Q: no heater

The lines to the firewall could be hot and yet the heater control valve is not admitting the heated coolant to the heater core in the dash. So, you want to inspect that valve and make sure it is moving in response to commands from the dash mounted controls. If the heater control valve works, and the blower motor functions, but only cold air blows, the only other possibilities are a plugged heater core or a blend door stuck such that no air blows over the heater core. If you desire that a certified mechanic inspect this issue and make the required repairs, the recommended diagnostic is a no heat diagnostic and that would be performed right at your location at any time that is convenient for you.