I have an older commercial upright Hobart freezer. The freezer seems to operate fine, cycling but always returning the compartment to zero degrees, until all of a sudden the temperature went up, well above 40 degrees and thawing everything inside. The fan is working and the fins are clean, but the evaporator was a solid block of ice/frost.

Defrost cycle usually comes on after about 7 hours. If it is happening every 30 minutes or so, the defrost timer is not working properly and your internal temperature will never have a chance to go down to zero as SOP.

When defrost cycle comes on, the compressor is automatically shut off and internal freezer temps can go up by close to 25 degrees or more. It is for this reason that commercial freezers do not have auto defrost - the latter extols a price, convenience at the price of higher average temps and less time to properly store foodstuffs in freezer.

it may be froze up, and require a defrosting. use a portable fan to speed up this process , also make sure the fan in the freezer compartment is working, with a little work and a zoom spout oiler, or some light motor oil, the motor can be sparked back to life.. freeze ups are common with the high humidity we are having these days.. jay the pool pump motor repair guy longwood Florida

https://www.repairclinic.com/RepairHelp/Refrigerator-Repair-Help

I’m happy to assist further over the phone at https://www.6ya.com/expert/david_dc2f9e0a44104720

sounds like you have an air flow problem maybe a fan out or blocked up return grille

dear sir
please make sure first your freezer is well clean if it is already clean then contect to maintenance personell for refilling the gas.
hope answer the question.
thank you.

Zero degrees F is the normal operating temperature for a freezer. The compressor should not come on until the time is above that.

Over what period of time did this variation occur?

The freezer should have an automatic defrost cycle. The temperature will rise while it is going through this cycle.

chest freezer operation is among the simplest available. if the compressor is running and its not cooling there are only 2 possibilities. low or no refrigerant and or a bad compressor. so if you have the ability to return the unit I would. could luck peyton

Check out "victory-refrig.com" for manuals, diagrams, etc.
Cap tube systems are "critical charge systems". Can be difficult to get right. You did not mention pressures that you had. The split on the evap vs.box temp is almost always 10 degrees for freezers. If you want a -10 box, your evap needs to be -20. As a general rule of thumb, you want the high side to be "ambient +30". This seems to work most every time. Your problem might be operations related. Them leaving the door open alot and so on. It could also be a problem with the evap fan or the control to it. It could also be that since there had been a problem with the unit, they are now constantly checking it. Could be checking during a defrost cycle. When your box is at 0 degrees, your pressure should be about a -10 and no less that a -15 degree for the evap. You may have to babysit this thing and see how far it will pull down. You may have a weak compressor. Also, I have had many Hobart freezers fail due to dirty condensers. This causes the oil in cap tube systems to gel and clog up the cap tubes. The only fix is replacing the cap tube. There are alot of variables here so be patient, it's not going to be a quick fix. One last thing, since R404A can fractionate, you really need to pull the charge out and weigh in the correct charge with virgin refrigerant. This will eliminate 2 things. 1, it will ensure the correct amount of refrigerant and, 2 no chance of having a poor mixture of refrigerant in the system causing inconsistant readings and performance.
Hope this helps. And remember, cap tube systems are performance measured by the amount of superheat just prior to the unit making temp and cutting off.

Hi dgh1,
The freezer should stay within 5 degrees of 0 while in operation. While in defrost the freezer may reach as high as 20 to 25 degrees. If any higher temperatures are reached your food will spoil as they are defrosted and refroze everyday. If you find that the temperature is allowing your food to defrost you may want to replace your defrost thermostat. This device is located on top of the evaperator coil inside the back of the freezer. It is made to turn the heater off when a temperature of 55 degrees is reached. One other possible problem could be your defrost timer is taking to long to restart the cooling cycle after defrosting. I hope this will help you and please send any other questions you may have, I am happy to help. and dont forget to rate me.
Mike Anderson
Anderson Appliance
(865)945-3808