I had this problem but found the gasket on my radiator cap was torn and pressure was escaping, I since replace the radiator cap with a new oem one and the heat now works, however when I come to an idle it seems to still blow cold air, not sure what that’s about. Maybe still air in the system?

 

It's the heater core Mr. Sorry to say but I just cut mine apart and those heater core passages are so tiny! I think Honda needs to recall the S2000 and replace every heater core! I changed my coolant often and mine still clogged up. My coolant has always looked very pretty and I used top shelf coolant conditioners like BG's coolant conditioner. Now it's not the additives fault, it's those super small passages that a tiny piece of scale would clog up. Also check the two aluminum lines on your firewall for corrosion. You may need to change them as well. Good luck!

 

^^^ hmm interesting, sorry to hear of your issue there, was it a bitch to get to the core? I'm not even sure where its located. Anyway my problem of blowing cooler air when at idle went away, i think I just had air bubbles in the system from the fuqed up cap I replaced.