An electric furnace may constantly blow cold air when it has a broken switch, thermostat problems or one or more broken heating elements. The high limit switch may be stuck open as well.
A temperature-sensing switch tells the unit when to blow out hot air. A stuck or broken switch will allow the fan to keep running, but the air coming out of the unit will always be cold because the burners will not be ignited.
The first thing a user should check is the thermostat setting. If it isn't set to "heat," the air coming out will remain cold. If the thermostat is at its correct setting, there may be a problem with its wiring.
Broken heating elements inside a furnace will cause the unit to consistently blow cold air because the unit never gets to the proper temperature to engage the thermostat.
The high limit switch may be stuck in the "open" position. Until the switch is repaired, the unit will continue blowing cold air.
Those not familiar with electrical systems or the workings of an electric furnace should leave the repairs to a professional. Working on electrical appliances can be dangerous without the proper knowledge and tools needed for the repair job.