Could be a bad thermostat.
engine not warm, low coolant, or stuck thermostat
I would change the thermostat first. If that dont work change water pump
check thermostat, might be stuck closed.
heat core. leak. thermostat. no antifreeze.
The heater valve could be failing to open for one reason or another.
Poor coolant circulation from a plugged up heater core. Try flushing the heater core and replacing the thermostat.
Possible causes are, low on coolant, thermostat stuck open, clogged heater core or vacuum scorce not getting to the heater control.
No, not if it's on a heat setting. Most home thermostats have a "Fan" setting that will blow cold air if the attic is cold.
Your thermostat is the likely culprit here it is probably stuck closed. Have it checked out.
Low on coolant. Stuck open thermostat. Poor circulation (plugged heater core).