From Longman Dictionary of Contemporary Englishheat up phrasal verb1 HOTto become warm or hot, or to make something become warm or hot The stove takes a while to heat up.heat something ↔ up I heated up the remains of last night’s supper.2 DIFFICULTif a situation heats up, it becomes dangerous or full of problems → heat→ See Verb table
Examples from the Corpus
heat up• The stones heated up in the sun.• As the gas heats up, it expands.• While the oven is heating up, prepare the sauce. 