Carrier Evaporator Cold & Warm Side

[Kabra]

I'm trying to diagnose a longstanding problem of the A/C running forever at night on hot days to cool the room the thermostat is in. Airflow is good but temperature out of the registers are in the upper 50s. Referring to the photo above the rear of the evaporator one of the things I noticed is the side the refrigerant enters runs in the low 60s and the opposite side the mid 40s. This is a 1.5 ton Carrier N-Type model CNPVP1814ALA. Is this temperature difference unusual?

I've looked at pictures of the inside of the unit and I see the way the "N" shaped coil sits in the the case that the left side may not cool as much as the other side. Pressure drop through the evaporator is .1 inches of water so it doesn't seem there's a restriction. Wondering if perhaps there could be a restriction in the refrigerant flow or some other reason for the difference.

Appreciate any thoughts on this or further diagnosis I might do.

 

[Fitter30]

Are those surface probes? Discharge temp doesn't mean anything by itself. Need the difference between return air and discharge air in the air steam for both. Discharge needs to be taken 18-25 " above coil 18° to 23° difference. Condenser is the coil clean? The coil could be double row where the inner coil catches a lot of the dirt. Is the larger insulated line ( suction) cold? Have a digital meat thermometer with a probe 1/8" round x 5"? Could use it for air temps.

 

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[John Gayewski]

Do you have a txv?

 

[Kabra]

Do you have a txv?

TXV is factory installed inside of the unit per documentation.

 

[Kabra]

Are those surface probes? Discharge temp doesn't mean anything by itself. Need the difference between return air and discharge air in the air steam for both. Discharge needs to be taken 18-25 " above coil 18° to 23° difference. Condenser is the coil clean? The coil could be double row where the inner coil catches a lot of the dirt. Is the larger insulated line ( suction) cold? Have a digital meat thermometer with a probe 1/8" round x 5"? Could use it for air temps.

First to answer your questions. Air intake is about 70*. Those are 8" probes but they're located about 9" above the top of the evaporator. The coil was cleaned a couple years ago by a pro so I assume it's pretty clean and the pressure drop across it is only .1 inches of water. I took a quick measure of the cold refrigerant line and it was in the upper 40s. That wasn't a surface probe though so may actually be colder. I think I have a surface probe if necessary.

After reading your comments I made a probe hole in the horizontal center of the plenum about 20" above the evaporator. I also used a simple k-type wire thermocouple that I taped to a narrow rod to insert and take measurements every inch across the plenum (right to left). It was mid 40s for the first couple inches and then went up from there to about 58 in the center and then back down a couple degrees as it got neat the other side.

Incidentally, I always measured evaporator output temp from the cold side not realizing what I see not that it's probably a cold spot. Because of that I'm using the highest speed tap so that of course explains to some degree the low temperature delta before and after the evaporator.

Last but maybe not least, something I observed when I went to take those corrected measurements after reading your comments. Today was much cooler than it's been the past week so that A/C (there are two separate systems in my house) had been idle all day. I noticed those same probes I sent the picture of last night were both registering 48* but it didn't last long after I noticed it and the warm side got warmer and the cold side stayed cold. I'm going to try that again later after the system rests for a while and see if that's repeatable or maybe just some kind of fluke. I'll report back on that if it's repeatable.

 

[Fitter30]

TXV is factory installed inside of the unit per documentation.

There are two reading for a txv to work properly. Superheat and subcooling. The txv bulb needs to be insulated and tight to the line and some times with the piping might have to be moved. Used to use heat transfer compound for optimal temp sensitivity. Superheat 10° subcooling 10° 12°.
Without taking all the reading with gage pressures line temps can't make a imform answer. Evap coils can read different temps. Air flow even with a matching furnace and coil cabinet isn't a perfect match. Even if you took a monometer and transverse flow across duct it wouldn't be the same it would be a average.

 

[John Gayewski]

You need to call a real hvac tech. They'll have to come take some measurements in your refrigeration system. If your airflow is good and the fan isn't super dirty. Then there's likely a problem in your refrigeration system.

 

[Fitter30]

Think your short on refrigerate by the different temps of the leaving air.

 

[Kabra]

You need to call a real hvac tech. They'll have to come take some measurements in your refrigeration system. If your airflow is good and the fan isn't super dirty. Then there's likely a problem in your refrigeration system.

Exactly and in fact I'm on Tech #4. I mentioned in the original post this has been going on a long time. These days it's so hard to find tech's that do real diagnosis. I know there are good diagnosticians out there but they don't seem to want to spend time on this kind of stuff and just want to sell systems. The first guy said I needed a pound of refrigerant and added that which changed nothing, and a guy that came out on Thursday said it's got a pound too much. The second guy concluded without putting any huge on anything that the evaporator was too large. Another guy wanted to put a bigger return in the room that has the thermostat before he did any system testing whatsoever. It's this kind of stuff that causes me to get involved and do my own testing to the degree I can. But this is definitely over my head. Both you and Fitter30 brought up the TXV and while I understand what it does I'm clueless to do anything about it. The tech I'm working with now has concluded "it's a complicated problem" and is thinking about what to do next. Knock on wood...

I really do appreciate the help and it was useful taking me to the point where as you said it's likely a refrigeration problem. So I'll keep plugging away in finding someone that is willing and able to fix it if the Tech #4 I'm working with now can't.

 

[Fitter30]

Exactly and in fact I'm on Tech #4. I mentioned in the original post this has been going on a long time. These days it's so hard to find tech's that do real diagnosis. I know there are good diagnosticians out there but they don't seem to want to spend time on this kind of stuff and just want to sell systems. The first guy said I needed a pound of refrigerant and added that which changed nothing, and a guy that came out on Thursday said it's got a pound too much. The second guy concluded without putting any huge on anything that the evaporator was too large. Another guy wanted to put a bigger return in the room that has the thermostat before he did any system testing whatsoever. It's this kind of stuff that causes me to get involved and do my own testing to the degree I can. But this is definitely over my head. Both you and Fitter30 brought up the TXV and while I understand what it does I'm clueless to do anything about it. The tech I'm working with now has concluded "it's a complicated problem" and is thinking about what to do next. Knock on wood...

I really do appreciate the help and it was useful taking me to the point where as you said it's likely a refrigeration problem. So I'll keep plugging away in finding someone that is willing and able to fix it if the Tech #4 I'm working with now can't.

Evap coils have more than one feeder tube coming off the the ex valve distributor. With the front access to coil off and blower disconnected the unit calling for cooling between 5 & 10 minutes the feeder tubes should frost up. A tube that doesn't frost up or not as much is either stopped up or has a restriction. Don't run the unit longer than 15 minutes with blower off. Other thing to check is the ex valve. Thin tube comes off the big end follow it to the line. Should clamped onto the line firmly not with plastic wire ties. If there is a build up of corrosion bulb needs to be removed line and bulb need to to cleaned with sand paper or steel wool. Stainless hose clamp work well for a bulb clamp and foam tape to insulate the bulb.

 

[John Gayewski]

Find out which UA local union's territory you are in. If you go to their website or even call them and tell them you have an ongoing HVAC problem, their website should have a list of contractors or they should have a contractor recommendation for you.

Some of these issues can be tough, but I would be willing to bet the UA has a local contractor that has a guy that can figure it out. The UA schooling is very good and there are likely a collection of really good techs hiding in your area. Calling random ones is tough cause there are a lot of people who recommend some guy who changed out a part for them ,but that doesn't mean the guy knows shit. A lot of guys go 25 years without really knowing shit and just change out parts until they get it.

Txv being stuck is a common problem, but you need someone to measure some things. Usually low refrigeration freezes the coil high refrigeration causes low cooling. But there are too many variables without measuring.

 

[Kabra]

Evap coils have more than one feeder tube coming off the the ex valve distributor. With the front access to coil off and blower disconnected the unit calling for cooling between 5 & 10 minutes the feeder tubes should frost up. A tube that doesn't frost up or not as much is either stopped up or has a restriction. Don't run the unit longer than 15 minutes with blower off. Other thing to check is the ex valve. Thin tube comes off the big end follow it to the line. Should clamped onto the line firmly not with plastic wire ties. If there is a build up of corrosion bulb needs to be removed line and bulb need to to cleaned with sand paper or steel wool. Stainless hose clamp work well for a bulb clamp and foam tape to insulate the bulb.

I got the balls to take you up on your suggestion. The photos below tell the story. I cleaned up the txv bulb but it was already quite clean and I just wiped the oxidation dust off the bulb and line before clamping it down again. As you can see the tubes frosted up nicely and I even checked the temps with a good instant read thermometer and the were all down in the teens and dropping. The coils look very clean and I was able to stick my phone in the one on the left and take a photo and it looked good.

So everything I can see looks good. Is there anything else you can think that I can test on the txv without special equipment before I button it back up?

Also that foam tape is pretty jacked up because it's adhesive back. It's mostly in one piece but the adhesive is useless. Is that something I can pick up at Home Depot or do I need to go to an HVAC supply store?

 

[Kabra]

Find out which UA local union's territory you are in. If you go to their website or even call them and tell them you have an ongoing HVAC problem, their website should have a list of contractors or they should have a contractor recommendation for you.

Some of these issues can be tough, but I would be willing to bet the UA has a local contractor that has a guy that can figure it out. The UA schooling is very good and there are likely a collection of really good techs hiding in your area. Calling random ones is tough cause there are a lot of people who recommend some guy who changed out a part for them ,but that doesn't mean the guy knows shit. A lot of guys go 25 years without really knowing shit and just change out parts until they get it.

Txv being stuck is a common problem, but you need someone to measure some things. Usually low refrigeration freezes the coil high refrigeration causes low cooling. But there are too many variables without measuring.

Thanks John. I found the UA local and will get in touch with them shortly. That's a great idea!

 

[Kabra]

Thanks John. I found the UA local and will get in touch with them shortly. That's a great idea!

No doubt you guys will notice the rust in the bottom of the pictures. I wondered about that and it dawned on me that there was a restriction in the drain and there was.

 

[Fitter30]

Never seen a coil in that configuration. Flat, A,V or slant. Coil looks like it's feeding correctly.