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 .

 

Gen 2 heat disappearing is almost always because air is taking up some space set coolant should be . Fill it up you have heat in my situation 45 min later cool air and level down again . Mine is HG seepage . Won't get fixed this is on my work vehicle with over 540 on it

 

As long as you're topping off under the radiator cap when the engine is cold for a few days in a row, you'll have successfully gotten all the air out of the system.

 

I you have to keep this up . You're losing a bit . If it's a seeping HG. My car will be out of heat in 45 min but car will run all month without issue in town etc . Wen I get on hiway going solid 65 mph . The triangle flashes etc like overheat . Get wer I'm going top off this car Been like this for months now it's old work vehicle so not fixing still drives 45mpg I use on colder months mainly and in town . In town it'll drive w no triangle for months you gotta get it up to speed 30 min or so .

 

I'm pretty sure in Gen2 Prius chronic coolant loss is usually attributed to engine water pump failure rather than Headgasket, which is a Gen3 problem.

 

If it was a gen3 it's an electric pump and so easy to replace, many do it pre-emptively. But on Gen2 it's a belt driven pump and way more hassle to replace and may people have double your mileage with still no need to replace it.

But if you keep losing coolant and it starts running really loud then replacing it would be wise.

 

Man I was hoping this generation 2 09. Was just that. previous this engine had lost its water pump belt and it probably took me 48 to notice no notices from car at all no coolant missing etc weep hole nada put new garages belt on . It's still on . Water moves great. In approx 45 min driving in town to store etc it loses about two litre bottle worth of coolant . Plastic is all missing under the car. I parked this car on very clean surface looking for any leaks they're not a hose or connection leaking on this thing . I've not scoped bores . All that's left .

 

I've replaced about 5 water pump like on Gen 2. But other Toyoda engines over 47 years of driving some those vehicles still on road today . those mech. Pumps can go forever . Not to tite on the belt.