TIP #6

RefrigeratorThermostat and its Hysteresis


In most cases it is best to replace refrigerator thermostats with the same thermostat type and differential (Hysteresis) setting. There are four basic types of thermostats used in pleasure boat refrigeration: Zone or area thermostats for coolers and spillover boxes, Refrigerator, Freezer and full range temperature controller with adjustable differential.


Differential is the difference in temperature between when the compressor stopping and restarting times. Zone area temperatures thermostats will have a narrow differential say 2 to 7 degrees while other thermostat differentials vary from 8 to 22 degrees. Originally Alternating Current (AC) electrical compressors would overload and heat up if there was not enough time between stopping and restarting the compressor so this, along with energy considerations, determined the compressor’s off cycle times. Modern air conditioning and large refrigeration units are equipped with timers to prevent short off cycling times. Direct Current (DC) compressors also require an off cycle of 3 to 5 minutes to allow refrigerant pressures to equalize before attempting to restart.


Small home refrigerators and all well designed 12/24 volt boat ice box conversion units with Danfoss BD compressors use a thermostat that controls evaporator temperatures. There are three good reasons for controlling evaporator temperature instead of box temperature:

  • Box temperature is more stable from 1 to 3 degrees as compressor cycles based on evaporator temperatures.
  • Evaporator temperatures are always15 to 20 degrees colder than box temperature and provide an area in the box to store or freeze a few items like ice and food.
  • The Most important reason to stay with evaporator controlled thermostats is that  they are more energy efficient than area thermostats. Snap action or electronic thermostats controlling a zone or area will tend to overpower the evaporator’s ability to absorb heat efficiently and will increase daily amp-hrs consumed.


When changing the thermostat on Danfoss BD 35 and BD50 compressor systems the amperage in that circuit is very small, from 2 to 5 milliamps, depending on the size of the compressor’s speed resistor, installed in the thermostat’s wiring. With no resistance in this circuit, 5 milliamps will cause the control module to operate the compressor at a minimum speed of 2000 rpm. By adding a 1500 ohm resistor the compressor will run at its maximum Rpm of 3500. Other compressor speeds between 2000 and 3500 rpm are achieved by using resistors smaller than 1500 ohms.


Recommended ice box conversion temperature control with standard thermostats



The Snap action standard manual thermostat control temperatures range from -13 F to +32 degrees F. Differential will range from 8, at dial setting, withnthe thermostat set to max cold. New Danfoss refrigerator thermostats Code No. 077B7001 thermostat service kits on EBay are generally under $20.


REFRIGERATOR COOLER- recommended thermostat with its sense tube is in free air and not touching a holding plate or evaporator. Use a +6 degrees F differential and temp thermostat range from +27 to +50 degree F. This is also a good spillover thermostat for controlling temperatures in the refrigerator side of the spillover box.



FREEZER -  The recommended thermostats for icebox conversions with standard evaporator temperature control. use a temperature range thermostat of  -26F to +5F with a Differential of  20 to 13 degrees F In many cruising boats it is desirable to have a full range temperature box so it can be used at times as a freezer or refrigerator or even a drink cooler. The best full range mechanical thermostats are listed as Temperature controllers with an adjustable range of -15 to +40 degrees F and an adjustable differential from 5 to 40 degrees.


Food in the freezer should not touch directly to the floor of the box as there needs to be support rails to allow air to circulate under frozen product. Natural air tumbling within the box allows warm air to rise and when cooled it will descend.


To calibrate the thermostat so that it is accurate in maintaining the center of the freezer, I use an inexpensive thermometer setting it on a cardboard box at mid point in the freezer. After the temperature stabilizes and with the controller set to zero, the next day I slowly adjust the controller temperature up until the compressor stops running and then I read the cardboard box thermometer. I am assuming there is a substantial divider that provides at least a 25 degree F Delta T between freezer and refrigerator. If  the temperature between the freezer and refrigerator area is less than 25 degrees, turn the controller knob to increase the temperature in both boxes to a point where items will not freeze in the refrigerator section.


Electronic thermostats gadgets are available but I do not recommend them for a serious cruising boat.


Box remote reading mechanical thermometers are a good addition to any refrigerated box but again I do not recommend electronics when mechanical is more dependable in marine applications.


PROBLEMS you might experience with mechanical boat refrigerator thermostats.


  • Thermostat’s temperature sensing small metal cap tube is easily kinked or broken. There is a special gas in this tube that expands with heat and closes thermostat’s electrical contacts starting compressor. Any excess cap tube must be coiled up in an out of the way place inside box. As long as the excess coiled tube does not touch the evaporator or its tubing it will not effect the refrigerator’s thermostat operation. The only time repeated compressor cycling can be blamed on the fixed differential thermostat is if the cap tube is touching the cold refrigerant line or cold evaporator other than where it is intended to be clamped.


  • It is important to understand that, Compressor speed provides the refrigerant cooling capacity and energy efficiency. It is the thermostat that determines when the compressor runs and the overall box temperature. The longer the compressor runs the colder the box temperature.


  • Repeated cycling of systems with thin plate evaporators can indicate the evaporator’s capacity to remove heat is not equal to the compressor’s higher output capacity. When this happens it means the System’s Coefficient Of Performance (SCOP) is not as good as it might be, if possible reduce compressor speed to improve SCOP. If compressor speed cannot be lowered convert evaporator from a static conductor of heat to convection heat exchange by moving air across the evaporator with a small fan.


  • Thermostats are most often located inside refrigerator box. If the thermostat is located very close to a cold evaporator, moisture or even frost, may form on electrical contacts changing compressor speed or preventing contacts from opening circuit if frozen.


  • The thermostat knob shaft can corrode and stick in one position. Many times this problem can be corrected with WD40.


  • On Adler Barbour CU 100 and CU200 refrigeration units the thermostat wiring contains a standard non marine telephone male and female plug connector. A special corrosion jell is provided in an attempt to keep moisture away from the four small connector pins. This standard four pin telephone phone jack is designed to accommodate either a two wire standard thermostat or the four pin pre-wired electronic thermostat. To test the thermostat and its wiring, turn the knob to any number and check continuity at the phone plug back through the thermostat to see if the circuit is open. It should be closed for the compressor to run. If the thermostat and wiring tests fails, my recommendation is to cut the male jack plug off the thermostat wire. Then connect the two actual thermostat wires direct to the Danfoss control module terminals  C and T removing original wires from module. The current in Adler Barbour’s  BD50 compressor thermostat circuit is from one milliamp to five milliamps depending on size of compressor speed resistor instilled in wire near thermostat . Should you bypass the thermostat at the module you will be testing it without a speed resistor in wire causing the compressor to run at minimum speed of 2000 rpm.






When installing a fixed differential snap action thermostat on an Adler Barbour evaporator, it is best to locate the complete unit inside the box.  On Adler Barbour thermostats a 2 ½  inch long U is bent at the end of a  small aluminum capillary tube so that 5 inches of temperature sensing tube can be champed to the plate provided on the side of the evaporator. Older model evaporators had a plastic flat plate between the U tube and the painted evaporator, but the new units use a plastic tube sleeve on the  thermostat tube to clamp against the evaporator. If the thermostat and its sensing tube are installed correctly and the control knob is not in mid range, then an adjustment can be made by changing the amount of the sense tube clamped under the plate at the evaporator. When the control knob is in mid range, reduce the amount of tube in the clamp. This will lower the box temperature. Adding more tube will increase box temperature.




Although more expensive than standard snap action thermostats temperature controllers  provide total control from low temperature freezer up to an energy efficient drink cooler temperature. Because of their longer temperature sense tube the control can be mounted inside or outside of box. Temperature controller's differential control at time of installation should be set to minimum setting of 5 degree and never adjusted again. Box temperature setting adjusted first to zero degree F. Thermostat settings are almost never relative to actual box temperatures this is why recommended adjustment is accomplished by waiting till box reaches desired temperature then turning controller knob till compressor stops.