Ongoing saga of the historic fridge

Dsisson · May 16

folks, historic fridge is not holding temp. I’ll do a small tweak to pressure (cut out) and get it to be 37ish (digital temp gauge) and a few days later it’ll migrate up to 40. Evaporator and valve are frozen. No condensation. Thermometer inside reads 36. Am I overreacting?

Calling the pros, but want to know what I might expect.

JUGHNE · May 16

The coil freeze up may be caused by air infiltration.

You have a lot of door gaskets that could be passing room air into the box. Also a lot of use with doors left open will compound the issue.

This looks like off cycle defrost, that is when compressor is off the coil should get warmer air to keep it clean.

But if compressor never cycles then defrost is lost.

EBEBRATT-Ed · May 16

I agree with @JUGHNE . Evaporator temp is usually about 10 degrees below the expected box temp. If you get the box to 36/37 the evap temp will be 26/27 which is below freezing and the coil will frost and ice up. This is normal.

But if you don't have electric defrost or hot gas defrost and are relying on time off defrost it will work but if you have excess humidity leaking in the coil will ice up. Check the door seals and other openings that can let humidity in. Keep the doors closed as much as possible. Normally the refrigeration runs a total of 16-18 hours out of 24 and defrosts during the off cycle.

Pressure controls can be finicky to adjust.

Dsisson · May 29

OK. I think I’m nearing success.

Looks like the icing was caused (as you predicted) by not enough off time.

I found a pressure temperature (PT) chart for R 409A, and used it as a starting point to dial in the pressures.

35 PSI cut in with a differential of 20 PSI resulted in icing, and the box being too warm.

40 PSI cut in with a differential of 25 PSI got rid of the ice, but was too cold in the box.

Both of these settings resulted in a low pressure (cut out) of 15 PSI.

I’m testing small increments on the cut in pressure, to see if I can get the box right. I don’t have proper testing tools, but I think I’m close anyway. I like that the on/off cycles seem to be good and long.

I’m not positive of the difference between coil temp and box temp, but maybe 15-20 degrees?

This is somewhat exciting. I wish I understood it better.

EBEBRATT-Ed · May 29

Every system id different and reacts differently. Tweaking the pressure controls takes time.

If you wanted to redo the controls, you would put a LL solenoid at the coil and control it with an electronic temp control which would operate the solenoid. This turns it into a pump down system and the pressure controls settings become less finicky. But if you have the time to dial it in that is fine.

ChrisJ · May 29

@Dsisson

I'd recommend posting on https://monitortop.freeforums.net/ regarding this.

We have a "Other Vintage Refrigerators" section and guys who specialize in such oddball equipment and I have no doubt you'll get unlimited help.

I'm hoping Erin doesn't take this as disrespectful. It's not intended as such in anyway. Our forum does antique refrigerators, not heating equipment or plumbing.