Onboard a modestly sized boat, the dog days of summer makes a skipper sweat. The sun drops, and now an icy breeze makes the captain shiver. The vessels’ inbuilt heating and cooling systems keep everyone comfortable as soon as the weather flips, of course. That’s not unusual. What is remarkable is how both of these temperature equalizing features are built into one marine appliance. Naturally enough, the crew would really like to know how it pulls off this function-reversing feature.

Tearing Down a Reverse Cycle Air Conditioner

Let’s get down to business. When a repair technician removes any intervening access panels, a reversing valve comes into view. In cooling mode, this valve operates normally. The compressor runs in a closed-loop system. It provides refrigerant gas (R410-A), which enters a condenser coil. Inside this coil, there’s a second pipe. That tubing carries raw sea or lake water. In sequence, heat is absorbed from the cabin, carried away by the refrigerant liquid, and then the energy is transferred into the water. A pump takes charge of the water flow. Remember, the AC unit isn’t bringing cool air into the cabin, it’s transferring and relocating heat to the surrounding mass of cool water.

Determining Reverse Cycle Mode Functions

Upon activating this warming mode, the reversing valve and compressor switch directions. Now the appliance is operating as a heat pump. And this feature works exactly as designed, with heat from the open water now being transferred into the refrigerant. The gas expands where it meets the water, then heat is transferred to an air handling unit and discharged into a boat’s inner compartments. Just like before, there’s no warming elements or gas burner pulling off this feat. No, the thermal energy is being transferred from the water surrounding the vessel. It’s a balanced heat exchanging mechanism, although it can go wrong if the temperature differential between the boat cabin and water are functionally inadequate. For cooling a boat, the dumping of its thermal load won’t take place efficiently if the outside water temperature is hotter than 32°C. Likewise, the craft won’t take on heat if the water is cooler than 13°C.

There’s a highly efficient heat exchanger architecture buried in a reverse mode marine AC unit. The compressor supplies fluid-collapsible R410-A gas. That gas condenses in a coil filled with raw water. Functioning as a regular air conditioner, the refrigerant and water transfer cabin heat to the surrounding water. Switching the reverse cycle mode to its “Heating” setting, the compressor and reversing valve changes the direction of the refrigerant. When this occurs, heat is now transferred out of the water and directed through and into a boat’s cooler quarters.