Heater temp.

Which temperature do you mean? There are (at least) 7 different temperatures that are worth monitoring. Engine, MG1, MG2, Inverter, Converter, Battery upper and lower. Inquiring minds will want to watch these change over time, so Prii-dash is a permanent installation in my car.

 

Your question is tough to answer partly because I almost NEVER use auto on the HVAC of my Prius.

See details about engine coolant temp towards the end of my post at Why mileage gets worse in winter | PriusChat.

I can tell you that when auto is off, the trigger points at which the ICE will turn on varies depending on the temperature setting (heat demanded). If you want to learn where those are, you'll need to use something to monitor engine coolant temp (e.g. ScanGauge II).

 

Just get a scan-gauge and you can select any temperature you like. Google it. H

 

For the Gen-II model of Prius, the threshold for the heater is 145°F.

That means if you have the heater on, the engine will stay off (if you don't need power for thrust beyond the electric motor) until coolant temperature drops below that.

For the Gen-III model, that threshold can be dropped to 114°F using the ECO mode.

For the PHV model (plug-in), the minimum I've observed it drop to the upper 80's with the right conditions in the dead of winter. And unlike the other models, it varies based on the heater temperature you set.

Long story short, block the grille and just drive it. If you want to squeeze out more, buy an aftermarket gauge to monitor coolant temperature.

 

When you say "block the grille", I am assuming that you mean shove a piece of cardboard in between the radiator and condesor. Is this correct?

 

You can stuff some water pipe insulation foam into the grill.