What would cause the heater not to work in a car????
It gets warm but never really heats up even after two or three hours of driving

 

1. Incorrect or sticking thermostat. Coolant that is not hot enought won't tranfer adequate heat to the air passing through the core.

2. Poor coolant flow. A plugeed core, restricted or pinched heater hose, or failing heater water control valve can inhibit coolant flow through the core. If you suspect the core may be plugging, you can try exchanging the heater hoses to reverse flow the core and possibly clean it out.

3. Temperature control air door not sealing. The "Heat-Cool slide control on the dash should cause the air door to fully divert to the heat position. A door that is not closed fully will allow unheated air to bypass the core and dilute the hot air fed through the vents.

4. It's just too damn cold outside. Heater cores will typically raise the air temperature up to 100°F, but if it's -30° and you are expecting 120°F air from the vents, dream on.