Usually this is not the case. It looks like a bad thermostat, because no way the coolant is not getting hot, but a rusted up thermostat would not open for the coolant to circulate. If I were you I will change the thermostat first than check the water pump.
If you are certain that you have sufficient coolant, and the heater is not warming up adequately, check the thermostat and/or water pump. If the thermostat is staying open the engine coolant will not heat up enough to provide heat inside the passenger cabin.
The thermostat may be stuck in the open position, so that the water is not getting hot enough.
If the engine is overheating, the thermostat is stuck closed. If the heater is not putting out enough hot air the thermostat is stuck open. Change the thermostat as a normal part of maintenance when changing coolant.
Low coolant or stuck open thermostat.Low coolant or stuck open thermostat.
Replace the coolant thermostat. It is probably stuck in the open position.
Spring side is always down to maintain constant contact with coolant-otherwise it would not open soon enough.
Thermostat fail to open. Waterpump is on the deathbed. None, or not enough Anti-freeze in radiator.
When you start the engine, the thermostat is closed. After the coolant warms up the thermostat begins to open up allowing coolant to flow thru the raditor. When first started the temp will go a little past what the thermostat is set to open at. After the water begins to flow thru the radiator, the temp will fall to whatever the thermostat is set at. The thermostat holds the coolant in the engine until it warms up. As soon as the engine is warm, the thermostat opens allowing cold coolant outside the engine to mix with warm coolant. The temp drops, the thermostat closes again, until all the coolant is warm.
The wax or gas element inside the thermostat expands in response to the increasing engine temperature, causing the thermostat to open and allow coolant flow to regulate the engine temperature.
If this happened immediately after changing the thermostat, then the most likely problem is the thermostat was installed backwards.The spring side of the thermostat faces the engine. It's the spring that is reacting to the coolant temperature and driving the valve to open/close.When the thermostat cannot open, that prevents any coolant from ever getting to the heater core. Thus, you shiver a lot!Pop it out, flip it over and you should be all sorts of warm and cozy.
If this happened immediately after changing the thermostat, then the most likely problem is the thermostat was installed backwards.The spring side of the thermostat faces the engine. It's the spring that is reacting to the coolant temperature and driving the valve to open/close.When the thermostat cannot open, that prevents any coolant from ever getting to the heater core. Thus, you shiver a lot!Pop it out, flip it over and you should be all sorts of warm and cozy.See "Related Questions" below for more
It will probably overheat. answer= thermostat will not open and will prevent the flow of coolant . which will in turn do damage to your engine