[chrisoffice]

I can understand why the liquid line is cold or frozen when the refrigerant is low, but can't understand why the suction line is hot.

can someone please explain?

 

[ammoniadog]

The warmest a suction line can be is whatever the return air temperature that is blowing across the evaporator is. I don't think it can physically be any warmer than that (unless we're talking about a heat pump when the suction line turns into the discharge line).

Also, the liquid line wouldn't ever be cold or frozen, unless the filter drier is plugged up, which is kind of a rare phenomenon. Are you sure you don't have what you think are the liquid and suction lines mixed up?

If a system is low of refrigerant then you would have zero subcooling, which would tend to make the liquid line WARMER than normal. At the same time the lower low side pressure would make the temperature of the refrigerant in the beginning part of the evaporator colder than normal, which could cause it to freeze up starting near the expansion valve and eventually the entire coil and suction line back to the compressor.

Are you just messing with us?

 

[beshvac]

.

 

[chrisoffice]

I am talking about a system without TXV, but using the old school piston. In such system, the liquid line will be cold or frozen when the refrigerant is low. And the suction line will be warm.

Again, it is easy to understand why the liquid line is cold, but difficult to understand why the suction line is warm.

The warmest a suction line can be is whatever the return air temperature that is blowing across the evaporator is. I don't think it can physically be any warmer than that (unless we're talking about a heat pump when the suction line turns into the discharge line).

Also, the liquid line wouldn't ever be cold or frozen, unless the filter drier is plugged up, which is kind of a rare phenomenon. Are you sure you don't have what you think are the liquid and suction lines mixed up?

If a system is low of refrigerant then you would have zero subcooling, which would tend to make the liquid line WARMER than normal. At the same time the lower low side pressure would make the temperature of the refrigerant in the beginning part of the evaporator colder than normal, which could cause it to freeze up starting near the expansion valve and eventually the entire coil and suction line back to the compressor.

Are you just messing with us?

 

[kdean1]

:couchhide:

 

[chrisoffice]

I think I can answer my own question now, correct me if I am wrong.

When the freon is low, initially, as ammoniadog said, the suction line (low side) will the frozen, and the liquid line (high side) will be warm. But if you keep the system on, then the frozen will go through the compressor and all the way through the liquid line back to the expansion valve (where the frozen started initially), i.e., the entire system is frozen up. AND THEN, because there is no freon getting into the suction line, the compressor is essentially making a vacuum on the suction line, which will make a hot. That's why when the system is low on freon and you keep the system running, eventually the liquid line will be frozen and the suction line will be warm.

 

[kdean1]

By Jove, he's penetrated the curtain.

You should apply for pro status now.

:D:

 

[rider77]

I think I can answer my own question now, correct me if I am wrong.

When the freon is low, initially, as ammoniadog said, the suction line (low side) will the frozen, and the liquid line (high side) will be warm. But if you keep the system on, then the frozen will go through the compressor and all the way through the liquid line back to the expansion valve (where the frozen started initially), i.e., the entire system is frozen up. AND THEN, because there is no freon getting into the suction line, the compressor is essentially making a vacuum on the suction line, which will make a hot. That's why when the system is low on freon and you keep the system running, eventually the liquid line will be frozen and the suction line will be warm.

That’s not what happens at all.
I’ve never seen a frozen liquid line on a conventional system, that was caused by low refrigerant.

 

[HVAC_Marc]

I dont think Ive ever seen a frozen liquid line unless there was a severe restriction.

 

[vstech]

To explain everything would be beyond the scope of this forum.

However...

The liquid line is always at or above ambient temperature. It cannot be frozen unless there is a restriction such as a plugged dryer etc.

The large suction line can be colder or warmer depending on the amount of refrigerant and the temp in the house.

As refrigerant pressure begins to drop, the system doesn't have enough capacity to handle the heat in the house. This will cause the line to be warmish...
As it drops further, the temp of the refrigerant gets colder still, and can begin freezing the humidity resulting in ice.
As it drops even further, the refrigerant has little liquid to change state and absorb heat letting the compressed gas be warm to hot leaving the condenser and as such it has no expansion to drop temp and passes through the evaporator actually warming the house.
Also, if the coil is in the attic it can pick up a lot of heat from there.

 

[chrisoffice]

Thank you all for the detailed answer! I learned a lot from this forum!

My system seems to have this weird reversible liquid line sweating problem. It only happens when the system has to run for an extended period of time, for example, when I return from multiple day trip, the room temp is over 90s, so the system has to work continuously to cool the house down. After several hours, I noticed that there were no air coming out of the grille, upon checking, the suction line was frozen, I believe the A-coil was frozen as well and that's why no air was getting through. And the liquid line was cold and sweating.

The first time it happened, I thought it was low on freon. So I had someone recharge the freon. He did not make any diagnosis, simply pull out a tank from his truck and recharge 3-4 pounds of freon (OK, that's what I was billed for, I don't know exactly how much freon he recharged). Afterwards the system worked fine. I thought the problem was the low refrigerant.

Then same thing happened when I came back from another trip. This time I simply turn the AC off for a while while leaving the fan on, and then turn the AC back on, the system started working normally. The liquid line turn from cold to warm.

I am puzzled! If there is a blockage on the liquid line, how could the blockage reverse itself? And why the blockage only happened after continuously running the system for couple hours?

 

[beenthere]

You probably have another problem, not a blockage. Call a service tech to come and diagnose the real problem.

 

[timjimbob]

Since this a novice home owner site, I will explain this as I do to my customers. Most believe the cold is going to the inside coil via an insulated line and the warm medium is the stuff returning to the compressor to be made cold again.

This is the misconception that most all my customers have. To the novice: The truth is that the cold suction line has already left the inside ac unit. The refrigerant flow is counter to what most of my customers think the circulation.

 

[kdean1]

It is better to let a technician make the diagnosis instead of trying to teach yourself how a refrigeration system works. Refrigerant should only be added if it is indicated after confirming air flow for both the indoor and outdoor units, and measuring pressures and temperatures of the refrigeration system.
Ask a technician for static pressure measurements, suction and discharge pressures, superheat, and subcooling. If they don't know what these are or don't measure them then they cannot effectively remedy the problem.

 

[Gary Warren Jr]

The warmest a suction line can be is whatever the return air temperature that is blowing across the evaporator is. I don't think it can physically be any warmer than that (unless we're talking about a heat pump when the suction line turns into the discharge line).

Also, the liquid line wouldn't ever be cold or frozen, unless the filter drier is plugged up, which is kind of a rare phenomenon. Are you sure you don't have what you think are the liquid and suction lines mixed up?

If a system is low of refrigerant then you would have zero subcooling, which would tend to make the liquid line WARMER than normal. At the same time the lower low side pressure would make the temperature of the refrigerant in the beginning part of the evaporator colder than normal, which could cause it to freeze up starting near the expansion valve and eventually the entire coil and suction line back to the compressor.

Are you just messing with us?

It can get pretty warm if the captube is soldered to it for heat transfer.