What is a pressure switch

Pump guide

A pressure switch is the electrical on and off switch for your water pump. Water pressure works a diaphragm inside it, and that diaphragm works a set of electrical contacts: when system pressure falls to the cut in setting the contacts close and the pump starts, and when it climbs to the cut out setting the contacts open and the pump stops.

That is the whole job. It reads pressure and nothing else, which is why it is nearly always paired with a pressure tank.

The short answer

1. What it is

A small box on the pump or the pipework beside it, with the pump power cable running through it.

2. What it does

Starts the pump at cut in, stops it at cut out. It reads pressure only, never flow.

3. What it needs

A pressure tank with a healthy air charge, or the pump cycles itself to death.

How does a pressure switch work?

Inside is a diaphragm or small piston connected to the water in the system, loaded against a spring. Water pushes one way, the spring pushes the other, and a lever between them carries the electrical contacts.

Open a tap and pressure drops. When it drops far enough for the spring to win, the contacts snap closed and the pump runs. The pump pushes pressure back up, the water side wins again, the contacts snap open and the pump stops. On most domestic single phase pumps the mains supply to the motor runs straight through those contacts, so the switch carries the pump's full starting and running current every time it operates.

Under the cover on most switches, a large spring and nut set the range, moving cut in and cut out together, and a smaller spring sets the differential between them. That same cover sits directly over live mains terminals. If a switch needs adjusting, the power comes off at the switchboard first, and if you are not certain what you are looking at, leave it to a registered electrician or a pump technician.

What do cut in and cut out mean?

New Zealand pumps are rated in kPa. Older gauges and switches are often marked in bar or psi.

Term	What it means
Cut in	The falling pressure at which the contacts close and the pump starts. Roughly the lowest pressure the system sits at, though the tap will read lower again once height and pipe losses are taken off.
Cut out	The rising pressure at which the contacts open and the pump stops. The highest pressure the system normally sees.
Differential	The gap between the two. Wide means fewer starts but more swing. Narrow means steadier pressure but more starts.
Units	100 kPa is 1 bar, roughly 14.5 psi, and about 10 metres of vertical head.

Settings vary by pump and installation, so read your pump manual rather than a number off the internet. What matters is that your pressure is never constant. It swings between cut in and cut out all day, which is why the shower changes while the pump runs.

What a pressure switch does not do

A pressure switch sees pressure. It does not see water.

This is the part that costs people a pump. If your tank runs dry, the foot valve loses prime or the suction line sucks air, the pump keeps spinning and never builds pressure, so the switch never reaches cut out and never opens. The pump runs on with nothing in it until the thermal overload trips or the seal cooks. A plain switch cannot tell a pump working hard from a pump running dry.

Some switches carry a low pressure trip that shuts the pump off if pressure never recovers, but that is crude next to a controller.

Do I have a pressure switch or a pressure controller?

Pressure switch

Box, spring, tank

A small box on the pump or on a tee in the pipework, usually a single cover screw, springs and adjustment nuts underneath. Almost always a pressure tank nearby, a vessel with a tyre valve on it. You can tell it apart from the outside, so leave the cover on. There is live mains behind it.

Distinct click as the pump starts and again as it stops, and it cycles on and off while you shower.

Pressure controller

Electronic module, LEDs

A slim electronic unit screwed into the pump outlet with the water passing through it. Indicator lights on the front, no springs, no adjustment nuts, often no tank at all.

The pump runs steadily while a tap is open and stops a moment after you close it, instead of clicking on and off.

A controller such as the Davey Torrium2 or the Grundfos PM PLUS and PM Start starts the pump on a drop in pressure like a switch does, but stops it on loss of flow rather than on reaching a set pressure, and most models shut the pump down if they detect a dry run. Protection features vary between models, so check the spec sheet for the one you are buying. That is what lets it hold pressure steadier than a switch and tank, and gives the pump a layer of protection a plain switch cannot. Full breakdown on our what is a pressure controller page.

Variable speed pumps go further again. The Davey DynaDrive, Davey EvoDrive and DAB E.SYBOX have the control built in and vary motor speed to hold pressure steady as demand changes, so there is no separate switch to fail.

Why does a pressure switch need a pressure tank?

Water does not compress, so with only a pump and a switch, every small draw becomes its own pump start. A pressure tank fixes that by storing water under an air charge. Between cut out and cut in the tank feeds the house on its own and the pump stays off. The volume it gives up over that range is the drawdown, and it is what buys the pump a decent run.

Rapid cycling

When a bladder tank loses its air charge, or the bladder splits and the tank fills with water, the drawdown disappears and the pump starts and stops every few seconds under any flow at all. Every one of those starts is a hit on the switch contacts, the motor windings and start capacitor, and the bearings and seal.

If your pump is clicking on and off every few seconds, switch it off and get the tank checked before you replace anything.

The air charge is set with the system depressurised and wants to sit just below cut in, so the tank still holds water when the pump starts. Check the figure in your tank manual, because it is tied to your cut in setting. We stock Davey Supercell pressure tanks and the rest in pump parts and accessories.

How do I know if my pressure switch has failed?

Symptom	What it usually means
Pump will not start, no click, power on at the board	First check the system is actually below cut in, because a pump sitting at full pressure is meant to stay still. If it is genuinely low, suspect contacts burnt or stuck open, or a failed diaphragm. Common on a switch that has done a lot of starts.
Pump will not stop, pressure climbing	Contacts welded shut. Kill the power at the switchboard before the pipework sees pressure it was never built for.
Pump will not stop, pressure low or falling	Usually not the switch. Look for a leak, a lost prime, an empty tank or a worn pump that cannot reach cut out.
Chattering, buzzing or arcing	Pitted contacts, a tired spring, or a sensing port blocked with silt or scale. Arcing contacts are a fire risk, so isolate the power at the switchboard and get it looked at rather than run it.
Starts and stops every few seconds	Almost always the pressure tank, not the switch. Check the tank first.
Water weeping from the switch body	Usually a split diaphragm or a failed port seal, and either way water is getting into a live enclosure. Isolate the power at the switchboard and do not touch it.
Trips the RCD or breaker	Moisture inside the switch or a motor fault. Electrician job, not a guessing job.

Check the tank before you condemn the switch. Plenty of failed switch call outs are really a flat tank.

Can I change a pressure switch myself?

The mains supply to the pump runs through the switch. In New Zealand, connecting and disconnecting fixed wiring like that is prescribed electrical work, so it belongs to a registered electrician. There is no way to get the switch off the pipework without the wiring coming off first, so this is not a job you can split in half and do the easy part of. Book the electrician.

If the switch has already failed twice, replacing it a third time is treating a symptom. Something is causing the extra starts, and until that is sorted the new switch goes the same way.

Either fix the tank and keep the switch, or fit a Torrium2 or Grundfos PM PLUS to the existing pump. If the pump is due anyway, a variable speed unit from house pumps covers both jobs.