What is a pressure controller
A pressure controller is the electronic box fitted to the outlet of a house pump that starts the pump when you open a tap and stops it when you close it. It watches two things at once, the water pressure in the pipe and whether water is actually flowing, and it shuts the motor down if the pump loses its water. Because it senses flow as well as pressure, one controller does the job a pressure switch and a pressure tank used to do between them.
Get the choice right and the pump comes on the instant you open a tap and shuts off cleanly. Get it wrong and you get a weak shower upstairs, or a pump clicking on and off all night.
The short answer
Pressure falls when you open a tap. The controller sees it and starts the pump.
The pump runs while water moves and stops when it stops. No cut out pressure to chase.
No pressure and no flow means no water. The controller shuts the pump down to save the seal.
What does a pressure controller actually do?
Three jobs, in one housing, on the pump outlet.
- Pressure sensing. A sensor reads pressure in the delivery pipe. Open a tap, pressure drops, and at the cut in point the controller starts the motor.
- Flow sensing. The part a plain pressure switch cannot do. The controller knows whether water is moving through it, so the pump runs while you draw water and stops a few seconds after flow ceases.
- Dry run protection. No pressure and no flow together means the pump has lost its water, so the controller kills the motor. A centrifugal pump relies on the water through it to cool and lubricate the mechanical seal, and running dry can wreck a seal very quickly, sometimes inside a minute. Never bypass or disable the dry run protection to keep a pump going.
Most of these controllers also carry a non return valve inside the housing, so water in the delivery pipe cannot drain back through the pump when it stops. Check the spec sheet for the model you are buying rather than assuming it, because it changes whether you still need a separate non return valve in the line.
Why does a pressure controller mean I do not need a pressure tank?
A pressure switch only knows pressure. It starts the pump at cut in, stops it at cut out, and that is the whole of its intelligence. Draw a small amount and pressure hits cut out at once, the pump stops, pressure drops and it restarts. That is cycling, and it is hard on the motor, the switch contacts and the pipework.
The pressure tank exists to fix that. It holds a cushion of water against a compressed air charge, so there is something to draw from between starts. A tank is a workaround for a switch that cannot sense flow.
Sense the flow and the problem disappears. The pump runs while you are using water and stops when you are not, so there is nothing to cycle.
That is why a controller replaces the tank on most home installs. One thing still earns a tank, and that is a slow leak. A dripping toilet valve draws too little to register as real flow, so the pump ticks on every so often through the night. A small Davey Supercell pressure tank absorbs it, though the better fix is to find the leak.
What is cut in pressure, and what do I need for my house?
Cut in is the pressure at which the controller starts the pump. It is the lowest pressure your system sees, so it sets the worst case at your highest tap.
The arithmetic is simple. Every metre you lift water vertically costs about 10 kPa, or 0.1 bar. Measure from the controller to your highest outlet, take that off the cut in pressure, and what is left is what the shower head sees when the pump kicks in.
| Cut in | Highest outlet 2 m up | Highest outlet 6 m up |
|---|---|---|
| 1.5 bar, 150 kPa | About 130 kPa left | About 90 kPa left, thin |
| 2.2 bar, 220 kPa | About 200 kPa left | About 160 kPa left |
| 3.0 bar, 300 kPa | About 280 kPa left | About 240 kPa left |
Pipe friction takes more on top, so treat these as the best case. Around 100 kPa left at the highest outlet is workable, 150 kPa or more is what a shower feels good on.
Single storey on a flat site is comfortable on 1.5 bar. Two storey, or a house sitting above the pump shed, wants 2.2 bar or more. That is why the Grundfos PM START comes in 1.5 bar and 2.2 bar versions, and why the Grundfos PM PLUS is adjustable from 1.5 to 3 bar with a gauge built in so you can see what you have set.
The Davey Torrium2 is chosen differently. You match the model to the pressure your pump produces rather than dialling in a number: TT45 for pumps working in the 150 to 450 kPa band, TT70 for 450 to 700 kPa.
A controller does not create pressure. If the pump is too small for the building, no controller will fix it.
Pressure controller or plain pressure switch?
Pressure switch and tank
Pressure only
Starts at cut in, stops at cut out. It cannot tell whether water is moving, so it needs a tank to stop the pump cycling on small draws. Pressure swings between the two set points while you shower, which is the rise and fall people call surging. Most switches have no dry run protection.
Pressure controller
Pressure and flow
Starts on a pressure drop, runs while water flows, stops when it stops. No cut out point, so no surging between set points. Dry run protection is part of the unit and the tank usually comes out of the job. Fewer parts on the wall.
A pressure switch is cheaper and simpler, and there is nothing wrong with one on a shed or an irrigation line where nobody is standing under a shower. For house water, a controller is the better buy.
Pressure controller or variable speed pump?
People get these muddled. A pressure controller is clever about when to run the motor. A variable speed drive is clever about how fast to run it.
Pressure controller
The motor is either off or running flat out. While water flows, the pressure you get is whatever the pump curve gives at that flow, so open a second tap and pressure eases off.
Cheaper, and it can be retrofitted to a pump you already own, though the fitting and wiring is a job for a registered electrician. Suits our Davey HM multistage and Davey HP pressure pumps, and Davey offers those models with a Torrium2 already fitted.
Variable speed drive
The drive measures pressure continuously and changes motor speed to hold your set pressure across the range of flows the pump can manage. Two showers feel much closer to one. It soft starts, and draws less power at part load.
Costs more up front, and the electronics are part of the pump. See the Davey DynaDrive, Davey EvoDrive and DAB E.SYBOX.
Rule of thumb: one bathroom, or an existing pump you are happy with, take the controller. Two bathrooms in use at once, or a long run up a hill, go variable speed. Submersibles are their own case. A Grundfos SBA has its controls built in, and a bore pump like the Grundfos SQE uses a CU 302 control unit instead.
When to call someone
Signs your controller is not right
The pump ticks on with every tap in the house closed. A leak somewhere, or a failed non return valve. Find it before you blame the controller.
It trips on dry run with water still in the tank. Usually a blocked foot valve, an air leak on the suction side, or the pump losing prime. Fix the cause. Do not disable the dry run protection to keep the pump running, because that is the part standing between a lost prime and a ruined seal.
Pressure is fine downstairs and useless upstairs. Cut in too low for the height, or the pump is undersized. Measure the height first, it is free.
Wiring a controller to a pump is mains electrical work. In New Zealand that is a registered electrician's job, not a DIY one. Isolate the pump at the switchboard before anyone opens anything, and leave the electrical side to the electrician.
More in pump parts and accessories and house pumps.
Talk to us
Tell us the pump you have, how high the top tap sits above it, and how many bathrooms are in the house. That is enough for us to tell you which controller suits. Or call in: 65 Kenny Street, Waihi, Monday to Friday 8 to 5, and 74 Kopu Road, Kopu, Monday to Friday 8 to 3. A real person picks up.
