Grease does not dissolve in a drain — it cools and sets. It goes into the sink warm and liquid, but the pipe in the wall or the floor sits at room temperature, often below 20 °C. Animal fat goes solid again at that temperature and sticks to the wall, where it collects food waste, soap and everything else that passes.
That is why hot water fixes nothing. It melts the grease where it is poured; the grease travels a few metres, cools and deposits there instead — in a section of pipe that is harder to reach.
How the deposit grows
The first layer is thin and almost invisible. Its rough surface catches the next layer faster than clean pipe would, so the process feeds itself. In a kitchen running daily, a measurable layer forms in two to three months; where there is frying and grilling, faster.
The first sign is not a blockage but slower drainage towards the end of a shift, once the day's grease has gone through. By morning the drain looks normal again, because nothing has been added overnight.
A grease deposit is not only a flow problem. It is food for biofilm and a breeding site for drain flies — two things that are difficult to explain in a HACCP record.
Why the usual remedies fall short
Hot water and detergent. Detergent emulsifies fat while it is warm. Further along, in cooler pipe, the emulsion separates and the fat settles again. The fat has moved; it has not gone.
Caustic soda. It works, but not the way most people assume. The alkali saponifies the fat — turns it into soap that washes away. The heat is a side effect: dissolving the soda in water is exothermic and warms the pipe locally. In a kitchen where this is repeated every month, PVC joints and rubber seals go first — more on that here.
Enzyme products. They break fats down slowly and need undisturbed time. Overnight in a house, that works. In a kitchen running sixteen hours a day, it rarely happens.
| Method | Effect | Limitation |
|---|---|---|
| Hot water | Melts grease at the inlet | Deposits further down the pipe |
| Detergent | Emulsifies while warm | Emulsion separates when cool |
| Enzyme product | Breaks fat down | Needs hours without flushing |
| Caustic soda | Turns fat into soap; gives off heat | Damages joints and seals |
| High-pressure jetting | Strips the deposit mechanically | Needs a contractor |
| Mineral granulate | Breaks the deposit down, no heat | Prevention, not removal of a thick layer |
What works in a kitchen running every day
The sequence that holds up in practice:
- Pan fat into a waste container, never the sink. The cheapest measure and the only one that addresses the source.
- A strainer on every sink — catches the food waste that gives the deposit something to grip.
- A grease trap maintained to schedule. It handles what reaches it; the pipework between sink and trap still needs separate attention.
- Mineral granulate on a set interval, after the last shift. It breaks fresh deposit down before it hardens, without heat and without risk to the joints. The manufacturer's guidance for prevention is one sachet a month; a kitchen under heavy load needs it more often.
- Annual high-pressure jetting where the layer is already thick — granulate will not strip it.
If the drain runs freely but smells
Odour with normal drainage is not grease but biofilm on the walls, or a dry trap in a floor drain that is rarely used. Those are different problems with different fixes — see the guide to drain odour.