Borehole Pump Airlock After the Municipal Water Comes Back On

The hybrid setup that's everywhere now

After years of water restrictions in Cape Town, Gqeberha and parts of Gauteng, thousands of homes installed a borehole or wellpoint alongside the municipal supply, with a changeover valve or non-return valve between them. It works beautifully — until the municipal supply returns after a shutdown and your borehole pump suddenly screams, trips, or delivers nothing.

That's an airlock, and it's one of the most misdiagnosed plumbing problems in SA right now.

Why it happens

When the council supply goes down, negative pressure can pull water back through a tired non-return valve, partially draining the rising main from your borehole. When the council supply slams back on, air gets trapped at the high point of the pump discharge line. The pump now spins against air instead of water — which means:

• No flow at the taps.
• The pump overheats within minutes (impellers rely on water for cooling).
• A submersible pump can burn out in under an hour.

Symptoms

• The pump runs but no water comes out, or pressure drops to almost zero.
• A whistling or gurgling sound from the pump head.
• The pressure switch cycles on and off rapidly.
• Borehole pumps trip on overload at the DB board.

How a plumber clears it

1. Switch the pump off at the isolator immediately — don't keep trying it.
2. Open the air-release valve at the highest point of the discharge line (most properly installed systems have one).
3. If there's no air-release valve, the plumber will crack a union fitting at the top of the rising main with a bucket underneath until water flows steadily.
4. Refill the wet-end of jet/surface pumps through the priming port.
5. Restart, and check the non-return valve is actually sealing — if it's not, this will happen every single shutdown.

What to ask for to stop it happening again

• An air-release valve at the high point of the borehole discharge.
• A modern spring-loaded non-return valve at the borehole head (not the old swing type).
• A proper break tank between municipal and borehole — many municipalities now require this by by-law to prevent contamination, and it eliminates the airlock issue entirely.

Don't DIY this if your pump is submersible

Pulling a submersible to inspect it is a two-person, specialist job — the rising main is often 40–80 metres long and full of water. Call a pump technician, not a general plumber.