Quick answer: Fit one pump where a failure means a nuisance you can live with for a day: a garden room toilet, a cellar that floods slowly. Fit two, as duty and standby, where a failure means sewage in the building, a flooded plant room, a lift out of service or a business closed. In a twin station the pumps alternate so they wear evenly, either one can do the whole job, both run together at high level, and an alarm tells you the first one has failed. The 24 hours of storage in a foul chamber is the backstop for the power cut that stops both.
The second pump roughly doubles the cost of the equipment and does nothing at all for 99 per cent of the installation's life. It exists for the day the first pump stops, and whether that day matters is the whole decision.
What actually fails
The pump may not always "die", but what stops a station could be a float that has stuck on grease or a cable, a rag of wet wipes round the impeller, a tripped protector after a run of dry starts, or a power cut.
A standby cannot help if the power fails. So a twin station is only half of a resilient installation. The other half is storage and an alarm.
An important distinction is whether you can control the inflow. With a small sewage pumping station, you may be able to stop using the toilet or connected appliances until a failed pump is repaired. With flooding, you cannot simply turn off the incoming water, and the rate and duration of inflow may be unpredictable. This makes redundancy particularly important for floodwater pumping, where failure could quickly result in damage. The decision should therefore consider how the system is used, whether inflow can be controlled and, most importantly, the consequences if the pump stops working.
How a duty and standby station works
Two identical pumps sit in one chamber. A controller starts them alternately, so each takes every other cycle and they wear at the same rate. If the level keeps rising with one pump running, the second cuts in to assist. If the duty pump fails, the standby takes over without anyone touching anything, and a high level float raises an alarm so the failure is found before the second pump is also lost. Each pump is sized to do the full duty on its own; two pumps that are each half the job are not a standby.
For grey, surface and flood water, the Iguazu Twin packages this in the same 190 litre chamber as our single stations, with two AMA Drainer pumps. For foul water and larger flows the twin stations are built to order with a deeper chamber, a control panel and sewage pumps such as the EVAK Hippo; the below-ground pump stations page is where those start.
When one pump is enough
A single station suits a garden room or annexe toilet used occasionally, a domestic cellar or garage sump where water rises over hours rather than minutes, a flood control sump behind a barrier that is only called on a few times a year, and any installation where the owner is on site, will notice the alarm and can drop a portable pump in for the day. The Iguazu 635 and the single Iguazu Foul Pro are the stations most of our domestic customers end up with, and they are right for them.
When you need two
Fit a twin station wherever the consequence of a stopped pump is unacceptable rather than inconvenient: foul drainage for a whole house or several flats, where failure means sewage backing into the lowest bathroom; a lived-in basement on a Type C cavity drain, where the sump is the only thing keeping the walls dry; a plant room, lift pit or server room; any commercial premises where a flooded floor closes the business; a flood control sump protecting a building during an event, where the pump is being asked to run for days; and anywhere unattended, such as a holiday let or a commercial unit at night. The design standard for pumping inside buildings expects a second pump wherever an interruption cannot be tolerated, and building control will often ask for it on multi-occupancy foul stations.
Storage is the other half
For foul water the regulations require the chamber to hold 24 hours of inflow at 150 litres per person per day, precisely so that a failure or a power cut does not put sewage in the building before someone arrives. For a flood control sump the equivalent question is how long the chamber and the floor can take the inflow before anything is damaged, which is minutes rather than hours, and that is the argument for a twin station on a flood installation even where the pump is only used a few times a year. Our guide to installing a packaged pump station for flood control shows one in use.
Alarms, power and the plan B
You may wish for the station whether, single or twin, to have a high level alarm or a battery backup, mounted where it will be heard. Keep the control panel and the sockets above flood level. For a flood control station, the plan B for a power cut is a petrol pump or a generator (EXTERNALLY). Test a twin station every month by lifting the floats in turn and confirming each pump runs. Most of the twin stations that fail in service had a standby pump that had not run for a year.
At a glance
| Installation | Single or twin | Why | Station |
|---|---|---|---|
| Garden room or annexe toilet | Single | Occasional use, failure is an inconvenience | Iguazu Foul |
| Family house, all foul drainage pumped | Twin | Failure means sewage in the lowest bathroom | Bespoke twin foul station |
| Cellar or garage sump, slow groundwater | Single | Hours of warning, owner on site | Iguazu 635 |
| Lived-in Type C basement | Twin | The sump is the waterproofing | Iguazu Twin or bespoke |
| Flood control sump behind a barrier | Twin, or single plus petrol pump | Minutes of warning, runs for days | Iguazu Twin or Microforce |
| Commercial premises, plant room, lift pit | Twin | Failure closes the business | Bespoke twin with telemetry |
The standby has to be as big as the duty pump
Where to go next
The standard single and twin chambers are in packaged pump stations, the pumps on their own are in sewage pumps, and larger twin foul stations are sized to order through the below-ground pump stations page. Tell us the appliances or the catchment, the lift and the run and we will come back with the right chamber.
Order online, or by phone. Order by 2pm for next working day dispatch.
