Because of bacterial contamination of the drinking water, the Stadtwerke Aachen recommended that the citizens of the city boil tap water as a precaution.
The contamination - the "Bild" newspaper reports of coli bacteria - was found during a routine control, the municipal utilities announced on Tuesday evening.
"If you let the water boil for three minutes, you can then use it completely harmlessly to prepare food, also for children, old and sick people, to wash food, to make ice cubes, to brush your teeth and for medical purposes", it was said.
The warning was also formulated in a corresponding Facebook post.
The tap water can be used for personal hygiene without boiling.
The cause of the contamination is still unclear, said a spokeswoman.
How long the recommendation will apply is currently not known either.
Around 245,000 residents of the NRW metropolis are affected.