Recently my 1999 Quest stopped giving heat; the van did not overheat and my temperature stayed low. My assumption was thermostat and I changed it: after I changed the thermostat my problem changed.

Now I have perfect temperature reading, but I only have heat when I rev the engine. It acts like there is a vacuum leak to a valve, and I did find reference to a coolant valve in the hanes manual however, no where can I find a location for such a valve. Is it possible that it is merely air in the system from when I changed the thermostat?