If admin. can figure out a better place to split it to, that's fine with me.

Done

 

I don’t know the internal arrangement of the heater core and the evaporator in different cars. I should look it up. Why would one configuration permit a smaller AC compressor over another?

 

If the heater core located in the lower section of the HVAC box is always hot thanks to continuous flow, the evaporated core located just above is always going to be getting a certain amount of waste heat. A mere plastic flap part of the air mixed damper is not going to isolate that. So you will need a larger capacity air conditioning system to provide the same level of cooling.

A TVC that cuts water flow when not needed will greatly increase the apparent capacity of the air conditioning system.

 

Why the TVC? Emissions compliance is one reason. Car engines are notoriously dirty during their first 1 to 2 minutes of operation when the system is operating in open loop. If you are trying to heat the cabin during warm up, you're delaying the onset of the sharp drop in exhaust emissions that's achieved when the system reaches full operating temperature.

TVC's have been on Honda and Toyota vehicles as early as 2016 to allow computer controlled cabin temperatures and favor engine warm up. This also helps prevent oil dilution by gasoline when the engine is not at full temperature and the rings fully seated.

That the Subaru design had a flaw is unfortunate, but it does not negate the desirability of the technology.

Subaru did use a bypass thermostat and two port housing on some FB engines to reduce water flow to the cabin until the engine reached temperature. A clumsy poor man's TVC that didn't work all that terribly well.

 

Are you referring to hot air leakage from the heater core region into the cooled air, or just conduction through the structure? A little of both?

 

Both. Conduction & convection.

 

My wife used to call this "Thermostat Soup". We had a Nissan that occasionally overheated, and I found that lazy thermostats played a big part. Some opened slow, others never achieved full lift and thus constricted flow.

Remember that the active elements are a spring that pushes against an expanding wax pellet. Technology from nearly a century ago! Note the range of temps listed in the Subaru service manual on the test procedure. No two cars will run at the same temp, especially during cold-start warmup. That's just not proper on a computer controlled engine when you have to guarantee to the EPA that all engines you produce will deliver the same emissions performance as the sample submitted for certification.

 

Prior to the electronic TCV there was a dual port thermostat used in some FB engines to limit flow to the heater core until the engine warmed up. There's a blurb from Tech Tips: