What is an acceptable differental tmp

[David A. Andersen]

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.

[Roy D. Cooke, Sr]

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.
------------------------------------------------------------------------


...Cookie

[David A. Andersen]

Carson Dunlop has been around longer the most Inspectors in this association and longer then NACHI by far.

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
http://londoncanada.ashraechapters.org/


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"?

[Roy D. Cooke, Sr]

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

[Robert J. OConnor]

See the following thread for a recent re-hash of this controversal topic ... http://www.nachi.org/forum/f20/ac-splits-yea-nay-3809/

[Roy D. Cooke, Sr]

Originally Posted by roconnor

See the following thread for a recent re-hash of this controversal topic ... http://www.nachi.org/forum/f20/ac-splits-yea-nay-3809/

I saw it I read it and I still like my way of doing it I can not and will not change .
I think many are well above the SOP .
I am just a simple Great Home Inspector.
........ Cookie

[Brian E. Kelly]

Originally Posted by dandersen

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.

Nice David, excellent points to consider.

[Jeffrey R. Pope]

David,

While I certainly appreciate your expertize in this matter, I freely admit that HVAC is still my weakest subject. So please forgive my over-simplification, as I'm sure the entire process is much more complex.

In my (relatively dry) climate, air, when it passes over an "exchange," will change in temperature. As Roy stated, there is a level of change considered to be within acceptable limits, based on a "generalist" inspection.

Until I have achieved a higher level of expertize in HVAC, I will follow the teachings of my predecessors.

I now see that this has been hashed and debated for quite some time in a thread that I did not participate in. I'll let you and the others continue the debate, while I sit in and observe.

[Charley L. Bottger]

I now see that this has been hashed and debated for quite some time in a thread that I did not participate in. I'll let you and the others continue the debate, while I sit in and observe.

Jeff this went back even further, back into Jerry Peck days I had a dog fight with him once over this very topic. Its like bringing a knife to a gun fight it does not work.

[Jeffrey R. Pope]

I seem to remember that one too. Apparently, I wasn't paying much attention then either

[Joe Farsetta]

David,

Your points are well taken, and are certainly based on fact. However, while researching the subject, and trying to stress the importance of performing a non-invasive inspection, I have found that measurement the temperature split between the high and low pressure refrigerant lines at the compressor, and looking for between 15 and 18 degrees is pretty universal, even amongst HVAC specialists. This method has proven to be pretty reliable for a very long time. We are not engineers, and have limited time with which to work with while performing the inspection.

Again, I have spoken to HVAC folks in several states, with varying climates. 15 to 18 degrees is a good measure.

Here's a simple explanation:


The most relevant test of Delta T is conducted at the refrigerant lines. Here’s why:


This Delta-T measurement is the difference between the refrigerant temperature entering and leaving the outdoor A/C condensing unit. The volume of air crossing the evaporator coil results in a increase of heat in the refrigerant return line to the compressor unit. When the refrigerant expands at the evaporator, it becomes super-cool (just above freezing). As the air passes through the evaporator, heat is removed from the air, and transferrs to the coolant, warming it clightly. Though actually warmer, the refrigerant return line is still quite cold (50-55 degrees F)This differential in temperature between the warm refrigerant supply line, and the cooler return line is the Delta-T.

The split in temperature across the coil should be around 15° to 20°, measured at the compressor/ condenser unit.

This is a good diagnostic check because it measures the latent heat of condensation as well as the sensible heat absorbed by the vaporizing refrigerant in the indoor evaporator coil. In other words, it helps to determine if the system is charged and cooling properly.

Barring any other variable, such as overcharging, undercharging, dirty condensor coils, defective blowers, defective ducts, or dirty evaporator coil, this measurement is pretty indicative of a properly coolling system.

[David A. Andersen]

Joe,

I've been writing reports all night! Your post just made me cross eyed!
I will look at this more closely after some sleep!

At this point I agree with you measuring the line-set.
The reason I agree (besides what you stated), is because refrigerant temperature is relative to it's pressure. It is a sensible heat measurement.

If we dig back in the "dead sea posts" I offered a solution of testing the HVAC by just touching the line-sets. This seems to go right along with what you posted. It is a heck of a lot more accurate than measuring air temps.

You can "feel" the capacity of the refrigerant by just touching the line!
For those who have taken the Building Science & IR course, you know that we can feel temp changes as little as 0.001C when an IR camera only detect 0.2 - 0.05C

I have tried to make this simple for all to be able to do. Some just want to take chances.

I think I will take my chances and continue my way of inspecting and write up split is 15% and move on.
Works for me ,8 years and still claims free.

And how many of us speed on the highway and not get a ticket?

[cgoyette]

My simple version on this topic.

I take temp readings at the return and the FIRST duct into the home after the air handler, and the most distant register.

I look at the supply and first register to see if the system is cooling air and in a measurable way, I use the 14-22 deg guideline. If the system isn't COOLING in these parameters then a HVAC service is recommended. If it is in parameters then I state that. (It shows I checked the system also)

The most distant register is to see how well the ducting is insulated and how well the system moves air, temperature is taken and a dollar test is done (If it blows my dollar around when held up...simple but it works). If the temperature is way off 5% or more at the end of the run it may indicate a insulation of duct issue, and eyes on in the attic or crawlspace usually confirms this. But this too is just a guideline to help me figure out whats going on with the HVAC system.

Using a thermal Laser gives you a a number that helps you make a judgment call.....and thats what we get paid for.



Curt

[David A. Andersen]

Thanks Joe! I was hoping someone would come up with something else to consider.

I made a couple corrections where you obviously didn't say what you meant. I just don't want anybody to get off track. I hope you don't mind.

Originally Posted by jfarsetta

David,

Your points are well taken, and are certainly based on fact. However, while researching the subject, and trying to stress the importance of performing a non-invasive inspection, I have found that measurement the temperature split between the high and low pressure refrigerant lines at the compressor (I am sure you meant condenser, the high side of the compressor is wicked hot), and looking for between 15 and 18 degrees is pretty universal, even amongst HVAC specialists. This method has proven to be pretty reliable for a very long time. We are not engineers, and have limited time with which to work with while performing the inspection.

Again, I have spoken to HVAC folks in several states, with varying climates. 15 to 18 degrees is a good measure.

Here's a simple explanation:


The most relevant test of Delta T is conducted at the refrigerant lines. Here’s why:


This Delta-T measurement is the difference between the refrigerant temperature entering and leaving the outdoor A/C condensing unit. The volume of air crossing the evaporator coil results in a increase of heat in the refrigerant return line to the compressor unit. When the refrigerant expands (changes state; from liquid to gas) at the evaporator, it becomes super-cool (just above freezing)(40 degrees F at 70 psig). As the air passes through the evaporator, heat is removed from the air, and transfers to the coolant, warming it slightly. Though actually warmer, the refrigerant return line is still quite cold (50-55 degrees F) (10-15 degree superheat is expected)This differential in temperature between the warm refrigerant supply line, and the cooler return line is the Delta-T.

The split in temperature across the coil should be around 15° to 20°, measured at the compressor (see above)/ condenser unit.

This is a good diagnostic check because it measures the latent heat of condensation as well as the sensible heat absorbed by the vaporizing refrigerant in the indoor evaporator coil. In other words, it helps to determine if the system is charged and cooling properly. (Very good!)

Barring any other variable, such as overcharging, undercharging, dirty condenser coils, defective blowers, defective ducts, or dirty evaporator coil, this measurement is pretty indicative of a properly cooling system.

Even under these adverse conditions your Delta T will go outside of the parameters!

The best thing about this procedure is that you are taking sensible temperature measurements with sensible heat test equipment.

The refrigerant line set is like a liquid filled thermometer, it responds proportionately to the change of heat energy added or removed, not so with air measurements.

[David A. Andersen]

I take temp readings at the supply and the FIRST duct into the home after the air handler, and the most distant register.

By supply, are your referring to a distance down stream of the evaporator coil?