Bought a VW ID.4

[Landshark]

This says modern heat pumps can work down to 5F or colder, but I don't know if the equipment size can be squeezed down to use in a car.

Yes, they can. They also operate in a static environment. At 20F, the coils on a residential heat pump are just sitting still at 20F. They aren’t moving at 60mph into the cold air or being pummeled by rain or snow, while attempting to produce heat from the cold.

On a residential unit, and I would imagine an automotive unit, two conditions can activate the heat strips. One is ambient temperature, the other is a call for a temperature increase of more than 2F above the current setting. If the house is 66F and the thermostat gets set to 70F, the heat strips will activate, even if it is 50F outside.

 

[leop]

During rain and snow, home heat pump are drawing in the rain or snow as fan draws in the air. And, the smart home HVAC thermostats allow the temperature split to turn on the auxiliary heat to be adjusted. Two degrees is not always the norm, and is usually not.

The main problem with heat pumps being used for heating vehicles has to do with the need for defrosting the condenser. In heating mode, the condenser will ice up when the condenser temperature is below 32F and the dewpoint is higher than the condenser temperature. Eventually, this ice blocks the air flow through the condenser to an extent that the heating ability is too low. As with home heat pumps, the system needs to be run in reverse to defrost the condenser. However, for home heat pumps, the fan is turned off in the defrost mode so that the air no longer cools off the condenser. The air flow through the condenser cannot be turned off in defrost mode for a vehicle. This makes the defrost cycle very inefficient, so much so that the defrost cycle time is very long and often does not complete.

The auxiliary heat needs to be turned on in the heat pump defrost mode. In a vehicle, the defrost mode is so inefficient that it is nearly always better to just solely use the resistance heat when the outside temperatures are not moderate. At least in a PHEV like the Clarity the engine can be run to provide heat.

LeoP

 

[Landshark]

During rain and snow, home heat pump are drawing in the rain or snow as fan draws in the air. And, the smart home HVAC thermostats allow the temperature split to turn on the auxiliary heat to be adjusted. Two degrees is not always the norm, and is usually not.

Some heat pumps are protected somewhat from the elements, and none are traveling 60mph down the road. I’m sure your familiar with wind chill.

Ours is set to to activate the heat strips when a rise in temperature of more than 2 degrees is requested. This is how the HVAC company that installed the system sets up all their residential installations. It is adjustable. Is it the norm? Who knows? I don’t claim it is or isn’t. It was an example of a parameter that activates the heat strips.

Frankly, comparing a home system to an automotive system is a waste of time. But conditions exist on both systems that can require more power in order to provide heat. Understanding that can help an owner operate the system in the most efficient way possible.

 

[rodeknyt]

Some heat pumps are protected somewhat from the elements, and none are traveling 60mph down the road. I’m sure your familiar with wind chill.

Wind chill only affects warm-blooded organisms. There is no such thing as wind chill for inanimate, non-living objects. Wind chill is the effect one feels when moving air removes the microscopic layer of air on our skin heated by our metabolism. It is an equivalent temperature one feels if there is wind. The temperature itself is still 20 degrees even when the wind blows at 30 mph.

 

[MrFixit]

Wind chill only affects warm-blooded organisms

Well, there is a similar phenomena... In an AC system, this effect is manifested by a wet-bulb temperature reading. The evaporator will have condensation on it, and when the fan blows air over the wet evaporator, it gets cooler than the ambient air temperature. This seems analogous to a 'wind chill' effect to me !