A commercial HVAC person and I were discussing temp. differentials and he stated that residential units putting out conditioned air at 57-59 degrees at the plenum were functioning properly. After that the differential had to do more with good/bad design and installation practice. This makes sense to me…I rarely get 18 degrees differential some older houses I’m glad to get 12-13 degrees.
A method of determining if the a/c is delivering the btu’s is measuring return and supply wet bulb temperatures. The thermometers used need to read out in tenths of a degree. Plot those two temperatures on an enthalpy chart. Subtract the lower reading from the higher = delta enthalpy. Multiply 4.5 x CFM (airflow) x Delta enthalpy = delivered BTU’s. The rep that presented this method showed that no matter what size unit if the delta enthalpy is close to, don’t run for the hills, 6.66 the unit is running close to capacity.
This does not mean that air infiltration/exfiltration from leaking ductwork is not adding to extra load to the system.
Gary, Any chance you could translate that from HVAC speak, into a language us common HI’s could understand. I don’t have a clue as to what you just said…
Come on Gary, I can’t get them to use the “cold beer” process!:mrgreen:
In speaking with a refrigeration tech, I was informed to use one of those thermometers that you may typically see in a mechanics shirt pocket and check the temp comming from the first vent in the distribution line. He cautioned to stay away from using infared thermometers as they can many times be very unrelieable.
he stated to place the thermometer and leave it for awhile.
Difference in temp from room to room is more related to system design, duct layout, air flow. A room on the end of the system may need a inline fan installed to assist is the distribution to that room. Ducts traveling through unconditioned area, or hotter areas will provide less cool air. In these cases insulation is important.
it is currently 112 here today…It better get cold in each and every room or something is wrong with the AC…
And I really do not care if it is the ducts, entropy, design, etc… and neither do my clients… 
They just want cold air…
I would say that lowering the attic temp is more important than insulation…
Before any insulation contractor replies; brush up on Q = U * A * Delta T
Yes, your were informed “wrong”.
Anyone know the best type thermometer to measure the temp of a fluid (maybe we should start with “what is a fluid?”).
Exactly! I use a thermal camera, just so I can show them that cold air and hot air was coming out of every register. They know I image all hot water, a/c, motors, dishwasher, stove/oven and electrical panel. With and image they can not prove something was not working.
John, I’m considering getting an IR camera partly because of some of the things you mention above. But what would you be able to see on a dishwasher? I run them through a cycle but I can’t figure out what the cameral would tell you?
Ok will try. Attached is a link to a wet bulb enthalpy chart. http://www.udarrell.com/wet_bulb_enthalpy_chart.pdf
Using an electronic wet bulb thermometer measuring the return and supply air temperatures. In this case the wet bulb temps are 51.0 and 61.5. The enthalpy for 51.0 is 20.86 and the enthalpy for 61.5 is 27.50. In this case the difference is 6.64 delta h. If this is a 2 ton system, 24,000 btuh, the air flow should be 800 cfm 4.5 x 800 x 6.64 delta h = 23,904 btuh. If the system is a 4 ton system, 48,000 btuh, required airflow would 1600 cfm. 4.5 x 1600 x 6.64 delta h = 47,808 btuh. the same 4 ton with a 6.66 delta h 4.5 x 1600 x 6.66 delta h = 47,952 btuh. As long as the airflow is correctly set on the unit for the tonnage, ie. high speed is not always cooling speed, if the enthalpy change is 6.66 the unit is delivering full btuh. If the air flow is set too high or too low for the tonnage the enthalpy change will not fall close to the 6.66 delta h.
If we where designing not confirming if it works all this information would be ok.
If we did that we would be charging a lot more.
Check Suction Line ( beer can Cold)
or if you prefer thermometer
Check supply close to the unit
check return air
check humidity if you wish
check temperature coming out of the outside unit.
Rule of thumb 20inside 30 temperature outside.
Move on
Feels comfortable in all rooms ( great)
Long and short Boys Leave the designing to the experts .
You do not have gages or the knowledge . Pressures, temperatures , super heat , CFM, size of ducting, Insulation , size of duct work can effect operation.
2 to 4 hours on a home inspection would not even be enough time to get this done.
Use the KISS method.
Agreed.
I perform a good visual inspection of the HVAC equipment. But more importantly, do the rooms get hot and cold as intended? Are there audible (??) and physical deficiencies of the installed equipment? Are there visual safety issues?
Also, as HVAC equipment is one of a home’s more expensive components, is a more detailed or in depth evaluation needed based on: age/condition etc.
I commend Gary’s attempts to enlighten and bring us out of the dark ages!
Though this bunch of numbers looks like hieroglyphics, it is very easy to standardize these formulas.
Don’t want to memorize the equation? Put it in a spreadsheet! Do it once and you don’t have to worry about it ever again. Just plug in the numbers and you have a result. You can do this on your telephone!
I have a spreadsheet that takes care of this scenario. I hate mathematics, but it is a necessary evil.
The most important point in Gary’s post is the use of a wet bulb thermometer. If you don’t own one, you should move as far away from HVAC inspection as possible.
The HAVC equipment does not function solely on dry bulb temperature. It also works on latent heat removal (that’s that water that pours out of the white tube).
Wayne has a good process when you’re not taking temperature readings. I don’t concur with his “designing versus confirming” scenario but his process is on the mark (so long is your not pulling out any thermometers). I can evaluate an HVAC system with 99% accuracy using Wayne’s process and I recommend everyone consider learning it.
However, if you insist on bringing out a thermometer, I highly recommend you get online with Gary’s approach!
You don’t even have to go out and buy a $300 hygrometer to perform the process. You simply need a small cotton sock to put over the end of your thermometer, a small bottle of filtered water to dampen the sock and a psychrometric chart (which you need anyway even if you have the expensive tools).
Just think of it! Gary’s process actually tells you the number of BTUs per hour the equipment is performing at!
You can compare his numbers with what you took all that time learning to determine the age and BTU capacity from the equipment data plate.
You combine Wayne’s approach with Gary’s approach and you can actually evaluate the air duct system!
It is so tiresome to read posts where inspectors refused to do the mathematics on the basis that they are not performing a “system analysis”, but in the same paragraph talk about pulling out a thermometer which is a tool to perform “analysis”. The problem is, the thermometer can cannot collect information needed to derive any conclusion as to whether the equipment is operating properly (without knowing how the system was initially designed).
HVAC contractors use the dry bulb thermometer temperature differential to assess performance because they know how they designed the equipment. They know that the air duct system is not leaking (because they just installed it). They know the system is operating properly because they have the refrigeration gauges hooked up to charge the system with refrigernat. They know the airflow because they have the equipment to test it. They checked the amperage draw on motors. So I’m not saying that this “rule-of-thumb” is totally without merit. It’s just that when a home inspector shows up on site with nothing more than a thermometer, they have absolutely no other information to base their assessment.
I don’t understand why it’s so hard to disclaim HVAC evaluations in a home inspection. We simply do not have the tools or authority to perform this task.
By disclaim, I do not mean that “further evaluation by a licensed HVAC contractor” is required. Simply state that in accordance with the standards of practice, you turned on the HVAC unit via the thermostat and recorded your observations.
If you’re in the house for two or three hours with the air conditioner turned all the way down and you’re not noticing enough comfort while conducting your inspection activities, then maybe you should record this observation in your inspection report.
Trying to do so much with so little is simply unthinkable. I think the major problem is home inspectors don’t like to say that there’s something they can’t do!
My point was that with the use of the wet bulb thermometer and the use of the wet bulb enthalpy chart if you come up a differential enthalpy close to 6.66 the system is performing at capacity. For a quick check a 6.66 delta enthalpy is close enough if one wants to calculate out the btuh then one would need the air flow measurement as well and perform the calculation.
That is exactly how Home Owners determine if their system is not working…
This is a great post and I appreciate all the information. I’ve always used the thermometer method and looked for the 16 to 22 degree differential, as I was taught. But just a few weeks ago I came across two different past clients. One told me they had to get the AC serviced as the house wasn’t cooling good enough, even though I observed an 18 degree differential and said the system was okay. Another, which I only had a 12 degree differential, I called out as needing service and the HVAC guy came out and said it was fine. After that, I knew I needed a better method of inspecting the system.
I’m okay with using the simple method described by Russell Ray and David Anderson. Would you guys mind sharing your verbiage on how you would write in your report that an AC system wasn’t cooling enough. It’s nice to have tools to provide the evidence in the report. If tools aren’t used, I’m finding it difficult to come up with the verbiage that a system needs service based off of, well a feeling…if that makes sense. Thanks.
I now reccomend that the client ask for the latest service records, if there are no service records, or the service records are more than one year old, I recommend the Unit/s be serviced by a HVAC Contractor…
AC guys here… well… they come in all degrees skill and integrity…You never know just what they will say…
The temperature; You can “see” that the dishwasher is hot in areas, you can “see” the water drops cooling.
You can then prove that there was water in it and it was hot.
I like it for running hot water, in the basin and in the drain lines.
I just make sure it is cold out of the vents.
[quote=“bkelly2, post:37, topic:50102”]
I now reccomend that the client ask for the latest service records, if there are no service records, or the service records are more than one year old, I recommend the Unit/s be serviced by a HVAC Contractor…
Good verbage…will add it…