True TDD-1 Compressor Run Times

[srletend]

I bought a 20 year old True TDD-1. I've read all I could find on this site and others and still have some questions about what I should expect for the compressor run times. Some facts/history:

• Door seals are good
• Evaporator pan had rust and associated pinhole leaks--fixed with a few coats of "Leak Stopper Rubber Flexx"
• Evaporator fan runs all the time like it should
• As part of trouble-shooting, I took out the manual temperature controller and replaced with an external controller (Inkbird ITC-308); it is still wired to have the evaporator fan run continuously (i.e., not connected to the temp controller)
• Compressor and fan cycle on and off based on temp controller settings
• Compressor gets very hot to the touch when running
• Condenser coils have been cleaned
• Unit was in the garage with ambient temp in the 90's and high humidity; for trouble-shooting I moved indoors with ambient temp around 78 F
• I am measuring the temperature in a 2 gallon bucket of water; external temp controller probe is in the water

Question: Compressor seems to run for a very long time. I have the external control set for 40 F with a 2 degree differential (i.e., compressor comes back on at 42F and stops at 40F). It will cool to 40F and then temp will slowly increase to 42F in 30-40 minutes. When the compressor comes back on, it runs for 3 or 4 hours before the temp drops back to 40F. I notice frosting/ice on the bottom section of the evaporator, but appears to mostly melt before the compressor cycles. Is it normal for the compressor to run this long? It runs even longer if I set the desired temp below 40F, in fact, I can't seem to get it below 39F without the compressor running continuously.

Thanks in advance for any help/suggestions!

[THE ICEMAN]

3 to 4 hours is definitely too long. I would say you have one of two things going on. Either the unit is slightly low on refrigerant or you have an inefficient compressor. With the unit running for a while, put your hand on the suction line near the compressor, should be the largest of the lines going into the compressor - 3/8". The line should be cool to cold. If it is not then it could be a low refrigerant situation. To check for a weak/inefficient compressor hold the suction line about an inch or so from the compressor & turn the unit off. If the line heats up slowly your valves are probably okay. If the line heats up quickly then you likely have bad/warped valves in the compressor. Whatever you find you will probably need to get a low temp technician to look at it - an A/C tech won't do.


THE ICEMAN

[srletend]

Thanks ICEMAN, this is exactly the kind of info I was looking for. I will check the suction line.

A bit of a deeper question (and sorry if it exposes my lack of knowledge about these things...). It doesn't look like there has been a break in the system (i.e., there are no valves added to the lines). How would it be low on refrigerant? Assuming a small leak would have completely emptied the system after 20 years--what things could cause it to be low on refrigerant now?

[THE ICEMAN]

If it turns out that the unit is low then you do have a leak. A leak can show up at any time due to vibration, food acids condensing on the evaporator, etc.


THE ICEMAN

[srletend]

Follow Up:

I ended up taking the unit to my local True service technician. He added a valve and charged with refrigerant. I've been running at 38F with 3F differential on the ITC-308 controller in the same bucket of water. Looks like things are working quite a bit better: during a cycle the compressor cuts in at 41F, temp still rises a bit and then drops back to 38F cut out in a little over an hour. No more frost/ice on lower portion of evaporator.