The short answer
Try hot water first, and step up to a chemical unblocker only if it fails. A flush of hot water — for plastic pipes, hot from the tap rather than fully boiling — softens fresh grease and can clear a mild, recent kitchen-sink slowdown for free, with no risk of harsh chemicals. A chemical drain unblocker (usually caustic soda based) dissolves more stubborn grease and hair and is the better tool for a firmer or established blockage, but it is corrosive and must be handled carefully. The catch with boiling water is pipe safety: poured into plastic waste pipes or onto a porcelain WC it can soften joints or, rarely, crack ceramic, so very hot tap water is the safer everyday choice. Hot water for prevention and light grease; an unblocker, used with care, for a real blockage.
When a sink starts to drain slowly, the simplest and cheaper first move is hot water, but it has limits and a couple of safety caveats. A chemical unblocker is stronger but harsher. Here is which to reach for and when.
Unblocker vs hot water
- Hot waterSoftens fresh grease, free
- Chemical unblockerDissolves grease and hair
- Try firstHot water
- Boiling water riskPlastic joints, porcelain
- Neither clearsSolids, wipes, roots
What each one does
Hot water works on grease. Cooking fats and oils that have cooled and congealed in a kitchen waste pipe soften when reheated, so a kettle or two of hot water — ideally with a squirt of washing-up liquid to emulsify the fat — can loosen a fresh, mild build-up and flush it through. It costs nothing, risks no chemicals, and is the natural first attempt for a kitchen sink that has slowed recently rather than stopped.
A chemical drain unblocker is a caustic-soda-based product (sometimes with bleach) that chemically dissolves grease, fat and hair. It is far more powerful than hot water and can clear an established blockage that water alone will not shift, generating heat as it reacts to help melt fat. The trade-off is that it is corrosive, hazardous to handle and harder on pipework, so it is the tool for a genuine blockage rather than routine use.
| Factor | Hot water | Chemical unblocker |
|---|---|---|
| Clears fresh grease | Yes, if mild | Yes |
| Clears firm blockage | Limited | Yes, grease and hair |
| Cost | Free | Moderate |
| Pipe safety | Good (avoid boiling on plastic) | Harder on pipes/seals |
| Handling risk | Scald risk only | Corrosive, burns, fumes |
| No effect on | Solids, wipes, roots | Solids, wipes, roots |
| Best role | Prevention, light grease | Established blockage |
Indicative comparison for guidance. Always read the unblocker's label.
The boiling water caveat
Boiling water gets recommended a lot, but it carries genuine risks worth knowing. Modern household waste pipes are usually plastic (PVC or similar), and water straight off the boil can soften the pipe or its push-fit and solvent-welded joints, potentially loosening a seal over time. Pouring boiling water down a toilet is a particular hazard: a sudden temperature shock can, in rare cases, crack the porcelain. For these reasons, very hot water from the tap is the safer everyday choice for a kitchen sink, and boiling water is best avoided on toilets and used sparingly, if at all, on plastic waste runs.
There is also a scald risk to you. Carrying and pouring a kettle of boiling water over a sink invites splashback, so pour slowly and keep clear. If a drain is fully blocked and holding standing water, tipping hot water in simply adds to the pool rather than reaching the blockage, so deal with the standing water first.
Choosing, and what neither can fix
The sensible sequence is hot water first, unblocker second. For a kitchen sink that has started to drain slowly, a hot-water-and-detergent flush is free, safe and often enough — and a weekly flush is good prevention. If that does not work and the blockage is grease or hair, a chemical unblocker used carefully is the next step: wear chemical-resistant gloves and eye protection, ventilate the room, follow the dose, and never mix it with bleach, acid or another product. A plunger or hand snake is a good mechanical alternative if you would rather avoid chemicals.
Neither hot water nor chemicals will clear a non-grease blockage. A solid object, a wad of wet wipes, tree-root ingress, compacted silt or a collapsed pipe needs mechanical clearing or proper repair. If a drain stays blocked after hot water and an unblocker, keeps recurring, or affects more than one fitting, the cause is mechanical or structural — time for rodding, jetting or a CCTV survey rather than pouring in more chemicals, which only leaves a hazardous chamber of caustic liquid for whoever opens it next.
Doing each safely, and a prevention routine
If you use hot water, handle the kettle carefully to avoid scalds: pour slowly and steadily into the plughole, keep your hands and face clear of splashback, and add a squirt of washing-up liquid first so the detergent emulsifies the softened fat as the water flushes through. Two kettles in succession often work better than one, giving the heat time to soften the grease before the flush carries it away. For plastic pipes, use water that is hot rather than at a rolling boil, which gives most of the benefit without stressing the joints.
If you move on to a chemical unblocker, read the label and treat it as the hazardous product it is. Wear chemical-resistant gloves and eye protection, open a window, keep children and pets out of the room, and measure or pour exactly the stated dose for the stated contact time — leaving it far longer does not help and is harder on the pipe. Never combine it with bleach, an acidic cleaner, or a second unblocker, and never pour it into a drain that is completely blocked and holding water, where it cannot reach the blockage and simply sits as a corrosive pool. Flush thoroughly with water afterwards.
Better than either is a routine that stops grease building up in the first place. In the kitchen, let pan fat cool and scrape it into the bin rather than pouring it down the sink, wipe greasy pans with kitchen roll before washing, and use a sink strainer to catch food scraps. A weekly flush of hot water with a little detergent keeps a kitchen waste clear, and a periodic enzyme treatment digests any film that does form. A drain looked after this way rarely needs a chemical unblocker at all, which is the safest outcome of all — no harsh chemicals, no risk, and no blockage.
Frequently asked questions
Is boiling water safe to pour down drains?
Use caution. On metal pipes it is generally fine, but water straight off the boil can soften plastic waste pipes and their joints, and pouring it into a toilet can rarely crack the porcelain through thermal shock. Hot tap water is the safer everyday choice for plastic pipes and toilets.
Which should I try first, hot water or an unblocker?
Hot water first. It is free, safe and often clears a fresh, mild grease slowdown in a kitchen sink, especially with a little washing-up liquid. If hot water does not work and the blockage is grease or hair, a chemical unblocker, used carefully, is the stronger next step.
Will hot water clear any blockage?
No. Hot water only softens fresh grease and fat. It will not shift a solid object, wet wipes, hair masses, compacted silt, roots or a collapsed pipe. For those, you need mechanical methods like a plunger, snake or rods, or for underground problems, jetting or a survey.
Sources & further reading
Figures on this page are typical UK ranges drawn from published cost guides and are intended as guidance, not a quotation.