Whirlpool Freezer Model #EV150NXENOO

This one sounds like the defrost timer is not working and you are not getting a defrost cycle every 6-8 hours (or when needed). Another issue could be that air (moisture) is getting in from a leaky door seal, etc. and causing the freeze up. If you have a sealed system problem where the cooling coils are not uniformly frosted up then the defrost thermostat will not "know" there is an ice condition and therefore not go into defrost mode (same issue if the defrost t-stat is bad). After you have ALL the ice out of the unit it would be a good idea to take the interior rear cover off so you can view the coils. Start the freezer back up and watch over the next 1/2 hour or so to see that the entire coil is frosting up uniformly. If it isn't you may have a plugged system, a leak causing low refrigerant, etc. If over time the coils look uniformly frosted up you may have a defective defrost heating element itself (no heat, no defrost sort of thing). The defrost elements can be checked with a multimeter for continuity if you know how to do that sort of thing. It's hard to know a person's ability by these forums or if they can tackle certain test procedures.

Above suggestion is good. As add-on, you can observe the defrost timer to see if it is turning. Place a mark at the current point and then check it later to see if it has moved. Another thing is to look at the terminal leads to see if any of them are burnt (blackened).

To test the thermostatic switch (bi-metal switch), remove it and place it in a freezer or bag of ice (when this switch gets real cold, it closes and you will get continuity). Use a meter to test continuity. Keeping it connected to meter, put it under a faucet with running water (as the temperature rises, the switch will open). You should observe it go from continuity to no continuity.