It looks like a conjuring trick the first time. You pour cold water into a tray, and ninety seconds later there's steam coming off it. No flame, no plug, no microwave. Here's what's going on.
The reaction
The heat pack is a sachet of flameless heater compound. Its safety data sheet puts it at calcium oxide 80~95% with aluminium 5~20% — quicklime, essentially. Add water and the calcium oxide slakes, a reaction that throws off a great deal of heat rather than needing any, and the steam arrives almost at once. Within 10 seconds of the water going in, in practice.
Nothing burns. There's no fuel and no flame, which is why these are allowed in venues that won't permit a chafing dish burner. The pads are labelled Food Flameless Heater Bag and they're single use — once the compound has reacted, it's spent.
What's in the box
- A heating tray — the black food-grade PP outer that holds the pads and water.
- A foil inner pan that holds the food and sits above the reaction, 35 × 25 × 7 cm.
- A lid, which traps the steam and does more work than people expect.
- 2 × 90g heat pads and a water pack.
The food never touches the heat pack. It sits in the foil pan, and the steam underneath brings it up. That's why the lid matters — leave it off and you're heating the room.
Using one without getting it wrong
- Pads go in the outer tray, flat and not stacked.
- Pour the water pack in, all of it. Cold water, from the pack — hot water makes the reaction violent and is explicitly warned against.
- Inner pan on top, food in, lid on.
- Hot food in 10–15 minutes. Open away from your face; the steam is the point and it's genuinely hot.
Don't activate before you travel. The reaction starts the moment water hits the pads and you can't pause it — set up at the venue, not in the kitchen.
What it can't do
This is the part the product pages skip.
- It won't cook. It brings hot food up to serving temperature and holds it. Cold food, raw food, anything out of the chiller — it isn't for that, and treating it as a cooker is a food safety problem rather than a disappointing result.
- It isn't adjustable. No turning it down, no stretching it. One defined service window and that's your lot.
- It won't heat during transport, because you can't start it early. Hot food leaving a kitchen still needs an insulated bag.
- It's single use, so the per-event cost never drops. If you cater weekly from a kitchen with storage, chafing dishes will be cheaper over a year and we'd rather you knew that now.
Disposal
A spent pad is slaked lime once the reaction has finished and cooled, and it goes in general waste. Let it cool fully first — a pad straight out of a tray is hot enough to melt a bin liner. Unused pads are a different matter: unreacted, the pack is classified UN2813 Class 4.3, a water-reactive solid, so store them dry and well away from a wet floor or a leaking chiller.