Also, when replacing the T-stat, many people overlook the opportunity to service the cooling system.
When I did my T-stat, I also took the time to add the flush before hand and follow instructions.
Then drain the system, remove T-stat, replace empty housing, insert water hose into upper radiator hose, open heater core valve and flush out for several minutes.
By sticking the water hose in the upper radiator hose is is flushing out the entire system, backwards.
Then remove T-stat housing and install new T-stat and gasket.
I figured I'd loose 1/2 my coolant anyways so I did it all at once, afterall, antifreeze is $12/gal.
I was surprised at what all flushed out, how cool it ran and how well the heater worked afterwards.