What is an acceptable differental tmp

is between 10 and 15 good?

I like to see 14 to 22 or so but many things can affect that including humidity and outside temperature.

Yep ,
I am happy 10 to twenty .
Others think we are doing it wrong.
… Cookie


Do you have a dart board?
Any number will do. It changes hourly and daily anyway.
A dry bulb temperature differential may work pretty accurately in Arizona because the relative humidity is low and changes are slight, but I would be far from comfortable with guessing about it in Florida.

This is how I break it down:

Less than 12ºF - inadequate differential. Recommend servicing.
12-14ºF - considered marginal operation. Recommend servicing.
14-16ºF - considered acceptable.
16-18ºF - good operation.
18-20ºF - very adequate operation.
20-22ºF - slightly excessive differential.
22-24ºF - excessive differential - air path may be restricted.

Hope this helps.

Can that not vary example Home temp is 65% or the home temp is 90%.

… Cookie

I agree with Mike’s breakdown.

Regardless of inside or outside temps (assuming summer conditions, not winter), the system should cool the air passing the coils to a difference of 16 to 20 degrees.

I have recently discovered that the IR camera gives very accurate information on these temps. You can actually see the air (through thermal imaging) moving into, and out of the system.

Please Lord is this thread subject going to start again I am going to set back and watch this one:) :slight_smile:

I would say that about 90% of the time I dont even report the temperature drop. If you have dirty sir handler coils, a dirty filter, damaged ducts, or dirty coil fins on a conenser unit, the machine is now no longer functioning as intended and needs to be cleaned / serviced. I get a look of surprise on a lot of the realtor faces when I tell them I will not report the drop. I tell tem that first they need to clean the system and then test the drop. These machines are made to run with clean filters, coils and non damaged ducts.

It appears you are both in an arid environment.

I am sorry for being such an a$$, but you can not determine BTU capacity through dry bulb temp measurements and you can not see air with the BCAM.

Flir makes a CAM that can see gas, but it does not operate in the same range as the BCAM.

You may be able to see moisture temps in the air, but if you have that much moisture in the air, dry bulb temps are more than worthless as an HVCA evaluation.

I sure wish someone with a masters degree in thermodynamics would get in here and shed some new light on this subject because my posts are about as obtuse as the damn latent heat your trying to measure with a dry bulb sensing device.

How can you say this? I expected more from you Jeff. You really dive into things. NACHI is now global. You can’t be telling someone from the South East (and places like that) that they can be doing these things! The only way you can tell what the unit is doing is by tapping into the refrigerant system! Yes, it is outside of HI SOP. It is also a violation of the Clean Air Act and you could be fined or go to jail! Why do so many SOP’s and State Laws say HVAC evaluation is outside of what we do? Because it is illegal to do it without certification!

Does anyone know what a high efficiency HVAC unit is?
Basically, it is a 2.5 ton compressor in a 3 ton condensing unit.
Larger coils lower head pressure, thus lower amp draw.
Units run longer with less of an amp draw.
Small units work on latent heat long before that can effect the sensible temperature.
You just can’t say that you need a 20 degree delta T. The unit is running at capacity, but because of OUTDOOR conditions, it has a lot of work to do before it can effect the dry bulb air temp inside the house.

A gas or electric heater is a sensible heat device.
You get a predetermined BTU output from natural, propane, oil and electric. Refrigerant systems don’t work anything like this! It is very VERY dependant on outdoor air temps (not referring to geothermal).

Funny Charley. ha. ha. I’ll join ya.
Any cold one’s. :smiley:

Marcel :slight_smile: :wink:

Going to need more than one this could get long and nasty again.

Great idea Marcel. Might be a good show again.:roll::wink:

An excellent idea Marcel, don’t mind if I do. :smiley:

I ran some figures through a Psychrometric Chart.

Can someone explain why it takes 2 more tons of equipment to achieve the same 20 degree split?

Test #1 (at 1,000 cfm)
Return Air 85 degrees dry bulb, 0% RH (0% because a db thermo has no input value)
Supply Air 65 degrees dry bulb, 0% RH
Results (btu removed) total: 21,600 btu
21,600 btu/hr Sensible
0 btu/hr Latent
1.8 tons of cooling required

Test #2 (at 1,000 cfm)
Return Air 85 degrees dry bulb, 65% RH (these are conservative expected values in normal a/c operation)

Supply Air 65 degrees dry bulb, 90% RH
Results (btu removed) total: 46,854 btu
22,077 btu/hr Sensible
24,777 btu/hr Latent
3.9 tons of cooling required

Do you notice that the sensible heat (which you are measuring with your dry bulb device) is about the same in both tests? The “temp split theory” is based upon this. If you have a “normal humidity” condition (for your area), a temp split test will be close to accurate if you know what delta T is “normal” for your area of the world.

If the HVAC unit has been running continuously for 12-24 hours prior to testing, latent heat load levels will stabilize and make the temp split method more accurate. Outdoor air has a large impact on test results.

A slight rise in rh values (higher %rh is experienced in many areas;
Currently: TN 70%, TX 79%, AZ 34%,FL 90%, UT 26%) in Test #2 substantially increases the work load of the equipment.

Electronic Psychrometric charts are free to download and use.
They are easy to use and understand (once you have done it once).
Hygrometers are not cheap. Sling psychrometers are, but are more complicated and time consuming.

My point is, you can do temp splits if you determine the split for your local climatic conditions. Failure to consider the wet bulb temp substantially increases the error factor. We all are trying to keep error factors to an absolute minimum in this industry.

I use the Carson Dunlop reporting system.
I am a Home Inspector who does a visual Inspection .
Do not have the Knowledge or equipment to go further then a Temperature drop.
Carson Dunlop has been around longer the most Inspectors in this association and longer then NACHI by far.

Here are the exact words from Carson Dunlop

Temperature Drop
The temperature Drop across the coil should be 14 to 22 F.If fins are not cleaned regularly,air flow across the coil can be blocked by dust and other foreign matter. Dirty fins are also a common problem where the furnace filter is dirty or missing. The fins are extremely delicate and can easily be damaged.
Low air flow can lead to excess temperature drop across the coil,resulting in ice build-up problems.Too low a temperature drop also indicates the need for service. The problems may include fan size or coolant pressures.

The plenum coil is not visible or readily accessable.


I will agree. But they have not been around longer than ASHRAE (American Society of Heating, Refrigeration & Air-Conditioning Engineers).

ASHRAE, founded in 1894, is an international organization of 50,000 persons. ASHRAE fulfills its mission of advancing heating, ventilation, air conditioning and refrigeration to serve humanity and promote a sustainable world through research, standards writing, publishing and continuing education.

I know your from Canada, but SI is the only difference.
ASHRAE® London Canada Chapter #116

I’m only trying to help here. Not trying to suggest that anyone evaluate HVAC equipment.

I just think it is a big slap in the face to a HI when they call out a problem HVAC unit only to have a mechanical contractor come in and tell your client that your crazy, there is nothing wrong with the unit (at a cost of $70? to the client/homeowner).

Mr. Cook, you live north of where I grew up and we didn’t use A/C.
Your cooling load is minimal. The chance of your “rule of thumb” being that far off is much less than elsewhere. Should we all be using the same “standard”?

I expect Carson Dunlop is the largest Home Inspection Company in Canada . ALL their Inspectors are Engineers ( Various, Not all Air) .
My point is we as Home Inspectors need to just confirm that the AC is cooling the home .
If I am found wrong and they did not need to bring in a Mechanic and I need to pay the service call of $70;00 so be I will gladly pay.
But I was wrong and the AC is bad I can see a cost of $2,000;00~.
I think I will take my chances and contiue my way of inspecting and write up split is 15% and move on.
Works for me ,8 years and still claims free.
You have much more expertise in AC and equipment then I do and if you like doing your pressure test’s fine ,
I can see more time used in the home with your method then mine .

In our area the bigest enemy of air Conditioneers is BOY DOGS!

They P P on them.

… Cookie

See the following thread for a recent re-hash of this controversal topic … http://www.nachi.org/forum/showthread.php?t=3809