The baking soda and vinegar trick is everywhere: a cup of each, a plug over the hole, ten minutes of fizzing, then a kettle of boiling water. It feels like it’s working. On a real blockage, it isn’t.
This guide explains what the fizz actually is, what boiling water and shop-bought drain cleaners do, and what clears a slow shower or basin. Most of it you can do yourself. Anything past the trap is a job for a Licensed Plumber or Drainlayer.
Does baking soda and vinegar unblock drains?
No. Baking soda is a mild alkali and vinegar is a mild acid, so they neutralise each other into salty water and carbon dioxide. The gas escapes up the plughole instead of pushing on the blockage, and nothing in the mix dissolves hair, fat, soap scum or wipes. We couldn’t find a published test showing it clears a blocked drain.
The US National Capital Poison Center puts it plainly: because drains aren’t airtight, “the gas generally does not build up enough pressure to release clogged drain contents”, and the mix “is unlikely to be strong enough to melt away tough drain clogs”.
What happens when you mix them?
Sodium bicarbonate reacts with acetic acid to make sodium acetate, water and carbon dioxide. The fizz is the gas leaving. Once the acid is used up, the reaction stops, and what’s left is a weak salt solution with leftover baking soda.
The usual recipe is badly out of balance. Vinegar sold in New Zealand must contain at least 4% acetic acid under the Food Standards Code, so a cup of it holds about 10 grams of acid. A cup of baking soda weighs over 200 grams. On those numbers, less than a tenth of the baking soda reacts.
The fizz also happens where you pour it, at the plughole, not at the blockage in the trap or further down. Putting the plug in, as most recipes say, doesn’t move the reaction any closer to it.
Does it do anything at all?
A little, at the surface. The bubbling and the hot rinse after it can lift loose film around the plughole, which is why a drain sometimes looks cleaner. But hot water and a brush do the same. It won’t harm your pipes, which is its main advantage over chemical cleaners. Its real cost is time while the blockage grows.
Is boiling water a safe way to clear a drain?
Not for fat. Watercare says hot water “merely transports the grease further down the sewer line”, where it sets again. And while a kettle now and then is within what PVC waste pipe can take, boiling water is well above the temperature the pipe is rated for, so it’s not a habit worth forming.
Marley rates its Optim PVC waste pipe for a maximum operating temperature of 60°C. Holman, another PVC drainage maker, puts the softening point of its fittings at about 80°C and says short, small hot discharges are fine because PVC is slow to heat through; large volumes over long periods are not.
Don’t pour boiling water onto a drain that already has chemical cleaner in it, where it can splash back. For a blocked toilet, see our blocked toilet guide instead.
Are chemical drain cleaners safe to use?
They work on grease and hair because they’re highly corrosive, and that’s the risk. Almost every drain cleaner sold in New Zealand is based on sodium hydroxide (caustic soda), at strengths that cause severe burns to skin and eyes. Used exactly as the label says, on a slow drain, they can help. On a fully blocked one, they sit there.
ESR’s 2015 health risk assessment for the Ministry of Health found three main types on New Zealand shelves:
| Product type | What’s in it | What to know |
|---|---|---|
| Crystals | About 55 to 60% sodium hydroxide, often with aluminium chips | The aluminium reacts when wet to stir the mix, and the crystals get hot as they dissolve |
| Liquids and gels | About 16 to 19% sodium hydroxide | Far above the 5% at which ESR expects severe chemical burns |
| Bleach gels | Under 5% sodium hydroxide plus 4 to 10% sodium hypochlorite (bleach) | Never follow with vinegar or another cleaner: bleach and acid release chlorine gas |
The labels carry the rest of the warnings. Drano’s Max Gel label says not to use it in toilets or with a plunger, and never to plunge “during or after use” because the product may still be in the pipe if the drain hasn’t cleared. The same applies to whoever opens the trap next. If you’ve used a drain cleaner and the drain is still blocked, tell the plumber what you used before they start.
If any gets in your eyes or on your skin, rinse with plenty of running water and call the National Poisons Centre on 0800 764 766.
Do enzyme drain cleaners work?
Slowly, and only on build-up. Enzyme and bacterial cleaners digest fats and organic matter rather than burning through them, and ESR found the enzymes in New Zealand products aren’t a health risk. Typical directions call for a dose overnight, repeated over several days, so they suit a sluggish drain or upkeep after one’s been cleared. They won’t shift a solid blockage, and like any drain product, they shouldn’t be mixed with another.
What actually clears a slow shower or basin?
Hair, soap and the gunk that binds them, caught at the waste just under the plughole. Remove the plug or grate, pull out what you can reach, and use a plunger if it’s still slow. That fixes most slow showers and basins. If it doesn’t, the blockage is beyond the trap.
- Lift out the plug or grate. Most basin pop-up plugs lift or unscrew out, and shower grates lift off. Clean the hair and slime off the plug itself.
- Fish out the hair. A plastic drain hair tool, a barbed strip sold at hardware stores, slides down past the plug and pulls hair up with it. Wear gloves, and don’t force it.
- Plunge. Put a few centimetres of water in the basin or shower and, on a basin, block the overflow hole with a wet cloth so the plunger can build pressure, then plunge up and down quickly until the water starts to drain.
- Run the hot tap for a minute to see if it’s cleared.
A kitchen sink is a different problem: fat and food rather than hair, and the trap under it is worth taking apart. Our guide to a kitchen sink that’s blocked or smelly covers it.
How do you stop a drain blocking again?
Catch hair before it goes down, and keep fat and wipes out. A hair catcher over the shower and basin waste, emptied often, does more than any monthly treatment. Watercare asks that fats, oils and grease go in the bin or your food scraps bin, and wipes in the rubbish, never down the drain.
When should you stop and call a plumber?
After two honest attempts, or straight away if more than one fixture is slow. A shower that backs up when the washing machine drains, gurgling from a toilet, or water rising in the gully trap outside all mean the blockage is in the shared drain, not the fitting. More chemicals won’t reach it.
Our gully trap guide explains what an overflowing gully means and who to call first. A drain that blocks again within weeks usually has a cause in the pipe itself: a dip, a rough joint, or roots in an older line.
What we do when you call us about a slow drain
We find out what’s draining slowly, what’s been tried and whether any chemical has gone down it, then clear it at the source with the right tool for the pipe. If it keeps coming back, we find out why. You hear what we found and your options, with the price in writing before we start.
Our blocked shower and sink page covers showers, basins, baths, laundry tubs and kitchen sinks.