Your thermostat which closes when cold to let the engine warm up and opens when the water gets hot and lets it circulate through the radiator. If they stick closed your engine overheats in short order. When they stick open it takes forever and a day for the heater to warm sufficiently. This also makes the engine use more fuel. If your thermostat is indeed stuck open it is easy to tell. With the engine cold, take the radiator cap off and start the engine. If it is stuck open water will immediately circulate as seen at the radiator cap opening. If the thermostat were operating normally you would not see water moving for 5 or more minutes of idling. You should always drain the coolant, flush and refill every 2 years. Never ever grab that garden hose and put tap water in your radiator. They come from the factory with distilled water (about $1 a gallon at any grocery store) which has no minerals in it. The minerals in tap water promotes rust and scale build-up in the cooling system reducing it's efficiency. Don't use tap water unless you WANT to have problems.