With temperatures plunging into the single digits, New York City tenants are being reminded of heat guidelines for their homes. 

Landlords are required by law to keep the temperature at least 68 degrees inside if it's below 55 degrees outside between 6 a.m. and 10 p.m. 

During overnight hours between 10 p.m. and 6 a.m., it must be at least 55 degrees if it drops to 40 degrees or below outdoors. 

The guidelines are set by the city's Department of Housing Preservation and Development. It says residents should talk to their supers or landlords if they think the requirements are not being met. If that doesn't work, call 311 or go online to file a complaint.