The heater will heat the engine once the heater core is up to temp. It also functions to heat up the engine more quickly when running. This is key for me as you can literally watch the engine temperature gauge drop while idling in -25°C or colder before I installed the parking heater. Now it is up to operating temperature minutes after starting the engine, and stays there.
All that being said, I know exactly what you're asking and I too would like to know if it is possible to switch from cabin heating to engine heating. Most generic installs of these devices do only that - heat the engine coolant. I would prefer this mode when it is extremely cold out - like -30°C or colder. Right now, operating the heater to warm up the cabin does nothing to ensure I can even start the engine. The heater saps the battery down and then I don't have enough crank left after a few glow cycles. When it's above -25, all is well. Warm cabin, engine starts and warms up in minutes. I had to jump start once this week when it was -36°C and the heater had run for 20 minutes. Interior was warm, but couldn't start.
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