Drain cleaners: when they help and when they cause damage

When chemicals work

• the water moves slowly, but it moves
• it's a fresh grease or soap deposit
• the pipework is plastic and in good condition

When not to use chemicals

If the water is standing still, the product sits on top of the clog and acts more on the pipe than on the plug. The risk is greater with old cast-iron pipework, since the inner surface is already worn.

Using several different products one after another is also dangerous. The reaction can generate heat and fumes exactly when someone is leaning over the sink.

Mechanical cleaning vs. chemicals

A drain snake and pressure flushing remove the sediment from the pipe wall, not just from the centre of the opening. That's why the result usually lasts longer, and the pipework's actual condition also comes to light.

Why chemicals often leave the problem in place

A chemical product moves wherever the water moves. If there's a plug in the pipe, the product sits on top of it and only acts on the surface. An opening may appear for a few days, but the sediment on the pipe wall remains and narrows again.

Another practical problem arises when a technician is still needed after using chemicals. Standing chemical water has to be safely removed before work can start, which adds time to the job.

What to use instead

• manually taking apart and cleaning the trap
• a plunger, when the overflow opening is covered
• a drain snake for the section inside the flat
• pressure flushing for the stack and drop pipe
• camera inspection, if the clog recurs in the same spot

Safety in home use

If you decide to use a product anyway, do it once, in a ventilated room, and protect your eyes. Don't use two different products in a row and don't pour hot water on top — the reaction can generate steam right when you're leaning over the sink.

Difference between an apartment building and a house

In an apartment building, a product poured in reaches further from the flat's section into the shared stack, where it dilutes and its effect disappears. If the clog is in the stack, home chemicals don't change the situation at all, but may end up sitting in a neighbour's section instead.

In a house the sections are longer and often underground. There the usual cause is an incorrect slope, a sunken pipe, or root ingrowth — physical obstructions that no product will dissolve.