AC differential temps

Gerald Wilcox · 08-22-2011, 09:01 PM

When I check for these temps I would hope that they are generally 17 18 degrees apart. I know that there are many things that our experts tell us that make all the difference when evaluating the AC differentials. Today's inspection at the return air was 70 degrees and over the evaporative coil was 60 degree's. I thought only 10 degrees...........Todays outside temps was 89 degrees with 55% humidity, The t-stat was set on 68 and the t-stat temp said we were at 70 degees. The return line was beer can cold. I think that it is working great does anyone differ in their opinion?

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**Dom D'Agostino** · 08-23-2011, 04:49 AM

That place must have felt like a meat locker compared to being outside.

Not enough info in your post to fully address the system. Although I hate to say it, 60 degrees at the coil "might be OK", with about a dozen qualifiers.

Don't rely on deltas, or temp splits. Too may variables.

Dom.

**Scott Patterson** · 08-23-2011, 05:27 AM

As Dom said, Delta-T's are just about useless for evaluating an AC system, it can be a tool but do not depend on it. This is crap technology that is taught and fostered by many of our home inspector training providers.

If the air is cool/cold to your hand when it is coming out of the register then it is cooling. If it is dropping the room temp then it is cooling.

Bob Spermo · 08-23-2011, 07:29 AM

Temperatures taken at the air handler across the coil can be a valuable tool. Temperatures taken at the grills and diffusers is probably not useful unless you also want to evaluate the duct system and its BTU gain/loss.

**Rod Butler** · 08-23-2011, 01:17 PM

Originally Posted by Gerald Wilcox

When I check for these temps I would hope that they are generally 17 18 degrees apart. I know that there are many things that our experts tell us that make all the difference when evaluating the AC differentials. Today's inspection at the return air was 70 degrees and over the evaporative coil was 60 degree's. I thought only 10 degrees...........Todays outside temps was 89 degrees with 55% humidity, The t-stat was set on 68 and the t-stat temp said we were at 70 degees. The return line was beer can cold. I think that it is working great does anyone differ in their opinion?

A couple of things: First, 89° and 55% RH seems awfully high for your neck of the woods. At that point the WB temp is around 75° or pretty close to temps in Houston or maybe Miami. But, if so . . .

Second, If the stat was set at 68° and you are maintaining a space temp at 70°, I would call that good regardless in those conditions.

Third, Beer can cold should probably NOT be used as a term in your report, but now I am just being ornery.

And last, the temp diff issue. Like the guys above I agree it is not a tell all way of determining capacity because the cooling coil is doing more work than just dropping temp, especially in the humid conditions you are referring to. However, I design systems at 17° to 20° delta T all the time so I think it could be useful as a quick check.

Bottom line goes back to my 2nd point, if you are maintaining space temps at 70° in peak summer conditions, why be concerned about a systems performance?

**Harry Janssen** · 08-23-2011, 01:48 PM

I also like to see a difference of about 17 to 20 degrees,but as other have mentioned,there are many other factors to consider,at least it is a starting point.
If the duct work feels cold above the coil,that is also a good indicator as well.

Gerald Wilcox · 08-23-2011, 04:06 PM

Thanks for all the help! It was undeniably working just a low temp differential. The house was empty, well insulated, cool when I got there.....

**Billy Stephens** · 08-23-2011, 04:22 PM

Originally Posted by Gerald Wilcox

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The house was empty, well insulated, cool when I got there...
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...

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Life is Good .

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Robert Alfred Moller · 08-24-2011, 09:48 AM

ABOUT TARGET SUPERHEAT (TSH)
Calculating Target Superheat is important because it is the first step in diagnosing charge related fixed orifice A/C system problems. The superheat needed to keep an air conditioner running depends on the entering dry bulb and indoor return wet bulb. A technician has to take two measurements, and then look up target superheat on a chart. A technician compares the target superheat to actual superheat on a fixed orifice system to determine if the system is properly charged. The actual superheat should be within five degrees of target superheat for a fixed orifice system. A properly charged system prevents damage to the compressor and runs more efficiently.

ABOUT TARGET EVAPORATOR EXIT TEMPERATURE (TEET)
The TEET diagnostic test (also known as “temperature drop” , “temperature split” or “delta T”) gives an indication of proper airflow to the system. The technician uses thermocouples to measure the wet bulb and the dry bulb temperature of the air entering the evaporator by clipping each thermocouple onto the air filter just in front of the evaporator. Then he measures the air temperature coming out of the evaporator. If the actual TEET is 3°F less than the Target TEET check for low airflow. If the actual TEET is more than 3°F above the Target TEET it is an indication of low capacity, and occasionally high airflow.

**Rod Butler** · 08-24-2011, 11:14 AM

Originally Posted by Robert Alfred Moller

ABOUT TARGET SUPERHEAT (TSH)
Calculating Target Superheat is important because it is the first step in diagnosing charge related fixed orifice A/C system problems. The superheat needed to keep an air conditioner running depends on the entering dry bulb and indoor return wet bulb. A technician has to take two measurements, and then look up target superheat on a chart. A technician compares the target superheat to actual superheat on a fixed orifice system to determine if the system is properly charged. The actual superheat should be within five degrees of target superheat for a fixed orifice system. A properly charged system prevents damage to the compressor and runs more efficiently.

ABOUT TARGET EVAPORATOR EXIT TEMPERATURE (TEET)
The TEET diagnostic test (also known as “temperature drop” , “temperature split” or “delta T”) gives an indication of proper airflow to the system. The technician uses thermocouples to measure the wet bulb and the dry bulb temperature of the air entering the evaporator by clipping each thermocouple onto the air filter just in front of the evaporator. Then he measures the air temperature coming out of the evaporator. If the actual TEET is 3°F less than the Target TEET check for low airflow. If the actual TEET is more than 3°F above the Target TEET it is an indication of low capacity, and occasionally high airflow.

Good post, thanks for that info.

Binford Tools · 08-26-2011, 08:53 AM

I like it "beer can cold" No I would not right that down

But it does show you physically reach down and grab a hold of the return line. ( larger of the two copper lines) The smaller line should be warm to touch. Does this give you the information to tell if the system is overcharge by 2 pounds. No. But it will tell you it is working. Just like Scott said, are the register blowing cold. Don't forget the basics!! Trying to provide an in depth report on wet/dry bulb, super heat, sub cool, temp differential is a bit much. You would really need your EPA license and hook up some gauges and a temp probe to see whats going on to provide a worth while in depth report of the A/C system.
It's 89 outside and its nice and cool inside I'm thinking the A/C is working and I would write those numbers down. What does a HO want to know. Sure they want to know it working. But they really want to know how old the system might be. Circa 1980's means a lot, as a 30 year old system can't last much longer. How dirty the coils are, is the insulation missing from the return line. Are the ducts dirty or missing insulation, or laying on ground under the house. Don't get me wrong the HVAC system is large $$$$ item. and HO needs to know what condition its in, just like buying a car. But at what point do you recommend an outside service provide a report? If the return line is covered in ice would mean the system is in need of service. The house is hot when it should not be would be another. If the buyer wants to really know they would have a HERS rater do some testing.

But in the end, the HVAC could be working perfect, subcool superheat WB DB, ect. But there is a design flaws in the system! Like the master bedroom does not cool like it should. So now your back to Scott analogy is the room getting cooler. This common approach will save you a lot of hassles. 2 maybe 3 years ago had this issue, in the end the new HO had to have duct work done to resolve the issue.

James Duffin · 08-26-2011, 09:38 AM

I see as many problems with systems overcooling creating condensation and mold problems as I do systems that are under cooling.

Binford Tools · 08-26-2011, 10:23 AM

Originally Posted by James Duffin

I see as many problems with systems overcooling creating condensation and mold problems as I do systems that are under cooling.

Lucky here its more of a dry heat. Less issues with mold.

You are correct that an over sized system will not remove enough moisture. The HO will compensate by turning the temperature down to remove more moisture from the air creating a cold and clammy environment.

Setting the t-stat to say 70° when its 90° outside an it 90% humidity. Your past the Dew Point. Meaning any warm moist air that can reach a cool surface will condense mositure out of the air. Normally created from a "hole" in the building envelope. Maybe the insulation dropped down in a wall and now you have an area where a cold surface is in contact with the warm moist air. Which as you pointed out leads to condensation and mold problems.

Good points.

A moisture meter might help, but a they have inferred cameras that can show moisture a lot better. If I lived in the southeast I bet I would own one.

FLIR Infrared BCAM - YouTube
check it out.

**Vern Heiler** · 08-26-2011, 10:38 AM

Originally Posted by Binford Tools

Lucky here its more of a dry heat. Less issues with mold.

A moisture meter might help, but a they have inferred cameras that can show moisture a lot better. If I lived in the southeast I bet I would own one.

You really do sound like Tim the tool man!

I agree with all who say we are giving an educated guess, based on many indicators of the condition of the A/C.
FYI, another indicator of the condition of the coil is the direction the air is blowing out of the condenser. Straight or nearly straight up is good, horizontal to 45deg. is an indication of a dirty coil.