RO not filling the tank? Empty and slow are two different faults, with two different lists.

Which fault you have: a five-minute test

Draw water from the tap until the stream dies, so the tank is empty and the purifier has every reason to run. Open the cabinet, plug in, open the inlet, and watch for five minutes. Three things can happen.

1. The pump hums, the drain line runs steadily, and the tap gives a trickle. The purifier is making water, just slowly. That is a flow fault and the second list below.
2. Silence. Nothing at the drain, nothing at the tap. The pump is not running. That is a control fault and the first list below.
3. The pump hums but the drain stays dry. The pump is running with no water to push, or the path is blocked. Start with the inlet in the first list, then the drain line in the second.

The drain is the tell. A domestic RO sends two to three litres to drain for every litre it makes, so a purifier that is working always shows a steady flow at the reject line. No drain flow means no water is moving, whatever the pump is doing.

Not filling at all: the control chain, in order

For the pump to run, five things have to say yes in sequence. Check them in this order, because each one upstream can mimic the one below it.

1. Power. The adapter's light is on, the socket works, the lead reaches the machine. If the light is off with the socket live, the adapter has failed. It is a mains part and is replaced, never opened; the service vs DIY guide puts it on the technician side.

2. Water at the inlet. The inlet tap open, supply in the line, the tube not kinked behind the machine. An overhead tank that runs low overnight is the most common reason a purifier is dead at six in the morning and fine by nine. Pull the tube at the purifier into a bucket and open the tap for a few seconds: a strong stream says the supply is there.

3. The low-pressure switch. Many purifiers carry a switch on the inlet that holds the pump off when the supply pressure is too low, so the pump does not run dry. A weak supply trips it and the machine looks dead. The fix is the supply, never the switch; a pump run dry is a ₹1,299 part lost to save a bucket test. A choked wall-mounted pre-filter trips it too, which a fresh candle in the two-candle kit sorts out.

4. The solenoid valve. An electric valve on the inlet that opens when the pump is told to run and closes when the tank is full. A coil that has burnt out stays shut and no water enters, with the pump humming against nothing. Its coil is on the adapter side and it is a make-specific part; a technician's job.

5. The float switch or pressure switch. The part that tells the pump the tank is full. On a gravity-tank purifier it is a float; on a pressure-tank machine, a high-pressure switch. Stuck at full, it never lets the pump start and the tank stays empty. Stuck at empty, it never lets the pump stop, the pump runs on with a full tank, and a gravity tank overflows onto the cabinet floor, which is the case the leak guide covers. Do not bridge it to test the pump: that is how a good pump runs dry or a tank floods the adapter.

Only then, the pump. Power present, water at the inlet, switches good, and the pump silent or humming without turning: it is the pump. The LUX-PUMP-100 is the LUXGEN replacement for 100 GPD systems at ₹1,299, and the booster pump guide explains how to be sure and which pump each membrane needs. Fitting it is a technician's job, since the adapter is mains and a pump plumbed backwards runs and delivers nothing.

Filling slowly: the flow list

If the drain runs and the tap trickles, the purifier is working and something is restricting it. Most of that list is on the slow flow guide, which walks the water path from the sediment cartridge to the membrane in order of cost, and its first step, a fresh sediment cartridge at ₹190, fixes most slow tanks. Three causes belong here because they are about the tank and the drain rather than the cartridges.

A kinked or crushed drain line. The reject line has to fall freely to a drain with an air gap. Crushed behind the machine, or pushed under the water in a sink trap, it holds the concentrate back, the membrane sits in its own brine, and production drops while the output TDS rises. Straighten it and the tank fills again; the installation checklist shows the right run.

Cold feed. A membrane passes less water when the feed is cold, and a purifier in a cold winter can take half as long again to fill with nothing wrong. If the slowdown arrived with the season and the TDS ratio has not moved, wait for the water to warm.

A membrane that is genuinely done. Last, because it is the dearest and because slow filling on its own does not prove it. Read the feed and the output; when the output has roughly doubled from the day-one figure, the membrane is due, and the 80 GPD at ₹650 or the 100 GPD at ₹800 behind a pump rated for it is the fix. The membrane replacement guide gives the test.

How slow is slow: the LUXGEN purifiers are rated at 15 litres an hour, so an 8 or 9 litre tank should fill from empty in well under an hour on fresh cartridges. A refill that takes three hours is a fault; one that takes an hour in January and forty minutes in June is the weather.

What the float, the switches and the solenoid do

Four small parts control a purifier, none of them a LUXGEN part, and all of them a few hundred rupees depending on the make. Knowing what each does stops the wrong one being replaced.

All four sit on the adapter side of the wiring or on the pressure side of the plumbing, and each one exists to protect something dearer than itself. That is why none of them should be bridged, taped or removed to see whether the pump runs. Find which one has failed by the order above, buy the part, and let the technician fit it with the machine off.

Both lists in one table