Is it safe to leave a slow cooker on when I go out?

This is one of the most searched kitchen questions there is, and the reassurance is genuine rather than polite: unattended running is the entire purpose of the appliance. What is worth knowing is which conditions make it safe, because the small number of incidents that do happen nearly all trace back to the same handful of things.

Safety

Step by step

1. Understand why the risk is low.A slow cooker is a sealed ceramic or metal pot inside a jacket with a low-wattage element wrapped around it. It draws about a tenth of what an oven does, has no naked flame and no exposed element, and its contents sit below boiling. There is very little there to go wrong compared with a hob left on.
2. Site it properly.A clear, dry, heat-resistant worktop with 15cm or so of space all round and nothing above it. Not under a wall cupboard where the steam will swell the carcase, not against a wall, not on the hob, not on a wooden chopping board, not near a tea towel, curtain or the toaster.
3. Plug it into a wall socket, not a coiled extension lead.Long-duration draw on an extension reel that has not been fully uncoiled is a genuine fire risk. If you must use an extension, uncoil it completely and use one rated well above the appliance.
4. Check the pot and the lead before every long run.A crack in the ceramic insert can spread when heated and let liquid onto the element. A frayed or scorched lead, a loose plug or a browned socket face are all reasons to stop using it until it is replaced or checked.
5. Fill it to between half and three quarters.Underfilled, the contents heat too fast and can scorch on the sides; overfilled, it can bubble out under the lid and run down onto the element housing. Between the halfway mark and about three quarters up is where it is designed to work.
6. Start with hot ingredients and do not lift the lid.Adding boiling stock rather than cold water gets the contents through the danger zone faster. Once it is running, leave the lid alone — each lift costs 20–30 minutes of recovery, and you are not there to lift it anyway.
7. Plan what happens at the end.Most slow cookers switch to keep-warm automatically, which holds food safely above 63°C. If yours simply turns off, food will drift down into the danger zone over the following hours — a plug-in timer that switches it off two hours before you get home is not the answer, and a machine that holds warm is.

Tips

• Have a working smoke alarm and test it. That advice is generic but it is the actual mitigation for every unattended appliance in the house.
• If you are nervous the first few times, run it while you are at home and put your hand on the outside of the jacket — it should be hot but not painful to touch briefly, and the lead should be cool.
• A slow cooker left on keep-warm for many extra hours will not become unsafe, but the food will overcook. Vegetables collapse and lean meat dries out.
• Do not put a slow cooker insert straight from the fridge into a preheated base, or from the base into cold water. Ceramic cracks from thermal shock and that crack is the failure that matters.

Common mistakes

• Leaving it under a wall unit or against a splashback — Slow cookers vent steam for hours. Kitchen units swell and delaminate, and paint blisters. It is not a fire risk so much as expensive damage you will notice months later.
• Cooking dried red kidney beans from raw in a slow cooker — Raw kidney beans contain a toxin that needs a full 10 minutes at a rolling boil to destroy, and a slow cooker never reaches boiling. Boil them hard first, or use tinned.

Questions people ask