heat stops after 10-15 mins - AC room temperature sensor?

It's a thermistor, not something you often hear about going bad. But a simple way to check would be to watch the room temperature sensor live data on a scan tool and see if it is or isn't showing a believable reading, before and/or after the heat stops coming.

 

That's the amount of time it takes for the heating system to transition from electric over to traditional engine heat via the coolant. So the system that does that would be a good place to start looking. Watching the the live data like @ChapmanF says will help... Also making sure you don't have any air in the coolant system and ensuring its running at a normal temperature needs to be checked too.

I think in reality though you don't have any problems with your car, just that its been down to freezing in Seattle area past couple days and setting your cabin temperature at 65 isn't going to warm up the car very well and you need to turn the heater all the way up to hi... As in a vehicle thermostat doesn't work like a house thermostat. For best results get the car as warm as possible as quickly as possible and control the vents and fan speed to optimize cabin temperature.

 

If this is a gen 2, though, I don't think it has any more electric heat capacity than gen 3, like somewhere around 700 watts.

That has never been enough to make a difference I can feel. It does make a noticeable difference in how fast the windshield clears, but the air still feels cool to my hand. The 700-ish watt electric element is just nowhere near close to the rated 5300 watts of heat that can be extracted from the coolant once the engine's warmed up.

 

The first thing to do is check the level of the coolant in the radiator. When the car is cool, remove the radiator cap and make sure the coolant is all the way to the very top. The next thing is to check the level of the coolant in the plastic overflow bottle.

 

Yes, both cooling systems use the same coolant.

After filling the radiator, come back and tell us if that fixed the problem.

 

Yeah... Once I realized OP was from the Seattle area, which rarely experiences weather down to freezing and OP said they keep their heater temp on the bottom end of its abilities and because we've had freezing temps the past couple days, I was certain that this was OP error not vehicle error.

 

Small coolant disappear in a gen2 puts heat off really quick car won't overheat . But no heat .