Should I pour drain unblocker down a blocked drain?

Drain unblocker is the first thing most people reach for and often the wrong first move. It is not that the products do nothing — caustic soda genuinely dissolves fat and hair — but the physics matters. A chemical has to reach the blockage in solution to act on it, and in a drain that is already full of standing water it neither reaches it nor acts on it. Meanwhile it makes every subsequent option worse.

Safety

Step by step

1. Establish what kind of blockage you have.A slow-running sink or shower, still draining but sluggishly, is a grease and hair build-up on the pipe wall. A completely stopped waste, or an underground drain backing up into a chamber, is a solid obstruction. Chemicals are only useful for the first.
2. Understand what caustic products do.Sodium hydroxide reacts with fat to make soap, and attacks hair and organic matter. In a partly flowing pipe it can restore flow, and the reaction generates heat that helps. In standing water it dilutes below useful strength and sits there.
3. Understand what enzyme products do.Biological products digest fat and organic matter slowly, over hours or days. They are far gentler, far safer, will not damage pipes or seals, and are genuinely useful as a monthly maintenance dose. They will not clear an actual blockage in an evening.
4. Try physical methods first.For a sink, a plunger with the overflow blocked, or removing and cleaning the trap. For a shower, a drain snake or a plastic hair-removal strip. For an underground drain, rodding from the downstream chamber. All of these actually remove the material rather than trying to dissolve it.
5. Never chemical-treat a fully blocked drain.The product cannot circulate, so it sits in a column of standing water at unknown concentration. Anyone who then plunges, rods or opens the trap gets a face full of caustic solution. This is the specific scenario that puts people in hospital.
6. Never mix products.Caustic and acid drain products react violently together, and either can react with bleach to produce toxic gases. If one product has already been used and has not worked, flush thoroughly with plenty of cold water before considering anything else — and ideally do not.
7. Use them where they belong.As maintenance on a slow-running kitchen or bathroom waste that still drains, following the container's instructions exactly, with plenty of hot water afterwards. Or better, as a monthly enzyme dose in a kitchen waste that habitually greases up.
8. Tell anyone who works on it.If a plumber or drainage contractor is coming, tell them what has gone down the drain. It changes how they approach the job and it is a genuine safety matter.

Common mistakes

• Pouring caustic into a sink that is not draining at all — It cannot reach the blockage, it sits there, and it makes plunging or opening the trap genuinely dangerous for whoever does it next.
• Using a second product when the first did not work — Caustic and acid products react violently together, and both react with bleach. There is no situation where mixing them is a good idea.
• Using strong chemicals in an old or plastic waste system — The heat generated can distort plastic waste pipes and damage rubber seals, and repeated use degrades them. A cleared drain that now leaks under the sink is a poor trade.
• Using them on an underground drain — The volumes are far too large for the chemical to have any effect, and the standing water dilutes it to nothing. Rods are the tool.

Alternatives

• Plunger: For sinks, basins and toilets. Blocking the overflow first is what makes it work, because otherwise the pressure escapes. Costs almost nothing and clears a large proportion of household blockages.
• Cleaning the trap: Unscrewing the U-bend under a sink or basin, with a bowl underneath, and removing what is in it. Deals with the location of most kitchen and bathroom blockages directly and removes the material rather than pushing it on.
• Drain snake or hair strip: A flexible spiral or a barbed plastic strip fed down the waste. Extremely effective on hair in shower and basin wastes, and cheap.
• Drain rods: For underground blockages between inspection chambers. The correct tool for anything beyond the building, and no chemical substitutes for it.
• Enzyme maintenance treatment: A monthly biological dose that keeps grease from establishing in a kitchen waste. Prevention rather than cure, safe for every pipe material, and the sensible long-term habit.

Questions people ask