Why do they ice up?

[Abe]

Most new domestic fridges are now the "frost free type"
A common fault with them is a frozen evaporator coil in the freezer section
Why does this happen
Assume the evap fan and heaters are fine
I say its the door left ajar

[BritCit_Juve]

Most have a minimum room temperature at which they are frost free. If the room is below that condition then the unit over condenses. Also if the room is humid and the door left open for longish periods then you'll get a frost up.

[frank]

Can you explain the term "over condenses" ?- I always thought that it was increased sub-cooling

[BritCit_Juve]

same thing I suppose. You have a condenser on the bach of the unit which is sized for certain conditions. When the condenser is in an area with a temperature above the design minimum then the evaporating temperature will be relatively high and the design of the unit will limit frosting. If the condensers environmenyt is too low then you will over condense, or get excessive sub cooling if you prefer. This will result in lower evaporating temperatures and usually excessive numbers of starts per hour. Fridge and freezer manufacturers usually have the 'Minimum room temperature' detailed in the installation pamphelet and it is usually arounf 14 to 16°C In larger systems hot gas bypas or fan speed regulating head pressure control would be used to educe the effects.

[fillrob00]

hi ,i have found most people do not level the unit causing a distortian. Because of this the doors do not seal properly

[Frosty]

Frank

Over-condensing is a term used when the saturated liquid temperature leaving the condenser is to low (not to be confused with non-condensables in the system!) Manufacturers/designers will size the expansion device...whether it be TEV, capillary or other, for a given pressure ratio across the device. This ensures the correct amount of liquid can reach the evaporator under any load.

If the saturated liquid pressure falls drastically on a DX air-cooled condensing unit (sudden arctic winds and no fan control) the ratio across the valve decreases, this effectively down-sizes the expansion device, culminating in decreased suction pressure and an increase in suction superheat. The liquid that would normally be fed to the evaporator will back-up into the condenser (capillary) or HP receiver (TEV)

Hope this helps?

Frosty

[BRAD]

greetings,
There are several factors that will cause the evap not to defrost completely. Aiyub was correct in noting that improper door seal is a major contributor, this will allow additional moisture to infiltrate the cabinet which the defrost system was not designed to handle. I believe, that if the door seal is OK, that the defrost timer or the defrost thermostat is most likely the problem. The contacts inside the timer often stick or become worn so that it does not function as it should. Ambient conditions play a major role how a refrigerator defrosts in congunction with the timer. Most US refrigerators defrost either after 8, 10 or 12 hours of compressor runtime. A 12 hour timer, which will save you energy due to fewer defrosts may not be sufficient in clearing the coil in areas of the country where it is hot and humid most of the year (i.e. Florida). A common fix for this situation would be to install a timer with a shorter defrost interval or rewire the unit to defrost according to elapsed time not compressor runtime.
The defrost thermostat often tends to drift out of calibration which could also cause ice formation on the coil. Also those devices have a very large set-point tolerance (+/_ 3 degress F) which could compound the calibration drift.
I could go into more detail but I'd check to make sure the timer is right for the given ambient conditions, and if so, that it's working properly, then check the thermostat. If those are OK there are other things to take a look at (i.e. evaporator cover, fan placement etc.) hope that helps.

BTB

[johnfarrow]

Hello,
I have a FF refrigarator that the coils freeze up.

I understand that the cause may be ..........

(1) The defrost timer defective

(2) The defrost heater defective

(3) the thermostat defective

Today when I defrosted, I took the cover off and removed the thermostat. I checked it with meter and its open. I jumpered accross the defrost thermostat and rotated the timer motor and the heater came on as it should. My question is.........What is the purpose of the Thermostat?

I mean the timer motor turns on the circuit to the defrost heater and turns it off. Is the thermostat a safety device in case the timer motor sticks in the "ON" position?
John

[BRAD]

Hello again,
John, to answer your questions:
The defrost thermostat is definitely a safety device. It is placed in series with the defrost heater and under normal operating conditions the thermostat is closed. In the defrost mode the compressor will shut off and heater will turn on. The thermostat will open the circuit and the heater will shut off when the t-stat reaches a given temperature. After the heater shuts down the compressor normally remains off for several more minutes to allow the coil to drip free of residual moisture and prevent water droplets from forming ice. It's common for the heater to stay on for 10 to 15 minutes while the total defrost cycle as determined by the timer would be somewhere between 20 and 30 minutes. When the t-stat is by-passed the heater will remain on for the full defrost cycle which is NOT GOOD. You run the risk of melting the plastic freezer liner and other styrofoam parts in that area, the potential for a fire is quite high. I would suggest the following: first make sure that your thermostat is making good contact with the evaporator tubing. If that's OK and you have a good door seal it almost has to be a defective timer (1st) or thermostat(2nd) if in fact they're the right ones.

BTB

[johnfarrow]

Brad,
Thanks for a most informative enlighting on the Defrost System. You made every thing perfectly clear to understand.

In my situation, after I manually defrosted the unit, I checked the heater coil circuit and found it open. Was a bad connection in a wire nut. Placed the T-stat back in circuit after your reply.

Thanks very much for taking the time to help.

John Farrow
Cloudcroft, New Mexico