What is Heat Mode in AC? Can air conditioners be used to make the room warm?

First let's talk about how air conditioners work.

The vapor-compression refrigeration cycle is a process that works via a chemical refrigerant that carries heat from one part of the system in your air conditioner to another and then releases it.

The chemical is compressed into a high pressure, extremely hot gas that is pumped into a long tube called a ‘coil’. As the refrigerant flows through the coil, heat is extracted through a fan. After it has cooled down, the gas is transformed back into a liquid state and is then passed into an expansion valve, which lowers the pressure of the chemical before pumping into the ‘evaporator’.

The evaporator is where the chemical turns into a gas by pulling heat from the air. As gas absorbs the heat, the cold air produced is blown into the room.

Now briefly

Vapor compression air conditioner's sole purpose in life is to move heat from one space to another. It could be moving heat from the inside of your refrigerator to your house, from inside your house to the great outdoors or from the inside of your car to outside.

If you want to heat with this cycle, "simply" turn it around. (Its really not that simple.) This is the basic concept of a "heat pump". They are air conditioners that in the summer take heat from your house and push it outside. In the winter, the cycle reverses and it pulls heat from outdoors and pushes it into the house. It works efficiently down to around 10 to 20F outside at which point it is typically to cold to pull the heat out of the outdoors efficiently.

Could you simply take a window A/C unit and install it backwards to provide heat? In theory, yes, but you will have a lot of problems.

First, most of those units are factory set to prevent them from trying to cool to a temperature less than 55F. That means if it is colder than 55F outside, it won't provide heat.

If you override this feature (search for controllers that can turn a window A/C unit into a walk-in cooler controller), the next problem will be when the air temperature is under 40F. To get heat to move, the cold surface of an A/C unit must be less then the air it is pulling heat from. So if it is 38F outside, the refrigerant must be around 30F. This is below freezing so moisture in the air will freeze on the coil. Real heat pumps have a defrost cycle to deal with this. Your window unit will just freeze solid and no longer provide heat.

Again, in theory it would work but you would need to know the various problems to look for to keep it running.

Thanks