The pump must be running during the backwash itself, because that is what pushes water backwards through the sand. But the pump must be OFF every time you move the multiport valve handle. Turning the handle while the pump is running can damage the valve’s internal gasket and cause leaks.

The correct pattern is: pump off → change valve → pump on → pump off → change valve → repeat.
 

Table of Contents

1. The short answer: pump on or off?
2. Why the pump has to be off when you turn the valve
3. Why the pump has to be on during the backwash
4. Safe backwash sequence
5. What can go wrong if you backwash with the pump on while turning the valve
6. Special cases: slide valves and variable-speed pumps
7. When backwashing doesn’t solve the problem
8. FAQs

The Short Answer: Pump On or Off?

People often ask whether to backwash with the pump on or off because both can be true at different moments:
 

Water only flows through the filter when the pump runs, so a backwash with the pump off does nothing. The rule exists for the valve, not the sand.
 

Why the Pump Must Be Off When You Turn the Valve

A multiport valve contains a rotating internal part (the spider or rotor) that seals against a gasket as it moves between positions. When the pump is running, water pressure pushes against that seal. Turning the handle under pressure can:

• Tear or wear the spider gasket, leading to water leaking into the wrong port
• Cause water to bypass the sand in the Filter position, leaving water cloudy
• Send a pressure surge through the filter and pipework
• Crack or loosen valve components over time

A worn gasket is one of the most common reasons a filter keeps leaking to waste or fails to clean the water properly, even after a correct backwash.

Why the Pump Must Be On During the Backwash

During backwash, the pump forces water down the pipe in the center of the tank and up through the sand bed. This reverse flow lifts and loosens the sand, and the trapped dirt is carried out through the waste line. Without the pump running, there is no flow, so nothing is flushed.

You’ll know it is working when you watch the sight glass (the clear section on the waste line). Dirty, cloudy water at the start should turn clear by the end, usually within about 2 to 3 minutes.

Safe Backwash Sequence (Multiport Valve)

1. Turn the pump OFF.
2. Move the valve handle to Backwash.
3. Turn the pump ON and run for about 2 to 3 minutes, until the sight glass is clear.
4. Turn the pump OFF.
5. Move the valve to Rinse.
6. Turn the pump ON for about 30 to 60 seconds.
7. Turn the pump OFF.
8. Move the valve back to Filter.
9. Turn the pump ON and check the pressure gauge. This is your new clean baseline.

For the difference between the two steps, see our guide on backwash vs rinse in a pool filter (link this once the article is live). More maintenance guides are in the Pearl Water blog library.

What Can Go Wrong if You Turn the Valve With the Pump On

• Damaged gasket and leaks: Water leaks to waste even when set to Filter.
• Valve cracking: Repeated stress can crack the housing.
• Cloudy pool water: Water bypassing the sand carries debris into the pool.
• Higher repair cost: Replacing a valve is far more expensive than turning off a switch.

Some pool owners add a shut-off switch or labelled pump control near the filter to avoid accidents. Whoever runs the pool, a clear label saying “Pump OFF before turning valve” is a cheap safeguard.

Special Cases

Push-pull (slide) valves: These work differently from multiport valves, so follow the manufacturer’s instructions for your model.

Variable-speed pumps: Many manufacturers recommend running backwash at a specific speed. Check your pump and filter manuals, because the right setting varies by model.

Large commercial pools (hotels, resorts, clubs): These systems often have several filters and valves. A written, step-by-step procedure for staff reduces mistakes.

When Backwashing Doesn’t Solve the Problem

If pressure stays high or the water stays cloudy after a correct backwash, the issue may be:

• Old or channelled sand that no longer filters well
• Oil or scale coating the grains
• A worn valve gasket, often from past mistakes
• A filter that is too small for your pool volume and pump flow

In those cases, replacing sand, servicing the valve, or upgrading the filter makes more sense than repeated backwashing. Explore the swimming pool sand filter range at Pearl Water to compare sizes and options.
 

Final Takeaway

You need the pump on for water to flow during backwash, but off whenever you move the valve handle. Follow that pattern and you’ll protect the valve, keep the water clear and avoid costly repairs. When your filter needs replacing or upgrading, see the Pearl Water pool filter collection, and browse more how-to guides on the Pearl Water blog.

FAQs

1. Can I backwash a pool filter with the pump on?
Yes, the pump runs during the backwash itself. But switch it off before you move the valve handle to any position.

2. What happens if I turn the multiport valve with the pump running?
You risk damaging the internal gasket, causing leaks to waste, cloudy water and valve wear.

3. Does backwashing work with the pump off?
No. Without the pump there is no water flow, so nothing is flushed out of the sand.

4. How long should I run the backwash?
About 2 to 3 minutes, or until the sight glass runs clear.

5. Do I need to rinse after backwashing?
Yes. Rinse for about 30 to 60 seconds before returning to Filter.