I made a short video on how an HVAC system works in your car.


I realize this is a manual HVAC system, and most Lexus vehicles have auto-hvac. The principal is the same, it would just have actuators moving the flaps instead of a cable, and an electronically controlled head unit.

The basics of the system are quite simple, but its amazing how complicated little mechanisms are buried under the dash, just for heating and air conditioning!

Here’s a few pics:










This is what’s inside the blower motor circuit:



Here’s an overview on how the A/C system works and where the evaporator sits:

Here’s a simplified version of the thermostatic expansion valve:

Here’s the evaporator chopped open:

Now lets look at the mixer box and heater core:

Controller:


Interesting mechanism to control the face/feet/defroster modes:



Here’s the basics of how the heating system works, and where the heater core sits:


And that’s pretty much all you need to remove if you need to service your heater core or A/C evaporator.

 

Impressive tutorial! Speaking of which you spelled ' Youtube Tuturial' in your CL Sig.

 

Thanks for an informative post.

One thing a lot of people don't realize is that the heater core, under the dash, is actually a small radiator, which, by circulating anti-freeze through it, takes heat out of the engine and re-distributes it, through the blower-motor and duct-work, into the cabin to warm our tootsies or defrost the windshield. In the winter, with an ice-cold engine, especially after it has been sitting all night, the sooner you turn on the heater, the more you use it, and the higher the temperature control and/or fan-speed is set, the longer it will take for the engine to warm up, giving you the best gas mileage and lowest emissions. You can actually watch this on the temperature-gauge needle (or the blue cold-temperature light, in vehicles without a gauge). The longer and warmer the heater runs, the longer it will take for the engine to fully warm up. I'm not saying don't use the heater (after all, that's what it is there for...your comfort and visibility)...but just remember that the quicker you get warm in the cabin, the longer it will take for the engine to. And, in general, the quicker the oil in the engine warms up (up to a point), the less wear the engine will have, and the longer it will last.

 

Thanks.
And thank you for the correction, I've had that for years and didn't realize it.
Actually you make a good point. I only drive cars with auto-HVAC and haven't thought about how turning on the heat on a cold engine would make it take longer to warm up. Auto-HVAC controls all that so I rarely touch the HVAC controls unless I need the defroster.

 

Definitely you need the defroster and if you have any confusion then seek help from local air conditoning companies. They will surely be helpful.

 

By know you would think they would have come up with instant heat, like what you get from plug in space heaters instead of relying on the engine to heat up the interior in the winter.

 

Great video and post, thanks so much for doing/sharing.

 

This "instant heat" is already used in electric vehicles that do not have a hot, combustion engine to provide heat; it is electric resistive element heating. It is also used in plug-in space heaters, electrically-heated homes and offices, as well as rear window defrosters, heated seats and heated steering wheels; but it uses a lot of electricity. If you turn up the heat in EVs, you quickly reduce your range. With a very limited battery capacity in normal ICE vehicles, electric heating would not work well.

There is, however, a new form of heating being introduced and is now used in the Nissan Leaf EV. It is a heat pump, essentially running the air conditioner in reverse.

Where the A/C system uses the temperature difference between the recirculating coolant and the cabin temperature to pick up heat in the cabin and dump it outside, the heat pump uses the temperature difference between the recirculating coolant and the outside air temperature to pick up heat from outside the car and circulate it into the cabin. As long as the coolant temperature is cooler than the outside air, it will work to provide some heat into the cabin.

 

This heating issue came up a few months ago when it was reported that tests showed an EV could lose up to 40% of its range on a cold day with generous use of cabin heat. That's a huge hit. So a heat pump is a very good idea but one which won't work well in a very cold environment.

A hybrid uses its ICE to heat the cabin. In order to speed up the warmup process, my RX450h hybrid even goes so far as to use an additional mechanism to extract waste heat from combustion: it uses a small heat exchanger in the exhaust pipe under the floor. As hot exhaust gas passes through, it warms up the coolant circulating through the exchanger, which then is pumped back to the engine block. Once the coolant temperature is high enough, a mechanical valve moves a plate that shifts the exhaust gases directly to the tailpipe.

I view this as one more thing that can go wrong down the road, but apparently it works reliably and it means the ICE warms up faster. But it may be part of the overall design for another reason. Because the RX hybrid gets such good mileage in stop-and-go driving, I've noticed there are many times during cold weather when the engine cools down significantly in city driving. It is, after all, providing all the cabin heat and that's a lot of energy. So the exhaust-recovery system helps to keep the coolant hot.