I have a friend who has a cottage that has gas fireplace for heat. The cottage was built as a beach house and probably is not well insulated. The gas available is propane and propane now costs more than resistive electric heating in the Pacific Northwest. And that is without considering the efficiency of the gas fireplace. One solution for her would be to replace the gas fireplace with a wood or pellet stove. And that is probably her best choice, even though it would require installing a new chimney ( the gas fireplace is vented through the wall and the height is considerably below the roof top). But she has decided against that.
If anyone built a heat pump that was just intended for heating ( that is no way to use it for air conditioning ) that would work well. There is no great need for air conditioning for a beach house on Puget Sound. But I do not know of any.
So the question is what are the major differences between a window heat pump and a window air conditioner? Besides the obvious things as the reversing valves.
Could one buy a used window air conditioner and installing it backwards be at all feasible. One would have to short out the thermostat in the air conditioner and install a thermostat inside the cottage wired to an outlet. And one might have to use a timer so it would not run long enough to ice up the evaporator. And it would not be useful when it was really cold. But there is a large portion of the year where the temperature is about 50 to 55 F. Not comfortable without some heat, but not a lot of heat needed.
Used air conditioners in the PNW are very cheap. In the range of $30 to $50 for a almost new 5000 btu to 10,000 btu window unit. Almost as cheap as an electric resistance heater. So there is no concern about having a guarantee.
Dan