2000 Civic 1.6L SOHC. Just flushed out the coolant system including the heater core using water. Drained water and refilled with 50/50 OEM honda coolant. I also replaced the thermostat from Honda. When at Idle, with heat on, I can feel the heat but it isnt that warm. When I hold it at 2000 rpm, it gets nice and warm. The temperature gauge stays the same. I had to running for quite some time and im not getting anymore air bubbles. Any ideas why my temp gauge stays at operating temperature but at idle my heat is cooler then when driving? I lean towards an issue with the thermostat but I feel like my temp gauge would be raising or lowering if it were stuck open or closed. I'm going to run it a lot longer tomorrow just to confirm I do have all the air out of the system for starters

 

not sure what the problem is, but my 1995 civic d15b7 does the same thing,

 

So I ended up using a coolant filter that attaches to the radiator, so it would be fully sealed. Ran the car for about 30 mins and rev'd to 3k and held for a few seconds at a time. Some more air bubbles did show up but no real change with the car. I can see why it would get hotter with higher RPM, as your water pump would be sucking a lot more coolant since its run by your timing belt but I just haven't really noticed this with other civics I've owned. The important thing is that its nice and toasty while driving so not really going to worry about it.