Isotherm Plus "SP" Water-Cooled Refrigeration Component System

Isotherm Kit

The Isotherm refrigeration kits come complete with everything needed for installation.


Isotherm’s SP Compact Cooling units are self-pumping refrigeration systems capable of producing either refrigerator or freezer temperatures. These units feature a self-pumping thru-hull fitting that eliminates the fan and the electric pump. This reduces power requirements and consumption. A broad range of sizes is available, handling refrigerator/freezer capacities as small as 4.4/1.5 cu. ft and up to 14.1/4.7 cu. ft.

The SP System

On hot days, the water-cooled Isotherm SP operates consistently. Isotherm ITC systems are designed for installations in existing cool boxes and primarily for use in boats where battery power is at a premium.

The key to the Isotherm SP system is the patented thru-hull fitting that replaces the conventional galley fitting and includes a full-flow coil heat exchanger. The SP system leads the ambient heat down and out through the hull by using the natural pumping action inside the fitting. Because this hose is not restricted by pumps or non-return valves, the flow inside is free.

Each movement of the water surrounding the bow reproduces a similar change of the water level inside the hose resulting in the pumping of a similar quantity of water in and out through the fitting, while the boat is underway.

The fitting is situated far enough from the boat’s center of gravity that the slightest hull movement will induce the pumping action. Even when the boat is hauled out, the SP System still functions to cool refrigerant by creating a chimney effect to carry away heat through natural convention. The alloy used in the thru-hull fitting is resistant to electrolysis and growth of barnacles or grasses.

The Sea Water system is powered by a 12/24-volt Danfoss compressor and features easier installation with the pre-charged system, click-on bracket and quick-connect lines. Warm water is continuously pumped out and is replaced by cold water, which prevents fouling. The thru-hull fitting is made of brass and the heat exchanger is copper-nickel alloy. The 1m (3.28’)-long pipe connecting the thru-hull to the compressor and the 3m (9.84 ‘) line from the cold-holding plate both attach with quick-connect fittings. A sacrificial zinc anode is included.

The cooling unit clicks in place with a custom-made bracket for easier installation.


The Isotherm Plus with Intelligent Temperature Control (ITC) module lets the user set the temperature he/she wants. Push the up or down keys on the control panel and the digital temperature setting in Celsius or Fahrenheit will appear on the display panel.

The Plus System

Isotherm says the Plus system replaces the ASU control module with the ITC in combination  with the Isotherm Holding Plate. It uses the same eutectic holding plate sizes and the same patented SP thru-hull fitting. The ability to provide freezing temperatures is new and the same SeCOP (Danfoss) compressors are backed by Isotherm’s five-year limited warranty. This provides the user with more control over energy output. The unit features digital temperature readouts in Celsius or Fahrenheit, digital temperature control, compressor speed control and battery protection control.

The stainless-steel holding plate can be fitted at any suitable angle high up in a compartment before being connected to the compressor unit by the three-meter long, 6 mm diameter flexible copper piping fitted with quick couplings. The compressor itself is small enough to be fitted in a locker within connecting distance from the holding plate. Isotherm says the unit is quiet when running and can even be fitted under a bunk. After its leads are connected to the battery, the pre-filled system is ready to go.

Defender Industries stocks a wide range of Isotherm compact cooling units including self-pumping and ITC featured models.