A heat pump is just an air-conditioner that can change the path of the refrigerant. In cool mode, the refrigerant goes from the compressor to the heat exchanger outside the cooled space (condenser), through the capillary tube (or thermal expansion valve, TXV), into the heat exchanger inside the cooled space (evaporator), and then back to the compressor. In heat mode it goes from the compressor to the heat exchanger inside the heated space (condenser), through the capillary tube (or TXV), into the heat exchanger outside the heated space (evaporator), and then back to the compressor. Remember, heat flows out of the evaporators and into condensers.
Here's a link that provides a more detailed explanation: http://www.heatpump-reviews.com/heat-pump.html
This unit uses a capillary tube to restrict refrigerant flow and generate the pressure needed to condense the refrigerant. Modern units use TXVs to regulate the pressure of system. While they are more efficient, they generally only work when the refrigerant flows in one direction unless they are specifically designed for heat pumps. A capillary tube is a long, thin tube that is a specific diameter and length to generate a specific pressure. There are no mechanical parts in a capillary tube, therefore refrigerant can flow through it in both directions. So in order to make this A/C a heat pump, I only needed to add a reversing valve and reroute the copper tubing.
Here's what I did:
-Remove unit from roof of trailer.
-Get part numbers of compressor and fan motor to find compatible replacements, if needed. (I did)
-Measure copper tubing diameter so you can but a compatible reversing valve. Reversing valves come in different sizes based on how big the unit they are designed for. Try to find a valve that has the same size fittings as the copper tubing in the A/C unit. If not, you'll have to get adapters to step up or down sizes. Remeber, refrigeration copper is "tubing" and is measured on the outside diameter and not "pipe" which is measured on the inside diameter.