Subzero 700TC pressure problems

[tinkerduck]

Model Number

700TCI

Brand

Sub-Zero

Age

More than 10 years

Hi all, new member here.

I've got a circa 2001 700TC that's got the symptoms of a plugged cap tube, but it doesn't seem to have a plugged cap tube.

I got it off craigslist for $100, the previous owner didn't want to pay for more repairs. He said the fridge would be warm some days and cold others. He didn't say what other problems he'd had, but it's had the evap replaced and has signs of some ham-fisted "repairs". I'm trying to fix this for an elderly friend of the family as a replacement for her fried 1980's 211RFD. I think I might need an exorcist.

I brought it home and plugged it in. The freezer hit -27F, the fridge was in the 50's. I naively figured it was a thermostat - easy fix. I took the panels off the fridge and freezer and found that someone had removed the foam strip ducting that channels air up into the fridge and all of the foam strips that separate the fridge from the freezer. They'd even cleaned the adhesive residue off - no sign the stuff had ever been there. All the air just blows around in the freezer compartment. That's not relevant to my problem, however, it's just a sign that a worse hack than me had been in there. I also found that the condenser fan wouldn't always start, so I ordered a replacement. I think the compressor is original, but I'm not sure.

On to the problem. I put the ducting back to what's shown in the manual. Now the fridge and freezer can almost hit the target temps and the compressor (this is a single compressor unit) runs full time, too hot to touch for more than a couple seconds - over 125F. I get some frost near the inlet of the evap, none on the filter/drier or other lines. I checked the pressure - 20 in/hg low, 150+ psi high, target is about 0/80. Subzero doesn't supply just a cap tube/heat exchanger, it's only available with the evap. I lopped a couple inches off the cap tube, replaced the filter/drier, evacuated, and filled with 8oz by weight of (automotive, which is what I can get easily) 134a. Same pressures within 30 seconds of startup.

I unbrazed the heat exchanger and removed it. Both lines pass air, although it'd hard to gauge flow with the skinny cap tube, so I guess there could be a partial restriction. There are no restrictions in the condenser, evap, or door loop. I flushed the heat exchanger with lacquer thinner, blew it out, and brazed all the lines back together with the lines full of argon (which is what I have). This time I filled the system only until I got the target 80psi on the high side, which probably took close to the spec'd 8 oz. The low side has the same 20 in/hg and I can only get about 4F in the freezer with 36F in the fridge.

I'm assuming the compressor is still good since it can hit those pressures. Does that make sense?

This has to be a cap tube restriction, right? I'd love to replace just the heat exchanger since I'm doing this as a favor, but I can only find it with the evap which is a $300 part. There must be another Sub Zero that can donate the heat exchanger, but I can't find any mention of the lengths of the ones I can get. Does anyone know how I can get just a replacement heat exchanger? I'd even be happy with just the tubing used for the cap tube.

Also, this is a long shot, but if anyone has photos of the ducting under the panels in the back of the fridge and/or freezer then I'd love to see them. I think I got it right, but the detail in the service manual is limited so I can't be sure.

Thanks in advance for any help!

 

[rickgburton]

I'd like to start by saying, please don't take anything I tell you out of context. I'm not being a jerk and I'm not talking down to you by any means. I can tell by reading your post you have some knowledge of sealed system repairs, just enough to be dangerous...lol. But it's not enough my friend. If I'm understanding/following you correctly this is the original problem and where you made your first mistake.

The freezer hit -27F, the fridge was in the 50's.

This is not a sealed system issue. This is either a thermistor or air flow issue. The freezer thermistor controls the compressor but if the freezer temperature hits it's set point before the refrigerator set point, the compressor turns off but the evaporator fan continues to blow cold air through the air baffle. If the freezer got to -27°F, you didn't have a sealed system problem. Except now you do because you went down that road. So now you have at least two problems, the original problem and the sealed system problem. I can't teach sealed system repairs online. That's something you need to do on your own. I can however point out some mistakes you made. Here's the first one:

I unbrazed the heat exchanger and removed it.

Never unbraze the cap tube because any moisture will turn to vapor and restrict it again.

I flushed the heat exchanger with lacquer thinner,

I wouldn't recommend this to anyone and it's never recommended by manufacturers. Any residue left behind will react to R134a. This can cause the refrigerant to break down to it's chemical components then you get restrictions. Even though you blew it out the cap tube diameter is so small there's a good chance you didn't get it all out.

brazed all the lines back together with the lines full of argon

You can't get a good joint brazed with any gas in the system. The gas will just blow out the silver while you're trying to braze it. Maybe it's not a restriction but a refrigerant leak?? I don't know if argon reacts to R134a, do you?

This time I filled the system only until I got the target 80psi on the high side,

That would be the wrong way to charge the system. What you're doing is experimenting. Always charge by the correct amount listed on the model tag. If the low side pressure goes into a vacuum and the high side pressure increases, there's probably a restriction that can be caused by any number of things. Here's two that you probably didn't think about; When you unbrazed the cap tube from the evaporator, if you got your heat too close to the aluminum you can restrict the entrance to the evaporator. If that happens you need to replace the evaporator.

I'd love to replace just the heat exchanger

The heat exchanger is the suction line and the cap tube. The cap tube is married to the suction line with solder because solder offers the best heat transfer. Soldering the cap tube to the suction line can be extremely difficult and you definitely need good soldering skills. That's why the suction line and cap tube is sold as an assembly, heat exchanger. There's another easier way. Cut a small piece of cap tube off and take it down to your local refrigeration parts supplier and match it to the correct size. Cut the proper size suction line to the correct length. Drill a small hole, the same size as the cap tube, about 6" from the bottom and 6" from the top of the suction line then feed the cap tube through the bottom hole and out the top hole, then braze the two holes. Now you have a heat exchanger.

 

[tinkerduck]

I'd like to start by saying, please don't take anything I tell you out of context. I'm not being a jerk and I'm not talking down to you by any means. I can tell by reading your post you have some knowledge of sealed system repairs, just enough to be dangerous...lol.

No offense taken, you hit the nail right on the head.

This is not a sealed system issue. This is either a thermistor or air flow issue.

Correct, the primary problem was an air flow issue. Like I said, someone had tried to "fix" the fridge the way one fixes a dog, with parts left over at the end. As I described, the air flow channels between the freezer and fridge had been removed. There were also stripped screws, bent evap flange, partially crushed return air duct, etc, so I don't know what other damage could be hidden. The thermistors check out ok.

The freezer thermistor controls the compressor but if the freezer temperature hits it's set point before the refrigerator set point, the compressor turns off but the evaporator fan continues to blow cold air through the air baffle. If the freezer got to -27°F, you didn't have a sealed system problem.

I'm pretty sure I fixed that the first problem (air flow), since both compartments now get cold, but not as cold as they should and the compressor still ran full time & hot. I didn't open the sealed system just for fun, I did it because the system wasn't operating as expected.

Never unbraze the cap tube because any moisture will turn to vapor and restrict it again.

Yeah, that was a calculated risk. The filter/drier had minimal run time on it and I didn't have another one. I did a pretty long evacuation before refilling, I don't think the cap tube is icing since the system is getting cold, but that's one of the reasons I asked people who know more than me.

wouldn't recommend this to anyone and it's never recommended by manufacturers. Any residue left behind will react to R134a. This can cause the refrigerant to break down to it's chemical components then you get restrictions. Even though you blew it out the cap tube diameter is so small there's a good chance you didn't get it all out.

Lacquer thinner doesn't leave residue, it evaporates pretty fast. I spent enough time running a vacuum and compressed air through it that I feel confident that there was no liquid left.
And, I understand it's not recommended by the manufacturer. The manufacturer recommends discarding everything that could have oil in it and replace them with new components, which I'm trying to avoid due to the cost.

You can't get a good joint brazed with any gas in the system. The gas will just blow out the silver while you're trying to braze it. Maybe it's not a restriction but a refrigerant leak?? I don't know if argon reacts to R134a, do you?

I don't know how to braze the joints without some kind of gas in the system, there's at least going to be air in there. I did a leak check after assembly and the system is holding pressure, so no signs of a leak.
And yes, I know for a fact that argon cannot react with R134A. Argon is an inert gas, it doesn't react with anything.

That would be the wrong way to charge the system. What you're doing is experimenting. Always charge by the correct amount listed on the model tag. If the low side pressure goes into a vacuum and the high side pressure increases, there's probably a restriction that can be caused by any number of things.

Yes, what I was doing is experimenting. You said there's "probably" a restriction. That's why I posted here - a restriction seems obvious, but I can't find one. I'm wondering the other possibilities are.

Here's two that you probably didn't think about; When you unbrazed the cap tube from the evaporator, if you got your heat too close to the aluminum you can restrict the entrance to the evaporator. If that happens you need to replace the evaporator.

There's no sign of the inlet to the evaporator having melted. But you said two things I didn't think about?

The heat exchanger is the suction line and the cap tube. The cap tube is married to the suction line with solder because solder offers the best heat transfer. Soldering the cap tube to the suction line can be extremely difficult and you definitely need good soldering skills. That's why the suction line and cap tube is sold as an assembly, heat exchanger. There's another easier way. Cut a small piece of cap tube off and take it down to your local refrigeration parts supplier and match it to the correct size. Cut the proper size suction line to the correct length. Drill a small hole, the same size as the cap tube, about 6" from the bottom and 6" from the top of the suction line then feed the cap tube through the bottom hole and out the top hole, then braze the two holes. Now you have a heat exchanger.

I'll try a refrigeration supplier and see if I can score the same gauge cap tube (or a replacement heat exchanger). That seems like the most likely candidate is a cheap and easy thing to try.

Thanks for the help!

 

[rickgburton]

But you said two things I didn't think about?

R134a cannot tolerate any moisture, any trace amounts of other chlorine based refrigerants or other chemical substances and cannot tolerate high temperatures for any length of time. The R134A refrigeration systems
utilize a synthetic ester oil in the compressor. This oil is capable of absorbing large amounts of moisture. When these systems are operated with excessive moisture contamination, a substance, sludge, is created and deposited inside the system tubing including the capillary tube. These deposits can partially or completely restrict the flow of refrigerant.

I don't think the cap tube is icing since the system is getting cold,

The cap tube separates the high and low pressures. The evaporator needs to be at a low pressure so when the refrigerant enters the evaporator it boils off and immediately gets cold. A small amount of moisture, and when I say small, I'm talking molecules not drops, will freeze as it enters the evaporator and can, and does, partially restrict the captube.

There's no sign of the inlet to the evaporator having melted.

There wouldn't be. How is the copper connected to the aluminum?

I know for a fact that argon cannot react with R134A. Argon is an inert gas, it doesn't react with anything.

I don't know that much about argon gas, mostly because I don't need to. Adding argon gas before you braze is an extra unnecessary step that just takes up time. No one else uses it before they braze anything. I don't use anything. I sometimes turn the vacuum pump on while I'm brazing to keep any left over refrigerant from blowing out the solder/silver

 

[tinkerduck]

R134a cannot tolerate any moisture, any trace amounts of other chlorine based refrigerants or other chemical substances and cannot tolerate high temperatures for any length of time. The R134A refrigeration systems utilize a synthetic ester oil in the compressor. This oil is capable of absorbing large amounts of moisture. When these systems are operated with excessive moisture contamination, a substance, sludge, is created and deposited inside the system tubing including the capillary tube. These deposits can partially or completely restrict the flow of refrigerant.

Moisture is one of my concerns, especially since I used automotive refrigerant. I've done evacuations of over an hour, hopefully that's enough to get any moisture left in there.
The solvent I ran through the plumbing came out pretty clean, but I can't see what's in the motor.

There wouldn't be. How is the copper connected to the aluminum?

It looks swaged.

I don't know that much about argon gas, mostly because I don't need to. Adding argon gas before you braze is an extra unnecessary step that just takes up time. No one else uses it before they braze anything. I don't use anything. I sometimes turn the vacuum pump on while I'm brazing to keep any left over refrigerant from blowing out the solder/silver

I've read about some techs purging with nitrogen in some cases. I don't know if r143a or the ester oil cares, but I knew it wouldn't hurt and it was quicker to do it than to look up whether or not it'd help, so I did it. This is a technique used when TIG welding hard vacuum scientific equipment where there's no filter to catch any copper oxide scale that forms on the inside of the tubes.

I went to a local HVAC shop looking for a cap tube or heat exchanger. I thought I was being conscientious by supporting a local business but they won't even sell me solder without a contractor's license. They suggested I shop online. Grainger had 12 feet of 0.031 x 0.082 cap tube on my doorstep 22 hours after I ordered it without any extra shipping charge. That's tomorrow's project.

I'm getting sick of taking this bloody fridge apart.

 

[rickgburton]

I've done evacuations of over an hour

That's what I would have done

techs purging with nitrogen

I use it all the time to blow out the lines and check for leaks in the shop. In the field I use R134a.

Moisture is one of my concerns, especially since I used automotive refrigerant.

PAG oil really sucks up moisture. I know you don't want to put any more money into that repair. When I run into any moisture issues on sealed systems I just replace the drier with a Sporlan C-136.

That's tomorrow's project ........ I'm getting sick of taking this bloody fridge apart.

I hear you brother. Been there, done that. Maybe I can help you out and save you a few aspirin. Before you do tomorrow's project, do a system purge and a sweep charge. You're going to need a service valve on the process tube and on the inlet of the filter/drier. You'll also need a rubber mallet and a heat gun (torch will work if you know what you're doing).

Turn compressor “ON” and purge for 8 minutes through the access valve on the filter into your recovery equipment/tank (yeah, right...Lol).

Heat compressor during entire purge

Turn compressor “OFF” and sharply wrap with mallet several times then purge for another minute. Turn compressor “OFF”.

Warm refrigerant tank or charging cylinder and add about 4-5 ounces R134a into the access valve on the drier and leak check with a bubble solution.

Run the compressor for 2-3 minutes to sweep the system.

Important: Check the evaporator for a frost pattern then purge the system for 5 minutes. Leave the compressor running and charge the system with factory charge. (process tube)

Check the condensor for heat.

 

[tinkerduck]

PAG oil really sucks up moisture. I know you don't want to put any more money into that repair. When I run into any moisture issues on sealed systems I just replace the drier with a Sporlan C-136.

I hear you brother. Been there, done that. Maybe I can help you out and save you a few aspirin. Before you do tomorrow's project, do a system purge and a sweep charge. You're going to need a service valve on the process tube and on the inlet of the filter/drier. You'll also need a rubber mallet and a heat gun (torch will work if you know what you're doing).

Turn compressor “ON” and purge for 8 minutes through the access valve on the filter into your recovery equipment/tank (yeah, right...Lol).

Heat compressor during entire purge

Turn compressor “OFF” and sharply wrap with mallet several times then purge for another minute. Turn compressor “OFF”.

Warm refrigerant tank or charging cylinder and add about 4-5 ounces R134a into the access valve on the drier and leak check with a bubble solution.

Run the compressor for 2-3 minutes to sweep the system.

Important: Check the evaporator for a frost pattern then purge the system for 5 minutes. Leave the compressor running and charge the system with factory charge. (process tube)

Check the condensor for heat.

I followed these instructions to the letter and got pretty much the same results, except that the high side pressure stayed around where it should be (~80psi) at startup instead of getting up to 150psi within 30 seconds of the compressor starting, so that's some improvement. The low side is still too low, however - around 20 in/hg. The manual says -2 to +2, it doesn't give units but I assume that's PSI.

There was no ice on the evap end of the cap tube after a few minutes of running, so I buttoned it back up and let it sit overnight. After about 12 hours the fridge had hit its 36 degree setpoint, the freezer was at 17 (set for 0), and the compressor housing was at 150. I didn't leave the gauges hooked up overnight so I don't know if it's back up to 150psi on the high side. The top of the evaporator is frosted up too.

I'm not sure if the fridge will do a defrost cycle if it hasn't hit its setpoints yet, there shouldn't have been any more moisture than what was left in the air when I closed the door and it wasn't especially warm or damp yesterday.

It's difficult (or maybe impossible) for me to remove the gauge hose from the high side (drier) without spraying some liquid, is that expected with all connectors or do I need a new seal?

If there is moisture in the sealed system then how long will it take for the evap end of the cap tube to ice up? I would think it'd happen pretty fast, basically as soon as the compressor built up enough pressure to make cold, so if I see the pressure rise from 0 to 150 psi over 30 seconds or a minute then could that be the cap tube icing up pretty much as soon as the system hits operating pressure?

 

[rickgburton]

Looks like a possible restriction in the cap tube but it's too hard to tell from the picture and the frost.

There was no ice on the evap end of the cap tube after a few minutes of running,

There shouldn't be any frost on the cap tube, only the evaporator.

.....for me to remove the gauge hose from the high side (drier) without spraying some liquid,

You shouldn't get any more than a "psssst" when you remove the hose as long as the schrader valve core and pin are OK. Did you remove the valve core when you brazed the drier? If not the seal is probably melted.

 

[tinkerduck]

Looks like a possible restriction in the cap tube but it's too hard to tell from the picture and the frost.
View attachment 50492

There shouldn't be any frost on the cap tube, only the evaporator.

You shouldn't get any more than a "psssst" when you remove the hose as long as the schrader valve core and pin are OK. Did you remove the valve core when you brazed the drier? If not the seal is probably melted.
View attachment 50493

I know there shouldn't be frost on the end of the cap tube, but I've read that that's a sign of freezing due to water in the system. Is that what my photo looks like to you?

I did take the schrader core out when I brazed the drier. It feels like the hose presses the schrader release before the seal is made to keep the gas from escaping from the hose fitting. There's no brand name on my manifold setup, it looks a lot like the Snap On one, so most likely it's an Asian copy of the Snap On (my plumber gave it to me a bunch of years ago when he upgraded his setup, he probably upgraded for a reason).

Diagnosing a sealed system over the internet may seem hard, but just after the lockdown started I saw a knee doctor via Zoom. I must have looked like a cancan dancer, balanced on one leg, holding the other up, flexing it, and showing her when and where it hurt.

And thanks, I appreciate the help.

 

[tinkerduck]

And, to go back to the first problem that I found with this fridge - someone had removed some of the ducting.

This is the rear panel and ceiling of the freezer compartment. The slot at the top of the back carries cold air up from the evaporator sump. There is a six channel passage that goes up to the fridge - four of them visible in this slot, plus one on the left of the slot that carries cold air up to the fridge, and one on the right that carries air back to the sump.

It seems strange that there are six channels that go up to the fridge, since the four visible here are supposed to be blocked off with a foam block at the bottom of the fridge compartment. I say "supposed to be" because someone had removed the foam in this fridge, so the air kind of went where it wanted to, and it mostly stayed in the freezer. Remember, I bought this fridge off CraigsList for $100 since the previous owner was too frustrated trying to get it to work.

My guess is that whoever was trying to fix the problem that I'm now facing had started randomly messing with parts without understanding what those parts were doing. I have no idea if it was an incompetent A/C tech or a homeowner with a screw gun and a drinking problem, but I'm thinking the latter. The insulation had been removed from the heat exchanger, the evap flanges had been bent, screws had been stripped, the ducting had been messed with, etc.

It's the "etc" that I'm worried about - I'm wondering if there's some other subtle damage hiding in there that could be causing me trouble, but I've got no idea what that could be or where else to look.

 

[rickgburton]

I've read that that's a sign of freezing due to water in the system.

It's a sign of a restriction, usually caused by moisture but it sounds like you got it all out. A wet system has a distinct odor if you ever smelled it. Too bad computers don't have smell-a-vision.....LOL

It feels like the hose presses the schrader release before the seal is made to keep the gas from escaping from the hose fitting.

You need a pack of these. Autozone has them:

The freezer zone has two vertical duct dividers behind the lower rear duct which separate the air to the refrigerator from air to the freezer. Air to the refrigerator zone is directed through a baffle in the left side of evaporator sump area, then ducted up the left side to the refrigerator compartment. Air travels up behind the back duct in the refrigerator compartment and is forced out at the top. Air then returns through an opening at the bottom right hand corner of the top duct, back down behind the right side of the lower air duct, to the evaporator sump area.
Air to the lower freezer zone travels up behind the center of the lower air duct, between the two vertical duct dividers. Air is then forced out at the top of the freezer compartment and returns to the evaporator sump area through the vents in the evaporator cover.