Reverse osmosis water treatment system with stainless tanks and pumps
5 components covered

Field manual · Water Softener

Under-sink reverse osmosis systems: field service manual

Covers: Residential under-sink RO units with a pressurized storage tank (typical 4-5 stage)

Cross-checkedBrands: All5 components

Feed water passes pre-filters, then a semi-permeable membrane that splits it into purified product water and a reject stream to drain. Product water is stored in a small pressurized tank and passes a post-filter on the way to a dedicated faucet.

Safety first

  • Close feed before opening housings
  • Keep spare O-rings; reused ones can stretch and leak

How it works

Pre-filtration: a sediment filter and carbon filter(s) protect the membrane from particles and chlorine.

Membrane: under feed pressure, water is pushed through the membrane; dissolved solids go to drain through a flow restrictor that sets the back-pressure the membrane needs.

Storage: the tank has an air chamber and a water chamber separated by a diaphragm; filling compresses the air, and that air pressure pushes water to the faucet.

Shut-off: the auto shut-off (ASO) valve watches feed and tank pressure and closes the feed when tank pressure reaches about two-thirds of line pressure. A check valve on the product outlet keeps tank pressure from pushing back into the membrane so the ASO can close.

Post-filter: a carbon polishing filter just before the faucet.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Reverse osmosis water treatment system with stainless tanks and pumps by Mas mufti, CC BY-SA 3.0

Diagnostic guides for this appliance

Guide · Water Softener

Water tastes salty after the softener regenerates

Softened water should not taste salty. A salty taste means brine was not fully drawn and rinsed out of the resin: clogged injector or brine line, restricted drain line or drain flow control, low supply pressure during regeneration, or a regeneration interrupted by a power or pressure loss.

5 steps ~60 minmoderate
Guide · Water Softener

Water softener uses too much salt or regenerates every night

Excess salt use is almost always a settings problem: hardness programmed too high, a timer set to regenerate every day, wrong capacity or salt dose for the household, or a lost program. Mechanical causes include a leaking brine valve, iron-fouled resin and a brine tank that overfills.

4 steps ~40 mineasy
Guide · Water Softener

Softener regenerates at the wrong time (often after a power outage)

Most timer and metered controls schedule regeneration by the clock, typically in the middle of the night. After an outage the clock can be wrong or reset, so regenerations happen when people are using water. The softener still softens, but the clock and settings need checking.

4 steps ~20 mineasy

Other water softeners & filters manuals

Manual · Water Softener

Control-valve water softeners (Fleck, Clack, big-box units): field service manual

Single-tank ion-exchange softeners with an electronic control valve (Fleck 5600/7000, Clack WS1, Whirlpool/GE/Morton cabinet units). Kinetico twin-tank units are non-electric (water-meter driven) and Culligan units are usually dealer-serviced.

7 components4 common failuresFleck (Pentair), Clack, Whirlpool, GE, Morton, Culligan, Kinetico
Under-sink reverse osmosis systems: field service manual · OnCall Tech