Originally Posted by
Mrmiata
Since you didn't go into detail.. You realize getting the engine hot to open the thermostat and then inducing cold water is not a good combination.. not to mention the thermostat will then close with the cooler temp from the water. So unless you intend to hook up to the home hot water tank..
Keep in mind that when you start your car on a cold day only the coolant inside your engine is warmed. Once the coolant warms up to the point where the thermostat begins to open it is replaced with more cold coolant from your radiator. Therefore, the running engine has to be able to tolerate water well below 0 deg F even after it has been running long enough to open the thermostat. So cool tap water will not cause any issues during a coolant change. As an example, many motor boats are powered by automotive engines (with only minor modifications) and are cooled by raw water.