Not sure if anyone encounted this for your 450h+ , during winter, when temperature is -12 C, the engine will turn on to generate heat, but at -11 C, a lot of times, the heat pump won't turn on (if it's on, you can hear the noise from engine compartment), so no heat, and the engine stays off, I have to manually switch to hybrid mode to turn on the engine.

 

I've seen this at temps below -14F which I found on page 285 of the manual. It seems use the ambient air temperature to do this and from the short time I've had my car, if it's 15F, it doesn't run the ICE to heat the car unless I select HV mode.
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In the following situations the gasoline engine may operate in order to extract heatfrom
the engine coolant via the heater even in EV mode.
• The outside temperature is approximately -10°C (14°F) or low
• Windshield defogger switch is operating

 

It takes time for the heat pump to turn on. It does not turn on immediate when the car is on. If you want to stay in EV mode, you can turn on the windshield defogger and the gas engine will turn on. It also depends if you have eco mode on and also what temperature you set.

 

Yes, I’m aware of that, normally the heat pump will turn on within a minute, but when this temperature, it’s won’t be on even I ran for more than 10 minutes, I know how to turn on the engine manually, but this should be automatic. It worked perfectly at other temperatures, either below or above that.

 

Perhaps Lexus needs to add traditional electrical heating coils in their Cold Weather Packages for their PHEV's and EV's. Heat pumps work by absorbing free heat from the air outside your car and then transferring it inside. This is more difficult when the temperatures are lower because they’ll struggle to keep your car comfortable even in a mild cold snap. That is why in homes and offices that have heat pumps instead of a traditional furnace and air conditioner system, they add electrical heating coils to the blowers. We had to do that in my office that has heat pumps.

 

Alas EV mode may be pointless with heating strips. It's more efficient just to run the engine if you have it. As there is no high power grid connection to a gas turbine miles away when in the car unlike for buildings.

 

Heat pump can't generate enough heat at low temperature. As stated in the manual, the engine will turn on if outside temperature is below about -10 C(14 F), in my case when I started the car at exactly -11 C, the system seems don't know what to do, both heat pump and engine stay off.