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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.

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  • what are the "significant problems with the system"? maybe its just a problem with how the temperature was sampled? – Steven Aug 11 '15 at 18:19
  • When you do a spot temperature check near the handler you can have widely different readings in temperature. Closer to the coil you get, the more wild the readings can be. By the time the air gets the vent, it is pretty well mixed and uniform. But low 40's for vent temperature seems way to low for a properly sized system. You are only looking for a 15-20 degF drop from air returns to vents. – diceless Aug 11 '15 at 18:36
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    Assuming it's summer and the environment (i.e. your house, attic/crawlspace, etc) the ducts run through isn't colder, it's not plausible for air cooled by your evaporator coil to get even cooler as it travels to a vent. Most likely there are anomalies in how this is being measured. At the air handler, are you measuring temp after the coil? Are the air handler- and vent-site measurements being taken actually simultaneously, i.e. is it possible the system is changing states in between measurements, such as by turning on/off or changing load in the home? – Shimon Rura Aug 11 '15 at 18:58
  • Outside temperature was about 95. Inside was 75. Significant problems = low R22 pressure, apparently indicating rusting evaporator coil is leaking coolant. At the same time, superheat was correct, which tech theorized meant evaporator coil problem was hiding the low R22 pressure. – Paul Aug 11 '15 at 19:04
  • How were the temperature readings taken? – Tester101 Aug 11 '15 at 19:27
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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.

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  • Thanks! This is exactly the sort of help I was looking for. – Paul Oct 26 '15 at 15:52
  • At my request the home warranty company sent out a tech from a different company. He measured the register temps to be more plausible values, as did I when I sample several times over the course of a few days. – Paul Oct 26 '15 at 15:53
  • He also replaced the capacitor that my original repairman (the one I trust) had said needed replacing. He would not comment on whether he had been able to demonstrate that it needed replacing or not, but I doubt he would have replaced it if he did not think it needed replacement. At any rate, the system works significantly better now, I assume because the compressor starts up at full speed. – Paul Oct 26 '15 at 15:56
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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.

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.
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  • Indeed, all of this is true. However, the repairman actually measured the temperature as being lower with his digital thermometer. He had given the measuring stick several minutes to cool down and adjust in the air handler so it should have reached a good approximation of the temperature of that air. The fact that he measured it as lower, and that his thermometer showed it as much higher until he pressed a particular button on it, was what made me suspicious. – Paul Oct 26 '15 at 15:58

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