Sounds like a flow, thermostat or water pump issue.

 

If you can source the heater pipes have a feel at them they both should be about same temp. If one is hot and tuther is cold the heater matrix might be blocked.
Disconnect the hoses at the easiest point and give it a good flush in both directions with a garden hose.

Make sure heater control is at hot setting. I am not sure if there is water valve on this type of heater or its just a deflector plate.

After you've done that I would flush out the whole cooling system just in case there is a load of crud in it.

Have fun

 

will try the suggestions and let ya know of the outcome, praying its not the waterpump as im guessing that means access to the timing belt? ouch!!

 

Good luck resolving the issue.

 

I had similar problem with my V6 when I got her, caused by leaking coolant expansion tank. You fill it up and very slowly it looses coolant and as heater sits quite high all fluid runs away from it. The leaking expansion tank is comon fault in early Sorento . Heating system is excellent now though. Hope this may be of some help to you!

 

kia has actually redesigned expansion tank due to leaking/splitting on bottoms
could be air pocket or failing tstat

 

thanks , will check the expansion tank soon as. hopefully its just a simple problem.