Why not use heat pumps in automobiles?

The technology of reverse-cycle air conditioners (heat pumps) is well understood and utilized. IIANM, all that is involved is a valving process that functionally (if not physically) turns the airconditioner around, delivering hot air to the “interior” and cold air to the outside. Why is it that automobile manufacturers, to my knowledge, do not use this technology to be able to “instantly” deliver heat to a just-started car during the winter season? It seems to me this would be a great marketing tool. Once the car has warmed up, the traditional hot water system would take over and the heat pump would be turned off to conserve fuel. Granted, heat pumps are not very efficient at temperatures below (about) 30 deg. F., but even a LITTLE heat when you climb aboard at 5 above would be a welcome, worth-paying-for feature (IMHO). Am I missing something obvious or fundamental?

I think it’s a lot of equipment for something that would only get used the first few minutes of each trip. There’s not a lot of room under the hood for more these days, and it would be in demand only in certain climates. Aftermarket heaters are available that are basically electric heaters that accomplish the same thing. Perhaps not terribly energy efficient but trivial to install and since they are only used for a small portion of travel time, who cares about efficiency.

The only thing you’r missing is TANSTAAFL, that old devil the second law of thermodynamics. Yes, you could install a heat pump at great expense but it would only work by putting a significant load on the alternator to power it. Efficiency wouldn’t be much different than if you just powered heater elements with the alternator directly.

Folks, you don’t have to install ANYTHING additional. Virtually every automobile on the road nowadays is equipped with an airconditioner. All that is (possibly) needed is a second set of valves and a pushbutton that says “Heat” instead of “Cool”. I had a window airconditioner with a “reverse cycle” that I used to heat a garage in Florida’s panhandle during the late 60’s. It was no bigger or heaver that a straight air conditioner of equivalent BTU capacity. The resistance heating elements seems to be a logical solution to the problem as well, bubt no one seems to be using it as Original Equipment. So why not??

I guess you’re right, but I don’t know why it hasn’t been marketed. However, this idea seems to be more important with the advent of hybrids, which don’t generate much engine heat.

It’s better just to have one of those remote car starters so you can have it start to warm up before you get there.

      • It is true that you can design an “air conditioner” to both heat and cool. You don’t even need to mess with valves: if it’s designed with a reversable pump, then the pump only has to change direction to switch between heating and cooling.
  • The reason that it’s not done is that for a heater, it doesn’t make any sense: the engine creates lots of heat it has to get rid of anyway. In practice you consume less energy by just drawing waste heat off the engine than you would cycling a heat pump.

I was trying to think of any way a heating air conditioner would be better (since making heat is generally the easiest and most efficient energy transfer).

Maybe because it could heat the air and blow it around quickly, instead of waiting for a hot engine to heat up the air??

You mentioned the inefficiency problem. When you REALLY want that heat, will the pump be efficient enough to actually warm you up?

I figure, it would take enough time to actually heat everything up with a secondary (less powerful) heater, that waiting for the normal heater to get going isn’t really that much worse. Getting a few extra degrees for the 5min it takes the heater to work isn’t really a huge benny.

Now, a heated steering wheel would be a dynamite idea!

I second that idea! but then again, I already got leather gloves to counteract that.

Some of luxury cars (in Europe at least) use a blow-heater for the first few chilly minutes. A lot simpler and cheaper than a reverse heat exchanger.

A heat pump simply wouldn’t work in the conditions where you most needed it.

Heat pumps suffer from a serious problem in very cold conditions. The evaparator ices up and the heat pump stops working.

So if you had one in your car, it would only work when temperatures were relatively mild, in which case, you wouldn’t need it.

That’s why the alternative is simple electric heating, as mentioned by CookingWithGas and jjimm.