Can the A/C temperature through a vent be significantly lower than in the air handler?

Question

Can the A/C temperature coming out a vent be significantly lower than the temperature in the air handler above the evaporator coil? If so, how does the air get colder as it comes out the vent? Or how does the vent get cooled below the temperature of the air cooling it?

(Background: I've had two A/C contractors out to my house. One was a friend my family trusts, who said there were significant problems with the system. One was an inspector from a company with no reviews who was paid by my home warranty company. He said everything was normal, which I'm somewhat doubtful about. He also measured the vent temperature as in the low 40's while the air handler temperature was 55, which made no sense to me whatsoever.)

Let me know if I should move this to physics SE.

Answer 1

Low R22 pressure (presumably suction pressure) combined with correct superheat typically means that there is too much compressor capacity for the airflow. This can happen when there is a clogged air filter or weak blower motor, or less often an undersized interior fan coil mismatched with a larger capacity condenser. I have seen systems where, unbeknownst to the owner, there were two filters installed inline, one was at the return air grill in the building and that one was being serviced; the other was at the air handler in the attic, and that one had never been serviced or removed and was completely obstructed.

The most likely cause I can think of for colder air at “the vent” than at the air handler is the temperature across the evaporator coil is very uneven, say 34 degrees on one side and 50 degrees on the other side. In some ductwork situations air supplying one set of registers comes primarily through one side of the evaporator coil while air to other registers comes primarily from the other side of the coil. A proper trunk and branch system with taps near the air handler discharge could do this, for example. Typically, an evaporator will be much colder on one side than the other if the system is low on refrigerant charge, because the refrigerant prematurely boils off completely before it crosses the coil. Low charge will also usually cause low system suction pressure, but the superheat will also be notably high (typically 30F or more at the compressor instead of 15F).

My take on your post:

1. Low 40s at a register is too cold for a properly sized system. High 40s is the safe lower limit for AC. Low to mid 50s is typical. Your system has either too little airflow or too much compressor. Look for airflow restrictions or an improper blower speed setting first. If in doubt, have someone measure the airflow. Your system should supply roughly 400 CFM per 12000 BTU of compressor capacity. For example, the air handler for a 48000 BTU system needs to push about 1600 CFM.

2. You cannot have a temperature at your supply vent that is lower than the temperature leaving the air handler. You are misreading the temperature at the air handler. If you are reading the temp at the suction line exiting the air handler, for instance, then the refrigerant inlet side of your evaporator coil could be much colder.

3. If you measured the air temperature leaving the air handler and had a warmer reading than one of the supply registers, then there is possibly a large temperature spread across your evaporator coil (30s-50s), and your system may be low on charge in addition to the airflow being too low for the cooling capacity. Superheat would also be high in this case. Have someone else recheck the superheat. If in doubt, crosscheck this with the subcooling at the condenser. The subcooling should be around 8F-12F. Less than 8F is very likely low on charge. If the system is not low on charge, then this inconsistency is simply a case of the temperature at the air handler being read inaccurately.

Answer 2

To address the part of your question which is more physics related:

how does the vent get cooled below the temperature of the air cooling it?

TLDR;

The vent is a better conductor of heat.

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You can think of this the same way as how grabbing an ice cube out of the freezer "feels" colder than putting your hand in the freezer air does. They're both at least the same temp, with the air probably being colder.

This page explained it well for me.

If the air and the water are the same temperature, what accounts for the difference 
that we perceive? It's a matter of heat transfer, the transition of thermal energy
from a hotter object to a cooler object.

As long as the temperature of your body is higher than the temperature of the 
surrounding medium (air or water, for example), your body will give off heat. As soon 
as the surrounding temperature becomes higher than that of your body, though, you'll 
start to absorb heat.

The amount of heat that moves between your body and the surrounding medium and the 
speed at which it moves, both of which are important to the sensation or warmth or 
cold that we feel, depends on how good a conductor the medium is. The reason the water 
feels colder than air is because water is the better conductor of the two. When you 
hop into that 60-degree pool, heat escapes your body much more easily than it would 
if you were standing beside the pool in 60-degree air. Because the water takes 
more heat from your body, and quicker, it feels colder.