I've had a number of slow cookers in the past and all seem to be roughly the same in this regard:

  • The outside of the unit is red hot to the touch when they're on
  • The crock pot inside the unit can actually move around. It has maybe 0.5cm of clearance on all sides from the heating enclosure.

They're not chef-grade (if such slow cookers exist), but they're not cheap (~£50).

Is there a reason for this? It's frequently said that this is an energy-efficient way of cooking food (amongst many other benefits) and I don't doubt that, but the two observations above seem wasteful in any case.

If the crock pot fit snugly into the unit then you could have more conduction rather than only convection heating, and surely the unit also could have heat better-directed inwards and reduce the amount lost on the outside of the unit? Once you've done that, you reduce the energy input from the unit to prevent overheating, and save more money. The closest I found was this but thermal expansion of the ceramic crock pot is way below the clearance given, easily evidenced by just moving the pot around after hours on the High setting.

I appreciate that this question is a mix of physics and food, but I imagine there is a practical reason for this because it seems like such an obvious flaw on the energy efficiency side, yet it's across the market - I'm surely missing another consideration.

  • I've often thought the outside could be a little better insulated, but all of mine, whether metal or plastic on the outside have been a reasonable temperature to handle, if briefly. So they're not getting all that hot
    – Chris H
    Commented Aug 2, 2023 at 17:15
  • 1
    @ChrisH all of mine, I couldn't reasonably handle for more than a couple of seconds. That is heat going the wrong way, though, to the outside; what a waste. I'll admit that all of the ones I've used are metal, though
    – roganjosh
    Commented Aug 2, 2023 at 17:27
  • I don't have plans to use mine in the next few days, but I'll try to remember to measure the temperature next time I do
    – Chris H
    Commented Aug 3, 2023 at 18:04
  • I've not had a slow cooker that got hot outside to the touch, either. At least here in the US it's not unusual to use the (obviously unplugged) slow cooker to transport the end product to another location, so possibly this is by design otherwise we'd all be burning ourselves and suing the manufacturer. Commented Aug 6, 2023 at 8:11
  • @RoddyoftheFrozenPeas it has two handles to move, and it's not going to shrivel your skin on touch, but there's absolutely no way you could move any of the ones I had using direct contact with the outer casing. Even with "asbestos hands" there's no way I could juggle skin contact with my fingers and hold the weight. It might be a difference between US/UK here if I'm honest and manufacturers here get away with selling substandard products. They're not so common over here
    – roganjosh
    Commented Aug 6, 2023 at 8:18

1 Answer 1


If the inner pot was a snug fit, it would get stuck, possibly when full of hot food.

That's not because of thermal expansion, but assume for a moment that the inner pot goes in damp (perhaps it's just been washed and not thoroughly dried). That water then evaporates, but condenses where it leaves the outer pot. Then you have a seal, which leads to a vacuum when you try to lift the inner pot out. This would be most likely when preheating, as the moisture will be driven off eventually during cooking.

Overflowing can have the same effect (and that can be due to thermal expansion of the food). Note that the outer pot is quite well sealed on the inside, despite the dire warnings not to put liquid in it. Overflowing can also mean sticky liquid, and being able to move the inner pot around would help with unsticking.

Note that even if the inner pot is metal, which can be made to finer tolerances than ceramic, there's still a comparable gap, so it's not to slow it manufacturing tolerances.

Convective heating does at least mean quite even heating. Conductive heating would lead to hot spots nearest the element.

The heat escaping out the sides could also be reduced by a little (more) insulation, but the heat escaping from the top dominates anyway - that's uninsulated and heated by steam from the food. To make them considerably more efficient you'd need a double glazed lid (I'm used to a glass lid, though had one that was thick ceramic) as well as better insulated walls.

  • Overflowing was in my mind but I couldn't connect it. I've had them boil over but didn't see the reason for the clearance gap
    – roganjosh
    Commented Aug 2, 2023 at 17:37

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