Problem with rlb20 model is that it keeps freezing up

At 21°C u must have an air flow blockage. It should not freeze up there.

Most fridges have a cooling vent above or on the side of the top shelf. This is where most of the cold air comes from, so keeping food away from here may preventfreezing. ... Some side-by-side refrigerators (particularly older models) feature a vent that funnels cold air from the freezer into the fridge.4 Aug 2015

How to Prevent Your Refrigerator From Accidentally Freezing Food ...

refrigerators.reviewed.com/features/how-to-prevent-accidental-food-freezing-in-fridge...or try this link... refrigerator keeps freezing things Google Search

low air flow or low on refrigerant.

your freeze must be branden company.

What is the model number of your unit?

You will need to clean the condensor coil. It is usually under the refrigerator on newer models and in the rear on older models. All you need to do is use a vacuum to clean it. Also try to leave some room behind the refrigerator, do not keep it too close to the wall. It needs to circulate the warm air it extracts.

Brand and model of set not indicated here?

Yes....not a big deal. These things happen regardless of make or model...parts are not expensive...what all do you need to replace besides the heater core?

The only way that a discharge line can freeze is if there is a restriction in it.
Are you sure it's the discharge and not the suction line?

Hi, Could you get me a model # so I can look this up and give you instruction. Some models require an after market drain heater.

Vic