Poor heat

I'm not sure where you are from, but I'm guessing Canada as you mentioned the temp in Celsius

If you think the Prius heat is bad at -15 C, wait until -25 C and colder. My first winter with my Prius, stuck in traffic at -40, I just about froze my fingers off. Even with the ICE constantly running, it cooled off in the car. Not to mention I only averaged 9.8 l/100km in that condition

Blocking the lower grille and two upper slits will make a world of difference. Even at -40, my Prius will briefly shut off at red lights. The fuel economy is really helped too, even at -40 I can maintain 6.5-7.5 l/100km

Other vehicles also benefit from a winter front. I made one for my FJ Cruiser and at -26 C, was toasty warm with good fuel economy. Well, good for an FJ, around 12 l/100km in mixed driving

 

As Jay says, you have to be driving to get the engine to warm up. BTW, this isn't just a hybrid/Prius thing. A friend with a car with a 1500cc aluminum engine has the same problem. My Prius starts blowing nice hot air after about 6 blocks of driving. Front grills are blocked. Climate system set on "auto". The car gets toasty warm inside after about 2-4 km, depending on how many times I have to stop for lights. It cools slowly at lights, whether the engine is running or not. Ambient temps here are around -20C.

 

cant begin to emphasize Jay's point. When i had my 04 with CANVIEW i tracked temps on a cold windy day it was only about 25F or like -4C... the engine temp would get to 175F... i turn on the heat and in less than a minute, engine temp would drop to 155-160... so as you see, even if you warm up your Prius, turning the heat on will cool the engine off and fast.

blocking the radiator makes a WORLD of difference. at a constant (as much as i could...keeping in mind no test track to do this on... just a early Sunday morning with minimal traffic) speed of 40 mph... temps ranging from 30-35F, grill blocking got me up to 175F in about 3 mins compared to 5 mins without... if heat was on and set to 70F, with grill blocking it took 4 mins, without it intially took about the same, 5 mins, but actually would not stay there. it took roughly 10 to 18 mins of driving before it would consistantly stay there. (hard to say because the course i had laid out was only about 8 mins chosen because no stops and one right turn involved and any stop caused the engine to turn off which dropped the temp like a rock)

 

it really depends. for around town driving, remove at 60+... there was a new thread by Evan (efusco) where he took a lengthy trip at 70 mph during pretty cold conditions in the low 40's and had some overheating issues. but realize, hundreds of people here have done this without issues, so i think his has to do with going 70 mph.

as for me, i remove when temps are above 50F if on freeway or 60F (last spring i admit it was consistently in the upper 60's before i got around to it)

i have taken MANY trips to Seattle with temps in low 60's no issues but i never do much more than 60 mph (too competitive on my mpg figures!!) we are talking roughly 60 miles one way

 

I have a hobby farm around 2 hours drive from my home. Even with the studded tires, I was getting stuck and sometimes stranded at the place. If I needed to get out, I had to then drive my 1984 Ford plow truck, which I equipped with a PowerTrax LockRight rear axle and a Detroit Locker front axle, and studded Cooper WeatherMaster tires

I wanted a second vehicle as a "toy" anyway, and got a very good price on my loaded FJ. I only drive it to the hobby farm, otherwise it sits in my garage. Have had it 10 months and barely have 8,000 km on it

Using the same techniques I use to drive my Prius, and putting Mobil Delvac Synthetic Gear Oil 75W-90 in the axles, I can easily get 27 MPG highway out of the FJ.

 

Unfortunately that doesn't apply to all vehicles though. While "pining for my Prius" last year, I tried to modify my habits driving the anti-Prius (2001 Nissan Pathfinder 4WD). I even got a Scangauge II to help. Couldn't get the mileage to improve, no matter what I did! The mileage wasn't "bad" for a 4400 lb mid-size SUV, it ran from 14 l/100 km to 17 l/100km (summer to winter). My Prius gets 4.2 l/100 km to (so far) 5.9 l/100 km.
Here's a pic. of my grill blocking.