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In Subarus the thermostat is located at the lower radiator hose, in one body together with the pump, effectively separating water inlet from the radiator to the engine block and core heater. Yet to drain the coolant usually the procedure is to use the drain plug at the radiator. With the thermostat at the lower hose I cant figure out how draining from there is useful or even necessary when we need to drain the engine block side of the circuit. If the thermostat is closed wouldn't coolant continue to remain in the engine-core heater loop anyways because cant get to the radiator drain plug? Does that mean that to completely drain and change the coolant I would have to remove the thermostat?

Here is one diagram of the cooling system, old but they havent changed it much (not for what concerns my question at least) enter image description here

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    I don't think you have to remove the thermostat; a number of Subaru coolant flush write-ups just involve draining at the plug, then disconnecting the radiator hose and heater core hose, and forcing fresh water through until it exits clear, then letting that drain out the hoses too. Some pull the thermostat though; subaruoutback.org/forums/99-do-yourself-illustrated-guides/… for example, there it pretty much recommends just replacing the thermostat as a regular maintenance item every time you do a coolant flush. – Jason C Sep 24 '16 at 5:48
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In a lot of cars, removing just the radiator drain-plug is not always enough to completely flush coolant from your vehicle, as you might suspect. You will most likely need to disconnect the lower hose to flush out more coolant. It's also possible that there are coolant draining plugs on the engine block as well.

The reason why coolant must be drained from both the radiator and the engine block is because portions of the coolant is still kept in the radiator when the vehicle is not running, hence to completely flush out all coolant from your system, draining from both points is necessary.

I recommend looking into the vacuum filling method for cooling systems, as to minimise the chance of air pockets in your cooling system and in turn to avoid air-locking it.

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