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2005 Honda Civic Heat Issue

I have a puzzler. My 2005 Honda Civic doesn’t seem to have much heat until I get going on the highway. Also at various times when I slow down. I’ve replaced the thermostat as well and the antifreeze is mixed correctly. When the heat does finally come on it seems to come on all of a sudden as if a valve is opening. Is there another valve other than the thermostat?

My 99 Civic has a water valve near the firewall. It is controlled by the dial on the heater controls that goes from cool to hot (blue to red). You may be low on coolant and have air trapped in the sysytem. Make sure your heater is set on hot, so the valve to the heater core is open. Take off the radiator cap and start the cold engine. As it warms up, be ready to catch overflow coolant (poison to animals and humans) and after it is fully warmed up, top up the radiator and cap it. Make sure the overflow tank has enough coolant in it. Check levels again after the engine has cooled down overnight and top up the rad and fill the overflow to the “cool” point.

You may be low on coolant and have air trapped in the sysytem.

Thanks, I’ll give it a try. I do remember in my old '88 Civic that there was a “bleeder” valve to purge the air bubbles in the coolant system. I did not find such a bleeder on this '05 Civic when I changed the thermostat.
As a side note, I was afraid I may have a blown head gasket so I changed the oil but did not see any antifreeze scum in it. I also tested the compression on all four cylinders and I got these readings: 100,110,130,130. I didn’t think there was enough disparity between them to cause concern but what is normal? The car has 128K on it as well.