Running bleach through a washing machine to clean it

Does it work?

Disputed

Disputed, with the dissent recorded, because the honest answer is that the technique addresses a real organism in the wrong place. The catalogue already holds a related claim about hot washes cleaning a machine; this one is separated from it because the bleach version carries a material warning and a mixing hazard the plain hot wash does not.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

Where not to use it

Why it works

Sodium hypochlorite is a strong oxidiser and an effective biocide against the organisms responsible for the odour, and a hot cycle circulates it through the drum, the pump and the pipework. Both parts are true as far as they go. The step the claim omits is that a biofilm is a structure rather than a suspension: the matrix limits penetration, the deeper organisms are metabolically slower and less susceptible, and the surfaces that hold the most film — the underside of the door seal, the drawer housing — are the ones the wash water reaches least. Killing the accessible fraction leaves the reservoir intact.

Background

A washing machine that smells has a biofilm problem, and understanding that is what explains why this technique disappoints. Research in this register on bacterial communities in domestic washing machines establishes that the organisms are established on wet surfaces rather than free in the water — the door seal's folds, the detergent drawer, the sump and the pipework. A biofilm's matrix physically limits how far a disinfectant penetrates and the organisms deeper in it survive, so the film regenerates from what is left. That is why the smell returns on a predictable schedule after each treatment. What actually works is mechanical: wiping the door seal and its folds with detergent, removing and washing the drawer, and clearing the drain pump filter, which one manufacturer specifies monthly. A hot maintenance wash then has far less to do. Two cautions attach to the bleach version. Chlorine in contact with stainless steel is a chloride exposure, and chlorides break the passive layer. And a drum that has held bleach and then receives an acidic or ammonia-based product is a mixing accident out of sight.

Conditions

• An empty hot cycle
• Manufacturer instructions govern what may be added

What it does not do

• It treats the water rather than the surfaces, and the surfaces are where the film is.
• It does nothing about the door seal folds, the drawer or the filter, which is where the odour is generated.
• The improvement is temporary by construction, because the film regenerates from what the treatment could not reach.

What to use instead

• Biofilm — Bacteria that have built themselves a protective matrix and stuck it to a wet surface. It resists disinfectant far better than the same bacteria loose in water.
• Washing machine drain filter — A trap that catches what should never have reached the pump. Two manufacturers give two different intervals and only one says to unplug the machine — which is the whole answer to whether there is a general procedure.
• Washing machine door seal — The folded rubber ring around the door. Its top and its bottom were measured as different environments, and only the bottom is a problem.
• Neutral cleaning — The default that four separate trades converge on independently: a neutral detergent, warm water, a soft tool and a thorough rinse.