AC differential temps

[Gerald C. 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?

[David A. Andersen]

Must we go through this all again?

Go back and read, then make your own decision based on your qualifications and assessment of the data you collected.

No one here can make an assessment for you from what you collected.

You can spend some money on an HVAC guy (I don't recommend you telling someone else to spend the money on your uncertainty), or you can report that you turned it on at the thermostat and it blew air...

[Bruce A. King]

Originally Posted by gwilcox

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?

Gerald,

Did you actually check the temperature right at the coil?
Most inspectors check it at a supply register for many reasons, the main one being lack of desire to drill a hole or remove tape etc to access the coil area.
If you actually checked it at the coil I would say the system does need service. Even if you checked it at a supply register that is not cold enough air based on the other information you supplied.

When you feel of the suction line get into the habit of also feeling of the liquid/pressure line to make sure it is not real hot. Liquid line too hot almost always points to a dirty condensor coil or airflow issue in the absence of other symptoms.

What you detect at these lines and how you interpret it is based on lots of experience and conditions that day so you have to have lots of experience using the limited tools we, as inspectors, can use.

If you have a vacant house, real hot humid sunny day and the house is over 85 degrees inside, a 10 degree split seen at the registers may be all that it can do until the sun drops down later in the day.

[Gerald C. Wilcox]

Thanks for the input Bruce. The high pressure line was warm, not hot and The temp at the registers was virtually the same as at the coil and the RA. Unit was only 12 years old and obviously was cooling the house. I appreciate your help.

[Gerald C. Wilcox]

Oh and Dave thanks for........

[David A. Andersen]

Yea, whatever.

If; "obviously was cooling the house. "
Then your response should be the same...

You talked about; "The return line was beer can cold". Gee where did you get that from?!
Now go back and read the rest of what I said.

I have been talking to a blank wall for several years now on this exact subject.
If anyone is offended for a frank answer, then quit asking the same question with undocumented criteria which can only result in inconclusive results...

If anyone is too lazy to read past posts and determine for themselves; "you can not determine the functionality of an HVAC unit by taking dry bulb temps".
There is no way around it.
If this is not good enough for you, go to school and work in the HVAC field for about 5 years and then maybe you will grasp the concept.

There are too many of you out there that think they can take a short-cut and second guess the engineers in this field with some "grand-ma's rule of thumb" that no longer applies to the industry.
It may have worked in the past, but you are not asking the same questions these rules applied to.

Simply put; most of you can't, and never will be able to evaluate the performance of an HVAC system.
Your insistence to try will only get you in trouble and your business in a world of hurt.
So, why does everyone persist so?

[Gerald C. Wilcox]

First I would say we are checking whether it is functioning as designed. Does it cool or not. Yes there are variables which the experts get all up tight about but as a generalist it's a simple question. When situations fall outside of a generalist scope we may call it out for further evaluation. But the plain and simple was even with a low temp differential it was obviously cooling well on that day. I don't calibrate or do exhaustive testing on any other appliance......just report on what I see. I Don't make more of it than what it is........

[Dan Bowers, CMI]

Gerald - Like David said ..........

What was the super-heat & sub-cooling? What SEER was the unit? After taking indoor drybulb, indoor wetbulb, did you plot this info to determine the RH% and expected TD (not measured but expected).

Then did you compare the SEER to the condensor TD to arrive at your desired SC?

Then did you take actual gauge pressures outside and convert them to saturation levels?

AND after taking the indoor dry bulb temperature and charting it compared to the outdoor temperature what was the SC.

Once he has that a home inspector is better equipped to talk reality's.

[LeRoy D. Holm]

I worked in the HVAC field and I only do temp drop test due to time constraints, I could spend an hour just on the air another hour on the furnace. If I get 15 - 20 degrees I am happy less or more I suggest having it serviced. I also give my clients a lesson about keeping the condenser level and clean to keep it the most efficient.
Furnace gets a exhaust gas test and the CO test of the supply air, this tells me a lot more than the type of flame it has. I can pretty quickly tell if there is good chance for a cracked heat exchanger and if the furnace is burning efficient.

[David A. Andersen]

I worked in the HVAC field and I only do temp drop test due to time constraints,

I realize this is no what you meant, but if you want to save time why do you do a test that tells you nothing?

If you "worked" in HVAC, you are certified.
Why not just stick on a set of gages and actually get something that tells you what is going on instead of guessing?

Just asking...

I never check air temp as a stand alone test. There are too many things going on between the registers and the AHU. I can show you hundreds of thermal images as to why this testing is wrong, even if you did it right...

It's a total wast of time, seeing you mentioned time.
It doesn't take 2 hours to test an HVAC unit. If so, you need more practice.

[Michael L. Bell]

As a Mechanical Contractor and Inspector, I say any one asking a Respectful Question deserves a Respectful Answer.
Just Saying.....

[James E. Braun, CMI]

Originally Posted by dandersen

So, why does everyone persist so?

Because we know it will piss you off!

I see a lot of home inspectors say incorrect stuff about mold but I let it roll off. Calm down some, my friend.

[David A. Andersen]

Sorry, I don't like to see wanna-be HAVC inspectors go off the deep end with their head stuck up their ***...

For one, it makes everyone in this industry look like an idiot.

Second, getting in trouble for following the blind and ignorant is not necessary to be part of the club.

As for respectful answers, they are posted in the archives for anyone that wishes to look it up instead of being a lazy *** and just ask the same nonsense over and over.

Seeing this is your #1 post and your a "mechanical contractor" jump right in there. No one is stopping your sweet answer.

You can start out telling me exactly the psychometric value of taking dry bulb temps to determine the working capacity of an HVAC unit. At what point I should call in another contractor to fix a definite issue with the equipment. I'm all for saving time in my inspections. I spent 9 hours on site at a home inspection today. I will likely spend 8 hrs writing the report tomorrow. If you can tell me how I can knock out 4 HVAC units by taking dry bulb temps, I'm all for it.

However, I must be able to tell my client that one unit has a low charge, one has a refrigerant leak at the reversing valve, one unit is leaking condensate all over the attic (and why), and the last unit has a detached return duct inside the wall...

I don't want to hear nothing like "recommend further evaluation and repair of 4 HVAC units that appear not to be performing correctly as determined by taking dry bulb temperature measurements. My client didn't pay me $1,300.00 for crap like that...

[Robert J. OConnor]

I do agree with David that dry bulb splits are not technically valid as a stand alone test or tool to gauge how well equipment is working, particularly with how most home inspectors get those readings.

However temperature splits can be a good indicator for an HI with some knowledge of how humidity and different types of equipment affects the readings and if the splits are taken correctly ... in the ductwork airstream flow just before and just after the equipment ... and if they do some other basic checks. Pretty much throw out the IR register readings, unless you are using that to supplement temperature measurements/splits at the equipment.

Getting splits is not the holy grail of cooling system inspections some may think it is, but I still think it provides useful information along with some other basic checks. Including checking: lineset temps (thats a big one, and you can just feel the lines); increase in condenser fan exhaust temp; condenser amp draw; lineset damage/kinks; ductwork leaks; ductwork insulation; adequately sized ducts (particularly the return); damaged/pinched ducts; clean filters, etc. All pretty simple and quick checks if you know what your doing.

With the family mechanical business we routinely do these basic checks (yes, including punching duct holes to get dry bulb TD readings with the fieldpiece) on an install or major repair. Granted we are also hooking up refrigerant gauges, checking for leaks with nitrogen, checking for contamination, checking superheat and/or subcooling, getting RH readings, etc. Buts those things are way beyond a basic home inspection.

JMO & 2-Nickels ...

[David A. Andersen]

Temp splits: OK (I checked just for U-guys, from ir scans of the "leaking" supply and return registers").

Refrigerant pressures: OK

Suction Line superheat: OK

Think this new house will continue to cool for long?

May even get thorough the heating season, just wondering if this high heat bill is normal for a 7k sf house.

Next season the other three HVAC units may even mask the condition and they can run the thing into the ground with no referigerant...

It took me 3 passes by this unit before I "saw it".
So even the best of testing may go unnoticed.
So why jack up your liability claiming your checking the HVAC (like I was being paid to do) when you don't have to and really are not allowed to?


BTW: The suction line should be wet, but not with oil...