Who Wants Faster Heat?

I just found a way to get faster heat from the vents when the vehicle is cold.

First, the airconditioning must work. Turn the A/C on. Set the mode for recirculation. Set the temperature to high. And turn the blower speed to high. And you must be going 20 MPH or faster.

I just tried this. The outside temperature is at 20 degrees. I had heat coming out of the vents within an eighth of a mile, and the temperature gauge was still sitting on the cold mark.

Try it! It works!



I can see how it would get warmer on recirculate, so I guess having the AC on prevents the windows from fogging up?

None of my cars have working A/C so I’ll have to take your word for it.

I’ll take your word for it (and will probably try it) but I’m racking my brain for an explanation - that hasn’t come to me yet.

On many cars you can set the heat on “recirc” without the A/C on and achieve the fast warmup, since you are not admitting fresh outside air.

On OP’s car turning the A/C on and defrost cycle with recirc. may be the only way to do this.

Okay. but where is the heat coming from??? If the radiator is still cold it can’t be from the heater core …then it must be from the AC but that makes no sense…what is generating the heat???

What OP really meant was that he got heat FASTER. Starting from cold you CANNOT GET HEAT in 1/8 of a mile, 1 1/8 of a mile maybe a little. OP’s car might have been warm to start with, or had been idling for a while.

The only thing that can generate heat in 1/8 of a miles is a heat pump.

“heat” may be a relative term.
Relative to the temperature the interior was initialy, the air is only getting warmer every second.
So I can see the value of recirculating every degree warmer to be warmed agian and with the fan on high I guess the 20 degree air is sooner 30 degrees than with a low fan. ( Somewhat converse to the knowledge that running your heater fan will aid in COOLING an overheating vehicle, must have to do with it being so cold to start with. )

Then, different vehicles have different potential for feeling the early yet slight temperature gain.

So, I’d need to test this theory on my own trucks to see if , maybe, just maybe it’ll work for me. ( My 08 Expedition has rear heater so it has a lot of coolant to heat up.)

A heater will put out heat quicker on “recirculate” if you can do it without the A/C working. Some cars can do that while others can’t. I can’t believe that with the
A/C on that it would warm up faster.

Tester, I am sure someone has gotten hold of your password:)

Well, a heat pump is basically an air conditioner…just depends on whether you want the heat pulled from the air to go out or in.

Okay, let’s do a thought experiment.

The refridgerant, heated by the AC compressor, might in cases where it’s in front of the radiator pass alightly warmer air through to the radiator. The AC compressor, ading a load to the crankshaft, might help the engine to warm up faster. The AC compressor is taking that energy from the gasoline via the crankshaft.

The evaporator, on the other hand, cools the air passing into the heater core, making that component’s output cooler than it would have been.

So, the question becomes, is the heat energy taken from the gasoline by the AC compressor via the crankshaft and added to the radiator’s airstream plus the engine’s slightly faster warm up speed caused by the compressor load enough to slightly offset the effect of the slightly cooled air passing through the heat exchanger (plus inherant losses).

It’s an interesting brain exercise. My guess would be that it does not. But I wonder…

The question for me is always “where does the added energy come from, how does it get converted to heat energy, and what are the energy costs”.

Of course, if it’s quite cold then the AC compressor would not run at all. (At least not until the engine compartment warmed up some.)

That’s true if you have a climate control system. I’m poor. My car only has an AC “on” button. The AC clutch is controlled by that and the system pressure. Oh, and the defrost position on the vent configuration switch. I wish I could put it in “defrost” position without the AC engaging to remove moisture. It would make it much easier to free the windshield of ice and snow after a storm. More heat.

I was/am under the impression that at very low temperatures the suction side pressure drops low enough to keep the compressor off, even on manual systems. I could be wrong about that, as my experience is with higher end systems.

It’s a closed system, and the temperature coefficient would be the same for both the high and low sides. As I understand the system, the clutch disengages when the pressure in the high side reaches a specific point. I did a bit of look-see, however, and discovered that some systems use a regulatory valve that also recognizes temperature (see below). Perhaps this works differently.


I’d be interested in the input of others here as well as Tardis. Guys, what say you?

And it seems like if the temp control is all the way on hot, none of the conditioned air is going into the cabin. Maybe it just makes the engine work a little harder? (But on the other hand, it should turn the fan on, which it seems would be counter productive).

"And it seems like if the temp control is all the way on hot, none of the conditioned air is going into the cabin."
All of the cars that I have dealt with, always passed all of the air past the ac evaporator. What the heat control did was to vary how much of the air that had already been through the evaporator also went through the heater core. (That is true for most of the cars, but some always passed all the air through both the evaporator and the heater core, the heat control varied how much hot water passed through the heater core.)
The heater core can always way overpower the AC evaporator.

I meant that the total absolute gas pressure in the AC system will decrease as temperature decreases. I don’t know how big that decrease is.
The link below does nothing to support my thought about the low pressure switch, but it is info on how one AC system works that you might find interesting.

Good link. Sincere thanks.

While I understand that your point is directed at the gas pressures in the system rather than a temp sensing device, I did notice this sentence

“An internal temperature sensing bulb senses the temperature of the refrigerant flowing out of the evaporator core and adjusts an internal pin-type valve to meter the refrigerant flow into the evaporator core. The internal pin-type valve decreases the amount of refrigerant entering the evaporator core at lower temperatures, and increases the amount of refrigerant entering the evaporator core at higher temperatures”.

Perhaps at some low temperature point that pin-type valve prevents refrigerant from entering the evaporator core altogether.

Perhaps at some low point the pressure becmes too low, the density too low, for the compressor to effectively pressurize the “high side” and create heat?

It works even without the AC on…I had warmer air within one minute while idling…(.4 cyl RAV) …The engine heats up the coolant very quickly and with the blower on recirculate the air has to get warmer …