing.

When to Replace Rather Than Repair

Flow controllers are not expensive components — typical retail prices in South Africa for a quality unit run from roughly R800 to R2,500, though you should confirm current pricing with suppliers as this market moves quickly. Labour to swap one out is usually under an hour for a competent plumber or pump technician.

Replace rather than adjust when: the controller has tripped on overload more than twice in a season (the internal components degrade after repeated thermal events), when the sensing port is corroded beyond cleaning, or when the pump itself has run dry and you suspect impeller or seal damage. In the last case, replacing the controller without inspecting the pump is an incomplete fix.

For context on the broader cost of a complete backup system — pump, tank, pipework and controls — the backup water systems pillar page sets out what South African homeowners typically pay and what the money covers.