Three fire brigades have teamed up to make a film highlighting the dangers of overheating wheat bags.
The footage, filmed by the two Sussex services at Surrey's training base, shows the hot water bottle alternative catching fire.
Roger Wood, from West Sussex, said it could take two to three hours before a fire started, by which time the person in bed may be fast asleep.
Last month, West Sussex crews tackled two wheat bag fires in under a week.
Recently, all three fire brigades - Surrey, East Sussex and West Sussex - have been called into action to tackle fires caused by wheat bags.
Wheat bags are heating pads that usually contain buckwheat or wheatgrass which can be warmed in a microwave.
Mr Wood, head of community fire safety for the West Sussex brigade, said: "Wheat bags should all come with manufacturer's instructions including maximum heating times and it's vital that people follow these.
"If the original packaging is thrown away there is sometimes no indication on the bag itself how long it should be in the microwave for, so timings could easily be forgotten or overestimated. Our advice is to keep any packaging and instructions stored with the bag."
He said it could take up to three hours for an overheated wheat bag to generate enough heat to cause bedding to smoulder.
Once smouldering, it only took a couple of minutes for a bed to become fully engulfed in flames, he added.
Mr Wood said by that time the occupant may be asleep and unable to react.