Window air conditioner, reversed.

[Externet]

Hi.

Always thought an electric heater is 1500 Watts in; 1500Watts of heat out; or 100% efficient...   Read about the ">100%" efficiency of heat pumps.  Read about the theories of reversing a window air conditioner to heat a dwelling; consuming ~400Watts instead of 1500W as the heater and yielding more heat...

Make me believe one or the other.

[swansont]

Cooling efficiency is not the same measurement. When you say that heating is (nearly) 100% efficient, that’s saying all of the energy is turned into heat. 100 J of electricity is turned into 100J of thermal. (temp goes up)

Cooling efficiency is really the coefficient of performance; it’s about moving stuff with energy around. You can remove, say, 250J of heat from space with 100J of input, but you haven’t destroyed any energy - the rejected heat from the system is larger. 250J is removed from the space (temp goes down) but that and the 100J of electricity is deposited elsewhere (350J total) - wherever your thermal reservoir is.

If you compare apples to apples, the COP of a heat pump is always bigger for heating than cooling, given the same input of heat and work

https://en.m.wikipedia.org/wiki/Coefficient_of_performance

[TheVat]

The name "heat pump," is a good reminder of what it does.  

I would guess that reversing an AC window unit for heat is ineffective due to its engineering specs - it is designed only to pull heat from air at or above comfortable room temperature.  

[Ken Fabian]

Our AC is "split cycle" rather than the window fitted variety but I would suppose, like our system, some of the latter can do heating as well as cooling.

The heating is not from conversion of electricity to heat, but from drawing heat from outside air with a fluid at lower temperature, then releasing that heat inside at a higher temperature. The trick (as I understand it) is to have a fluid that is liquid with a boiling temperature/condensation temperature that is higher than the outside temperature but lower than the desired inside temperature - compression causes it to condense and it gives off heat, then outside it is warm enough (despite being "cold") to boil, absorbing heat. The heat absorbed outside is converted to higher temperature heat inside. (The reverse of what AC does). The amount of heat transferred from outside to inside can be several times the electricity used by the compressor and the fans.

Different working fluids and the pressures they work at make heat pumps possible that work in outdoor temperatures well below freezing.

[studiot]

7 hours ago, TheVat said:

The name "heat pump," is a good reminder of what it does.  

I would guess that reversing an AC window unit for heat is ineffective due to its engineering specs - it is designed only to pull heat from air at or above comfortable room temperature.  

Here is the rating plate on my heat pump.

Note it is not the window or protable variety.

 

The cooling mode has about 10% less capacity than the heating mode.