Defrost Termination Thermostat

[Jovanna Ponder]

Inboth of these situations, the goal is to get the ice off the coils as quickly as possible but to stop the defrost cycle as soon as the coil is ice-free, yet no sooner. We don't want to terminate or stop the defrost too early and leave ice on the coil, but we also don't want to keep adding heat to the coil for no reason.

There needs to be a time limit to how long a defrost can go before it goes back into refrigeration to prevent catastrophic product loss in the case of defrost termination failure. This is part of the defrost clock and is often called the fail-safe or fail-safe time.

The fail-safe time can be a wide range of times, depending on the application and frequency of defrost, but 20 to 40 minutes is a common range. If your fail-safe time is 30 minutes, this means that once a defrost cycle begins, the LONGEST it will remain in defrost is 30 minutes, regardless of the defrost termination thermostat.

The defrost termination/fan delay control is a temperature activated, single-pole, double-throw (spdt) switch controlled with a remote sensing bulb. The control shown also happens to be an adjustable type.

Time clocks can be programmed for certain defrost duration periods. This time duration is set at the time clock in minute increments. For example, a defrost time clock on a freezer could be programmed to defrost every 6 hrs (four times daily), and have defrost increments of 40 minutes.

However, there will be times throughout the year where the coil does not need the entire 40 minutes of defrost heat. These times could result from low usage of the freezer, when door openings are at a minimum, or when the humidity is low and not much frost accumulates on the coil.

Delaying the fans prevents the suction pressure from getting too high after defrost and subsequently overloading the compressor. It also prevents warm, moist air from being blown on the product load in the refrigerated space.

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I have a Frigidaire refrigerator (model # Fftr1821qwo ) that I believe has a bad thermostat. This unit has the defrost thermostat wired directly into the wiring harness. I am trying to prevent having to purchase the whole harness and plan to cut the thermostat out and wire in another one. Unfortunately I cannot locate the open and close temperature perimeters on the thermostat in order to replace with the proper replacement. I have several good thermostats lying around but from what I understand it is very important to replace the thermostat with one that has the same or very similar open and close temps. I have tried contacting parts companies as well as Frigidaire to get these specs...to no avail!!

242213501 this is the part number for the entire harness. Does anyone have any ideas or advice?

The defrost bi-metal on this unit isn't what stops or controls the defrost - it is just a safety in-case the defrost doesn't turn off correctly to keep the freezer from getting too hot and melting the plastic liner down.

This bi-metal should probably open some where over 90 degrees. If you check the continuity on this defrost safety bi-metal at room temp it will be closed - will only open in the case of defrost run away temp.

Do you actually have a defrost problem where the complete evaporator was frozen-up in frosty ice or was just the first part of the evaporator where the tiny capillary tube enters the evaporator covered in an ice ball?

Hello Mr. Willie. I am still trying to repair this unit (Frigidaire refrigerator model # Fftr1821qwo). Your reply was super helpful. I didn't realize that some units worked this way (with the bi-metal not being the controlling factor for the defrost cycle). So it appears the bi-metal thermostat is working properly. However the unit is still freezing up. The entire evaporator was frozen up in frosty ice. Now I am trying to determine whether it is the thermistor or the control board that is causing the issue. Any suggestions on how to determine this or the next step I should take?

They don't show the defrost safety bi-metal or the defrost theremistor/sensor on the parts pictures but they are listed in the parts list ref 13* as NI -Not illustrated part, (in the cabinet section).

There are many separate components in a refrigerator's defrost system that must work in concert for a frost free system to work properly. We will attempt to explain the workings using simplified electrical schematics.

The heart of the defrost system is the defrost control. The most common control is a mechanical defrost timer which is a motorized device that opens and closes several electrical contacts. Each contact can be thought of as a simple light switch but instead of a light, one connects the defrost heater circuit, another connects the cooling system. When one of these is switched on, the other is switched off. A motor on the timer (NOT illustrated) turns a cam that opens and closes these contacts at set intervals (see below for other types).

Once the defrost termination thermostat (a.k.a. defrost limit switch) senses a set temperature, it opens the circuit to the defrost heaters, shutting them off. The timer remains in the defrost cycle until the timer advances back to the cooling mode. Since the limit switch is open, the heaters are no longer on for the rest of the cycle.

When the timer again advances back into the cooling mode, the compressor will start to run along with any air circulation fans. The defrost limit switch will remain in the open condition until it is reset by cold temperatures.

Once a set colder temperature is reached, the defrost termination thermostat closes again. This is OK since the defrost timer is no longer supplying power to the defrost circuit, the heater does not get energized.

When the defrost timer again advances into the defrost mode, the limit thermostat will already be closed and will allow power to be supplied to the defrost heater to melt any frost that has developed on the evaporator coil again.

Normally the interior evaporator and exterior (if present) condenser fan motors should run whenever the compressor is running and vise versa. If the timer is stopping operation of the cooling system, neither the fans nor compressor should usually be running at that time.

The most common symptom of a defrost system failure is a complete and uniformly frosted (not iced) evaporator coil. Frost may also be visible on the panel covering the evaporator, usually in the rear of the freezer compartment.

Excessive frosting can be cause by the defrost heater or limit thermostat being open (ie. defective), a mechanical defrost timer sticking and never advancing into the defrost cycle or a problem in an electronic defrost control or one of its sensors failing to allow the defrost heaters to be energized.Sometimes (but fairly rarely) both heater and cooling system can be energized by the timer at the same time. This can result in thawing then refreezing of food in the freezer compartment often leading to freezer burn on that food. In most cases the evaporator coil will remain mostly in an un-frosted state. The defrost heaters will cycle on and off as the defrost thermostat opens and closed due to the temperature it senses.

If the evaporator coil is only partially frosting (see illustration above) or a ball of ice develops on just a small area of it (see illustration below), it is usually a sign of a refrigeration system problem in which case a trained refrigeration technician will be required to determine the cause and correct it. These conditions are not caused by a defrost system failure.

The defrost heater is mounted onto or woven right into the evaporator coil in the freezer. The defrost termination limit switch is usually mounted on the side of the evaporator coil or on one of the connecting tubing.

The defrost timer can be in various places including behind the kickplate at the front of the cabinet, inside the fridge compartment possibly in a control panel along with the thermostat or on older models, at the back in the motor compartment by the compressor. Some strange Kenmore refrigerator models had the defrost timer built into the icemaker unit.

The defrost heater is basically a wire filament enclosed in a quartz, glass, aluminum or other material, tube sheath which gets hot when powered. It will either have resistance (show continuity) and be good or will have infinite resistance (no continuity) and be defective. How much resistance it has is irrelevant as its resistance will not normally change except to being open (infinite resistance) when it fails.

The defrost termination thermostat (aka defrost limit switch) is basically a small SPST (single pole) electrical switch which is actuated by temperature. Depending on the temperature it is, it will either have no resistance (show continuity) and be good or will have infinite resistance (no continuity) and be defective. At room temperature it will usually be open (which is normal and not a sign of being defective) and only close when it gets cold. How cold it has to be to close will depend on its particular calibration but usually near or below freezing point.

Some newer model refrigerators (Amana and Frigidaire in particular) and some older models (GE included) run power for the evaporator (freezer) fan motor through the defrost heater element and defrost limit switch. If either of those components should fail, remaining open, the fan will not run which will stop the circulation of cold air throughout the refrigerator.

On that design, the evaporator fan motor will not start running after a defrost cycle until the evaporator has had a chance to begin cooling again. While it is generally a good design idea so as not to blow the warm defrost air throughout the refrigerator, a failure in one part of the defrost system will usually render the whole refrigerator ineffective because of the lack of air flow.

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