Is your new thermostat OEM or aftermarket?

How about a block in the heater core, or one of the lines running to it?
 A chunk of rust or other debris could have come loose inside the
radiator and made its way to the core.

Try disconnecting the two heater lines *from the block*, force water
into one of them (making sure the heat control is set to full hot), and
see if it comes out the other.  Then force water into the other hose as
well, so if something is clogging it, it may be forced out.

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The coolant valve in the heater line under the hood near the firewall may
not even be opening. Those things get stiff in almost every car made,
probably because they are mostly plastic. A squirt of silicone on the rotary
shaft couldn't hurt, but it is fairly likely the operating mechanism is out
of whack now. It doesn't even get mentioned in Haynes, but the air mixer
assembly is at the very bottom of the center console and is easily
accessible from the passenger side by pulling the carpet aside and
unfastening the duct that covers it... a couple of screws, IIRC. Both the
operation and adjustment (by loosening the housing clamps and refastening
them when the cable is in the right position) are fairly obvious if you look
at it and twist the temperature knob. At that location you can also unfasten
the actuator cable that goes to the valve and operate it by hand to check
the resistance of the cable/valve pair. If the temperature control knob is
broken, it is available from Honda for something around $10 CDN. Changing
the valve is a more serious thing and demands you be very careful to avoid
slashing your hands and wrists. Don't ask me how I know :-(

Mike