Problems with an older Fisher Isotemp plus 98 model, the defrost circuit is not working correctly. The temp is normally around -20 degrees celsius and the temp is now at -08. I have manually defrosted the circuit and it helped, the temp went back to -21 for a few days but is now back down to -08. I do not have any service documentation for this unit either.

Hi,

Best way I can answer this is to purchase the right equipment for the job. A domestic freezer is not designed to work like that, although it may get there in temp it won't hold it. Recovery is slower then the industrial unit you seek. Domestic unit warranties are void if used for commercial settings (i.e. lunch rooms, offices, lab's, etc...) I wish I had a better answer for you but if you need a specific temperature get the correct equipment for the job. Good Luck

I’m happy to help further over the phone at https://www.6ya.com/expert/gere_bf68e6055dd61249

http://www.searspartsdirect.com/partsdirect/part-number/Frigidaire-Parts/Freezer-Parts/297216600/0046/253/Model-LFFH2067DW7/1428/0142000?keySuffixId=NA&productTypeId=0142000&searchModelNumber=LFFH2067DW7&subCompDesc=System&brandDesc=FRIGIDAIRE&modelDesc=FREEZER&blt=11
Thermostat is what I would suspect first.

The defrost timer is the next suspect : http://www.searspartsdirect.com/partsdirect/part-number/Frigidaire-Parts/Freezer-Parts/297216600/0046/253/Model-LFFH2067DW7/1428/0142000?keySuffixId=NA&productTypeId=0142000&searchModelNumber=LFFH2067DW7&subCompDesc=System&brandDesc=FRIGIDAIRE&modelDesc=FREEZER&blt=11

I think the defrost heater is probably okay and it is just getting the wrong signals from the above components. But it is item 14 on this diagram: http://www.searspartsdirect.com/partsdirect/part-model/Frigidaire-Parts/Freezer-Parts/Model-LFFH2067DW7/1428/0142000/R0805140/00003

The first thing that you should do is plug the AC cord of the freezer and open the upper portion on top of the two doors and insert the key on the slot and turn it on. Open the freezer door and locate the temperature adjustment panel inside and adjust it to your desired temperature.

This is normal. All commercfial freezers must defrost 3 to 5 times a day, or they will freeze over at the coil and then stop freezing. A timer in your freezer turns off the compressor and turns a heating element on for 10 to 15 minutes. During that time the coil heats and melts frost from the coil. Usually within 30 minutes the temperature will briefly rise to 20 t0 25 degrees F and then quickly drop down to 0 degrees F. The food rarely defrosts in this short period of time..

Freezers should defrost as perscribe to prevent build up of ice.However it would not be long enough to cause listed problem.
If things are defrosting as stated, there is a problem other then the defrost

DO NOT TRY TO MODIFY TIMING CYCLE.you would need to completely modify printed circuit board and inbuilt computer......some freezers cycle around 6 to 8 hours.the temperature can only be checked say every 12 hours to get an average reading

On my upright freezer I use pans or trays I have and set them in the bottom where it catches nearly all the water. I don't have to empty them till it's all melted because it till hold all of it.

If this is a self defrosting freeze,then it is normal to periodically see an elevated reading during a defrost cycle. At 20 degrees, it was probably ready to complete its defrost cycle and come back on to return it to your cold setting.

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

If the temperature fluctuations occur (at least the ones to 20-30 degrees) at intervals of 6 hours or so then you are catching it in it's defrost mode which occurs on a timer. The timer shuts off everything except lights every 6 hours for an average of 20 minutes. This usually isn't enough to thaw things but I have seen it cause some crystale. Look closely at the back inside wall of the freezer for an accumulation of frost. If you see it there then you may have a defrost problem. Usually this is accompanied by steadily rising temperature in the freezer and fresh food secions both. The temps you listed (0to10) are fine. Good luck, AJ

turns out the coils were not defrosting or at best working intermittently. I defrosted the coils and ordered a defrost timer the old timer continued to pop from intermittently.I replaced the defrost timer
and it quit popping and noew the freezer works fine temps range between 9 degrees F and
14 degrees f.