Problems With Your Refrigerator’s Defrost System?
Back in the old days a refrigerator’s freezer would frost up fairly quickly if you were opening them frequently to retrieve the food they keep. The moisture within the air, confined in the freezer compartment, quickly collects on cold surfaces forming frost. The more moist air allowed to enter the freezer compartment, the more frost it accumulates. That’s one reason why keeping a good door seal is important.
The same is true today. However, if everything is working properly you never see any of the frost because of the invention of the defrost cycle. Needing to manually defrost your refrigerator every two months is not very convenient, so most modern refrigerators have this automatic defrost cycle.
There are three main components in this defrost system. A timer that keeps track of compressor run time, a small bimetal thermostat that senses if the evaporator needs defrosting, and a heater that melts accumulated frost by generating thermal energy when it is supplied with 120 volts.
It’s not all gumdrops and rainbows though. Food cannot be preserved quite as long in an automatic defrosting refrigerator freezer as it can a manually defrosting one due to the short rise in temperature from time to time to eliminate ice.
One other issue is that you expect it to function properly. In the days of manually defrosting refrigerators, people would understand that they needed to let their appliance melt down every now and then to work properly. But today, if this unseen automatic system fails and the frost appears, or that cool glass of milk you just poured is suddenly room temperature, people freak out because they don’t know what to do.
Airflow is crucially important for you refrigerator freezer to work properly. All cold air is generated in the freezer compartment and then it is circulated and forced into the refrigerator compartment by a freezer fan called the evaporator fan.
Cold air from the freezer is forced into the refrigerator and the slightly warmer air in the refrigerator is forced back into the freezer area to have the heat that it contains removed. If either of these air passages becomes blocked with frost, or the air temperature controlling duct door, called a diffuser, sticks closed, the refrigerator will not receive any cold air from the freezer. And even if the evaporator fan is blowing air directly into the refrigerator, if the air return is frozen closed it’s like blowing through a straw with your finger on the other side.
The defrost timer is supplied power by the refrigerator’s main thermostat whenever the compressor is running. In this way it calculates how frequently a defrost cycle may need to be performed. Most timers are set to initiate a defrost cycle after the timer has run a collective eight to ten hours. Because the compressor is cycled on and off as needed by the refrigerators control thermostat, this could take ten hours if the compressor is running continually, or a few days if the doors stay closed. Most timers are mechanical, however some models use a circuit board to perform the same task.
A defrost cycle starts when a defrost timer sends electricity to a small thermostat attached to a cooling coil called the evaporator. For more information about how a refrigerator works visit ApplianceAssistant.com.
This thermostat is called a bimetal thermostat because it has two small contact strips that flex with temperature change to open or close the circuit. In this case the bimetal thermostat closes allowing the electricity to flow through when it is cold. In this way the refrigerator will not energize the defrost heater and there is no ice to melt.
This thermostat is basically an automatic switch that is on when it is cold. If the thermostat is cold enough to be closed, electricity passes through it allowing the heater to be energized. The heater temporarily becomes red hot melting all the accumulated ice until the timer stops supplying power or the bimetal thermostat warms up enough to open, breaking electrical flow. So now you know a little bit about how it works. Let’s see what can go wrong in order of likelihood.
The most common cause of defrost problems, or ice build up, is user error. Let’s just say you have one too many cold ones and forget to close the refrigerator door. In the morning you sluggishly enter the kitchen for a cup of coffee and notice that the door is open, so you close it. All that moist air accumulated in the refrigerators diffuser duct and became frost, blocking cold air from entering your refrigerator.
Later that day you’re thinking about a nice cool beverage only to discover that the milk is warm and smells funny. You could call a repair service, but this scenario can usually be corrected by unplugging the appliance and allowing it to thaw out the old fashioned way.
The most common mechanical failure is the bimetal thermostat failing to close and allowing power to flow through to energize the heater. The thermostat can be checked for continuity when it is cold. If everything is frozen and the thermostat has infinite resistance, it needs to be replaced.
Sometimes the defrost timer will fail to send electricity when it should. Mechanical timers can be manually advanced with a flat head screwdriver in the clockwise direction, slowly, until a click is heard. At this point electricity should be sent through the heater to the thermostat. Some simple voltage tests can determine where the problem lies.
The circuit board style of the defrost timer is a little more complicated to trick into a defrost cycle so I’m not going to explain that. Doing it wrong will definitely destroy the timer, even if it was fine before, and if you’re not careful you can be electrocuted.
The last and least likely to fail is the heater itself. To test the heater, unplug its contacts and do a resistance test. Defrost heaters should have some resistance, however, an infinite resistance reading would mean that it should be replaced. The entire defrost circuit can be tested when the freezer is cold by checking for continuity from before the thermostat through the defrost heater. When the freezer is closed you should have a low resistance reading and you can assume the defrost timer is not sending electricity.
I hope this article was helpful and saves you a bunch of money. But if you are ever unsure about what to do to get your appliance working properly, you’re better off calling in an experienced and trained repair technician than just winging it on your own. Today’s appliances are just too expensive to ruin needlessly.