Why do I not have heat in my 2000 town and country.Changed thermostat and now the heat will only work when accelerating

the cooling jacket is partially obstructed by corrosion...as someone has put garden hose water in that during a PANIC refill....and NOT immediately replaced with the proper 50/50 ethylene glycol and de-ionized water mix...what we must do is a cooling system flush with the heater control ON while hot and the thermostat OPEN (or removed)~ actually this does sound like a stuck open thermostat...but if it were replaced recently can only suggest that it is jammed open....as this will produce this condition....really I do think that someone has panic refilled with ordinary tap water that was not immediately replaced with the proper mix~ hope you're able to bring it in for a coolant flush and replacement/check the thermostat~