'05 S40 heating system not working

I recently replaced the thermostat on our 2005 S40 and solved a "check
engine" problem. Recently however we have noticed that the heating
system is not working at all. No matter what the temp dial is set to,
it's blowing cool air. The dash gage shows the engine is getting warm
but the cabin heating system is not picking up warm air at all. I've
read in other places about a cabin air filter but the air velocity is
not affected it's the temperature only.

One other point to note - the fans under the hood run a great deal
once the car has shut off. I'm not sure if that is at all connected
to this issue but thought I'd point it out since it seems to be a
relatively new condition.

I don't believe it's the transistor pack and hope it's not the heater
core. Any ideas how to troubleshoot and what may cause this chilly
condition?

Thanks in advance.

 

My first thought is that you have an air bubble in the cooling system.
Some cars are a lot more prone to this than others.

 

Thanks James. I will try to flush out any bubble that may be there.
A new symptom today - the car was idling in a parking spot with the
heater on, and the engine temp skyrocketed. Once the cabin temp
control was set to cool, the engine temp returned to normal. Perhaps
a blockage in the radiator or does this lead back to a possible
bubble. I suppose it's possible I introduced a bubble into the system
when I replaced the thermostat.

Thanks again