Technical Training Guidebook of Domestic Refrigeration

8/17/2019 Technical Training Guidebook of Domestic Refrigeration

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DOMESTIC REFRIGERATION

TECHNICAL TRAINING GUIDEBOOK

Teacher: Luis Valera

2013

INTRODUCTION


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How does a frost free refrigerator's defrost system work?

There are many separate components in a refrigerator's defrost system that must work in consort 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 bethought 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 thetimer (NOT illustrated) turns a cam that opens and closes these contacts at set intervals (see below for

other types).

Cooling Cycle

During the cooling mode, the defrost timer closes a contact to the compressor circuit so it will run. The

circuit to the defrost heater is open.

While in this mode, the thermostat (a.k.a. cold control) cycles the compressor and fan motors on and off tomaintain an appropriate temperature.

Defrost Cycle

The defrost timer eventually switches into defrost mode and supplies power to the defrost heater(s) to

melt any frost that has accumulated on the evaporator (cooling) coil.

The cold control contacts remain closed but since the defrost timer is no longer feeding power to thatcircuit, the compressor does not run.

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 therest of the cycle.


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Cooling Cycle

When the timer again advances back into the cooling mode, the compressor will start to run along with anyair 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 theevaporator coil again.

Fans

Normally the interior evaporator and exterior (if present) condenser fan motors should run whenever the

compressor is running and vice versa. If the timer is stopping operation of the cooling system, neither thefans nor compressor should usually be running at that time.

Defrost Problem Symptoms

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. Thedefrost heaters will cycle on and off as the defrost thermostat opens and closed due to the temperature it

senses.


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Refrigeration System Problems

NOT a defrost system problem

If the evaporator coil is only partially frosting (see illustration above) or a ball of ice develops on just asmall 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.

NOT a defrost system problem

Defrost Component Locations

On most frost free refrigerators, the evaporator (cooling) coil is inside the freezer compartment coveredby a panel. The freezer fan motor is usually in the same general area.

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.


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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

The defrost heater is basically a wire filament enclosed in a quartz, glass, aluminum or other material, tubesheath 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

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 haveno 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 orbelow freezing point.

Some newer model refrigerators(Amana and Frigidaire in particular) and some older models (GE included) run power for the evaporator


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(freezer) fan motor through the defrost heater element and defrost limit switch. If either of thosecomponents 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 thedefrost system will usually render the whole refrigerator ineffective because of the lack of air flow.

Defrost Timers Types

Early production mechanical defrost timers would go into the defrost cycle after a set amount of time.

Common timing periods were 6, 8, 12 and 24 hours. This meant that say every 6 hours, the refrigeratorwould go into defrost whether it needed it or not. The duration of time it would remain in the defrost cycle

was fixed and could be anywhere from 18 to 30 minutes depending on the timer design but it would alwaysbe the same length of time.

As stated above, the defrost heaters may not be on for that full length of time, thanks to the defrost limit

thermostat, but the cooling cycle would not start again until after the complete defrost duration wasended.

This original design was wasteful as the refrigerator would defrost regardless if it was necessary or not. A

later design tried to help this a bit.

Cumulative Run Timers

The next design was called a 'cumulative run' timer. These timers were installed in a way were they wouldonly count the time that the refrigerator (the compressor) was actually working. This makes sense since no

frost could build up if the compressor was not running.

These later mechanical model timers would only advance the into defrost when the compressor had actuallybeen running for a certain length of time, usually 6 to 8 hours of accumulated compressor operation.

Adaptive Defrost Control

The latest, energy saving variation is electronically controlled and called an adaptive defrost control. Not

only does the period between defrost cycles change but also the time duration of the defrost cycle itself.

The device is programmed to keep track of the appliance usage and how long it takes for the evaporator coi

to be thoroughly defrosted. It will then calculate the amount of time required and adjust itself accordingly.

GE's 'Mother Board'

Newer GE made refrigerators are almost totally electronically controlled. Their motherboards take the

place of both the temperature and defrost functions even controlling DC fan motors that can operate at

several different speeds. On this system only the defrost heater is the same as described above. A defrost


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cycle is ended when a thermistor detects a temperature rise of the evaporator which is the s ignal to the

main control to terminate the defrost time and start the cooling. The termination thermostat on this

appliance design only acts as a safety device to shut the heater(s) off in case of a malfunction before theplastic interior liner melts, otherwise it is never active. The control cannot be manually put into defrost

mode.

Timer Testing

The mechanical timer types described above will usually have a screw on their underside which can be

turned clockwise to manually change its present cycle. If running, slowly turning it until it clicks once shouldput the timer into 'defrost mode'. When in the defrost mode, turning it until it clicks once should put the

timer into 'run mode'. It can be left in the defrost mode to see if it will advance by itself to the run mode(like it should in less than 30 minutes) or the defrost heaters may be able to be tested to see it they arereceiving power or not at that time.

NOTE: Once the compressor has been turned off, it should be allowed to sit idle for several minutes

(5-10 minimum ) before attempting to restart it. This is required to allow the internal refrigerantpressures to equalize so the compressor is able to restart again without putting excessive strain on it and

damaging it.

Test points and how to manually switch adaptive defrost controls into the different cycles vary frommanufacturer to manufacturer. For these, check the appliance's 'tech sheet ' for instructions. It is usually

included in the appliance's wiring diagram which is most often located behind the kick plate at the bottom

front of the refrigerator or inside the control console along with the defrost control. Defrosting (orthawing) is a procedure, performed periodically on refrigerators and freezers to maintain their operatingefficiency. Over time, as the door is opened and closed, letting in new air, water vapour from the air

condenses on the cooling elements within the cabinet. It also refers to leaving frozen food at a higher

temperature prior to cooking.

Procedures for manual defrosting

The resulting ice inhibits heat transfer out of the cabinet increasing running costs. Furthermore as the icebuilds up it takes increasing space from within the cabinet - reducing the space available for food storage.

Defrosting the unit is achieved by:-

●

Temporarily removing all food from the cabinet.

●

Turning off power to the unit.

●

Leaving the doors to the unit open

●

Waiting for the ice to melt and draining it appropriately. Using a towel is advisable when completing

this step.

The process may be speed up by mechanical removal of ice, or the introduction of gentle heat into the

cabinet. Placing a pan of hot water in the cabinet and closing it is an effective method. Using a fan to blow in

room temperature air will also greatly speed up the melting process as well as help to evaporate the damp


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surfaces. Note that the fastest manual way is to use a vacuum cleaner: simply insert the hose into the

exhaust port (nearly all are designed for this), and use the wand to blow on the coils; this method is muchfaster than any other. Any mechanical removal of ice should be done gently so that the equipment is not

damaged.

It is generally recommended that defrosting should be done annually.

Hot Water Method Take a pan and fill it with water, get it nice and hot on the stove or use a bowl in the microwave. Place the

hot water inside the freezer and close the door. After about 5 minutes remove the pan or bowl and reheatthen put it back in the freezer. Continue until ice is melted.

I personally hate the hot water method of defrosting the freezer. I find that the ice under the hot watermelts first and the pan or bowl sticks to the ice as it cools off. It’s annoying to come back every 5 minutes

and it doesn’t seem to help the ice melt that much faster.

Fan Method

Set up a fan outside the freezer with the freezer door propped open so that air circulates faster. More air

flow will help the ice to melt faster.

However, you might not have a fan that you can angle properly and if your thawing a unit above a

refrigerator it might not be worth the effort.

Blow Drier Defrosting Method

Lets think about this for a second. The goal is to turn ice into water. Blow driers and water are not a good

combo. This is not the safest method. It is not recommended for safety reasons.

That said I’ll admit I’ve tried it. At first glance it doesn’t seem to do much. But what happens is the heat

goes to the back and then up.

So after a while the ice on the top will defrost away from the freezer, however with thick ice you most

likely won’t see this. Instead large pieces of ice just start to break off the top.

In my experience this method along with the scrapper method and lots of towels is the fastest way to

defrost the freezer.

Scrapper Method

Scraping the ice is probably the most common way of speeding up the process of defrosting the freezer.

But we have to make sure to do it safely. And that, my friend, means no sharp objects.

There are two reasons for this. First you could hurt yourself. Second you can scrape or puncture thecabinet of your freezer. Well, it doesn’t sound like a big deal, puncturing the stuff behind the freezer

cabinet could cause a leak and make your whole freezer useless. It’s not worth the risk.

A dull object, like a plastic spatula or spoon, works better. Think of it less as ‘ scrapping’ and more

as ‘assisting’ large chucks of ice to come out before they melt.

This method works well with the blow drier because as the ice starts to defrost off the wall or ceiling you

can get the spatula under that and take out larger chunks.


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Clean It

Use a mild detergent or 3 tablespoons baking soda in 1 quart of water to gently clean the freezer while it’s

already empty.

And do not rub oil over your freezer like some sites suggest, because over time even at cold temperature

that oil will go rancid.

Turn It On and Let It Cool Make sure the freezer is dry or you’ll be starting to build up ice again before there's even food inside. Turn

the freezer on, set the ideal freezer temperature for your needs, between 0F and -10F.

I would wait about an hour before adding food that is still frozen, longer for refrigerator temperature

food.

Repack Frozen Foods in Good Condition

Check the foods you have before returning them to the freezer. Anything too old or freezer burnt should

be tossed out. Any meat that has thawed needs to be cooked before it can be frozen again. Learn moreabout how to refreeze food if you're not sure.

Put like items together so they are easy to find. There's a method to organizing your freezer contents.

While we are on the topic there is a myth I would like to dispel.

Myth: Frost in a freezer contributes to freezer burn on your food.

Truth: Frost in the freezer usually means there is an air leak. However, foods that are properly wrapped in

moisture-vapor proof freezer materials will not develop freezer burn simply because the freezer doesn’t

seal properly.

While repacking the freezer I highly suggest you add every item to your freezer inventory sheet. This way

you know exactly what is in there.

You can get a free printable freezer inventory sheet as a gift when you sign up for the Favorite Freezer

Foods E-zine, a newsletter full of tips and tricks about freezer cooking.

Many newer units employ automatic defrosting (often called "frost-free" or "no frost") and do not require

manual defrosting in normal use. Although, in some cases, users of Frost Free fridge/freezers have noted

ice blocking the vent that allows air into the refrigerator compartment. All refrigerators and/or freezerswhether or not frost free, should be defrosted at least every 6 months (or more if you live in humid

conditions or if the door is opened excessively). Frost Free units should be defrosted every year, as frostcan form on the evaporator covers, which can look unsightly and degrades freezing performance, sometimesto the point of the freezer thawing out due to degradation of defrosting. This can happen when the defrost

timer is not set up correctly/malfunctioning or if the heater units or temperature sensors are

malfunctioning.


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Self-Defrost Freezer

Definition: Self-defrost is a term used to identify a freezer that is designed with an automatic defrost

cycling system that requires no intervention from the homemaker. A self-defrost freezer cycles on/off

periodically to melt and remove ice that would otherwise build up in the freezer's interior. This water isthen discharged to a drip tray positioned at the base of the unit. There is no need to empty this water tray;

it tends to evaporate on its own. The drip tray is usually located behind the bottom trim plate of an upright

freezer. Most (not all) upright models of freezers have a self-defrost feature. Also known as a frost-freeor auto-defrost, this is a feature many consumers love for ease of cleaning and in terms of saving time and

labor. A self-defrost feature does increase the price of a freezer and consumers should confirm thepresence of this feature before buying, since it's not always easy to detect the presence in a freezer's

design.

A freezer without a self-defrost feature is known as a manual defrost freezer. Most chest (not all)

freezers have manual defrost. Defrosting a freezer manually involves a hands-on, time consuming and

tedious task.

Though manual defrost (chest) freezers require more cleaning, they are cheaper to buy andcost less in energy to operate. However, they lack the organizing potential and retrieval convenience of an

upright self-defrost model. A self-defrost feature reduces freezer maintenance considerably, but it does

have a down side. Because of the on/off cycling routine, temperatures tend to fluctuate and the freezermust work harder to restore the ideal interior temperature. These increases operating costs, leads to a

noisier operation and foods are also more prone to freezer burn than in a manual defrost freezer.

Technician and consumers should consider all freezer features along with operation, energy and maintenancebefore making a freezer buying decision.

How to Repair a Refrigerator

Servicing the Defrost Timer

If the compressor doesn't run, it is likely that the defrost timer is malfunctioning. This part is located near

the compressor. To test the defrost timer:

Step 1: Unplug the refrigerator.

Step 2: Disconnect the wires from the timer and timer motor. Remove the timer from its brackets by backing

out two retaining screws.

Step 3: Test the defrost timer with a VOM set to the RX1 scale. Clip one probe of the VOM to each defrost

timer -- not motor -- wire, and turn the timer control screw shaft until it clicks. If the defrost timer is

functioning, the meter will read zero. If the needle jumps, the defrost timer is faulty. Replace it with a new

one of the same type.

Step 4: Connect the new defrost timer the same way the old one was connected.

To check the defrost timer motor, clip one probe of the VOM to each motor wire, setting the scale to RX100.

If the meter reads between about 500 and 3,000 ohms, the motor is functioning properly. If the meter reads

higher than 3,000 ohms, the timer motor is faulty. Replace it with a new one of the same type. Connect the new

motor the same way the old one was connected.


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Servicing the Defrost Heater

This component is a heating element located on the evaporator coil. When the refrigerator or freezer switches

to the defrost cycle, the defrost heater is turned on to melt the frost in the compartment. Failure of the

defrost heater causes failure to defrost. Test the element with a VOM set to the RX1 scale. To gain access to

the heating element, remove the compartment's wall panels. Clip one probe of the VOM to each element

terminal. The meter should read between 5 and 20 ohms. If it doesn't, the heating element is faulty and should

be replaced. Replace the heater with a new one of the same type and electrical rating. Connect the new heater

the same way the old one was connected.

Servicing the Condenser Fan

The condenser fan is located under the unit. If the fan is malfunctioning, the refrigerator or freezer won't

cool properly, or it will run continuously or not at all. Test the fan with a VOM set to the RX1 scale. Disconnect

the electrical wires to the fan motor and clip one probe of the VOM to each fan motor terminal. If the meter

reads from 50 to 200 ohms, the motor is functioning properly. If the meter reads higher than 200 ohms, the

fan motor is faulty and you should replace it. While you're working on the fan motor, make sure the fan blades

are clean and unobstructed. If the blades are bent, carefully straighten them with pliers.

Clearing the Drain Ports

The drain ports are located along the bottom of both the freezer and the refrigerator sections of the unit.

These holes can become clogged with debris or ice, causing a drainage problem when the unit is defrosting. To

clear the ports, use a short section of wire that will fit the holes. Do not use a toothpick, because the wood

may break off in the port and become stuck. On some refrigerators, the drain ports are located near the

defrost heater at the evaporator coils. A lot of disassembly is required to clean this type of unit. If therefrigerator or freezer is this type, you may be better off calling a professional service person to clear the

ports. On some freezer compartments, the drain is located under the freezer compartment and shaped like a

shoehorn. This type of drain can usually be unscrewed so that the drain area can be cleaned.

Servicing the Drain Hose and Pan

The condenser fan is located under the bottom of the refrigerator. During the defrosting cycle, water may run

through a small hose into the drain pan and is naturally evaporated. On some refrigerators, the drain hose is

rubber instead of metal. This type of hose can become cracked, causing leaks. Examine the hose. If it's

damaged, replace it with a new one of the same type. If you spot water on the floor, the drain pan may be

tipped on its brackets, or the pan may be cracked or rusted. To eliminate the leak, realign or replace the pan.


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Servicing Ice Makers

Freezers with automatic ice makers sometimes malfunction because the water inlet valve strainer that feeds

water to the ice maker becomes clogged. To correct this problem, unplug the appliance and disconnect the

water supply. Remove the water line where it enters the valve--usually at the bottom edge of the unit. Locate

the wire strainer and remove it. Clean the strainer with a stiff brush, using mild household detergent.

Reassemble the component in reverse fashion.

Servicing a Refrigerant Leak

Coolant leaks are identifiable by their acrid smell. There is nothing you can do to repair a coolant leak except

call a professional service person to deal with the problem.

Servicing the Motor/Compressor

The compressor and motor of a refrigerator or freezer are contained in a sealed unit. If you trace problems to

either of these components, do not try to fix the unit yourself. Call a professional service person.

As you've seen, some common refrigerator problems (like faulty gaskets) can easily be repaired at home, while

others (like a motor or compressor) are beyond the scope of the average handyman. Now you know how to makethe small repairs, as well as when you've met your match.

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Technical Training Guidebook of Domestic Refrigeration

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