The link below will give you a chart to use. You’ll need an electronic thermometer that can measure both dry and wet bulb temperatures in the return and a thermometer to measure the supply air temperature. If the return air dry bulb temp is 76 and the wet bulb temp is 76 then the temp drop across the evaporator will be 6.5 degrees. Some will gawk why so low of a temperature drop, but at those db and wb temps the water would be almost fog. But it is a good chart to use. Target Temperature Split for Airflow
When someone takes a temperature at the return air grill or at a supply air register using an IR thermometer such as a Raytek, they are NOT measuring the temperature of the “air” but rather the temperature of the metal grill/register. That seems to be the thing that keeps getting missed in this never-ending saga of temperature splits. The IR thermometer can not and will not measure the “latent” heat in the air. You need a sling or electronic psychrometer to get the Wet bulb temp. I think David said that earlier. The latent heat can add as much as 20% to the “sensible heat” and if it isn’t detected can skew the hell out of this mystical 20 degree split everyone seems to be looking for. I do not remember who said it but trying to determine if the system is performing by using this method is like trying to determine the horsepower of your truck engine by measuring the temperature of the exhaust gases. Before anyone goes into a conniption fit over my use of the word “heat”, everything in HVAC is “relative” and is about the transfer of HEAT from one place to another.
To illustrate I took some pics at the closest supply register and at the return air grill. Note the significant difference in the two temps and also note the humidity at the two. If you really want to get into the physics of it, note that the humidity is “higher” at the supply registers than at the return. I also went and took the readings at the furtherest register and they were pretty consistant to the closest.
Very educational, thank you. Greenie
Curt
That would be 100% Relative Humidity (RH) … essentially almost raining in the house … just not realistic.
Common AC design parameters are return air of 75°F @ 50% RH, which gives you 62[FONT=Verdana]°F WB, and a target TD of 19.3[FONT=Verdana]°F. Or 70°F @ 50% RH, which gives you 58[FONT=Verdana]°F WB, and a target TD of 18.7[FONT=Verdana]°F[/FONT][/FONT][/FONT][/FONT]
[FONT=Verdana][FONT=Verdana]Even for 75[FONT=Verdana]°F @ 70% RH, which gives you 68[FONT=Verdana]°F WB, it’s a target TD of 14[FONT=Verdana]°F … and those are not considered comfortable indoor conditions (but possible if just starting up the system on a muggy day).[/FONT][/FONT][/FONT][/FONT][/FONT]
[FONT=Verdana][FONT=Verdana][FONT=Verdana][FONT=Verdana][FONT=Verdana][/FONT][/FONT][/FONT][/FONT][/FONT]
[FONT=Verdana][FONT=Verdana][FONT=Verdana][FONT=Verdana][FONT=Verdana]P.S. Dave … if you ran the examples with more realistic numbers (0% RH just isn’t realistic) you won’t see those kinds of differences. Although I understand the point you were trying to make, which is that humidity is a big factor. Just something to keep in mind if your indoor conditions are at the extremes of humidity levels (either very dry or very humid).[/FONT][/FONT][/FONT][/FONT][/FONT]
[FONT=Verdana][FONT=Verdana][FONT=Verdana][FONT=Verdana][FONT=Verdana]JMO[/FONT][/FONT][/FONT][/FONT][/FONT]
Target Temperature Split for Airflow
Thank you Gary!
A very productive approach to this subject.
That is why this subject should be rehashed.
The only problem, a wet bulb temp is needed and there are Inspectors that will consider spending $10k on IR, and there are those that won’t kick in for a CHEAP hygrometer for $35 (but still feel the need to “evaluate” HVAC performance).
That would be me!](*,) :bandit:
Mr. Edwards, that is correct. Another thing that is not considered, is emissivity. The IR thermometer is just like the IR Camera (for those that have been to school), the reading displayed is an “Apparent Temperature”. It must be adjusted.
I have read here where the IR Camera was verified with an IR Thermometer!
Actually, it is very realistic! Gary was talking about measurements “across the evaporator coil”, for us HVAC guys that means inside the A/C Coil. When you warm the 100% rh air from process equipment by discharging it into the room of a house, you must perform an “air mixture” evaluation.
As Doug was pointing out, putting the air through the A/C raises the rh seen in the supply air. But when it mixes with the room air the rh goes way down below the return air rh. This is why consideration of the rh (latent heat) of the process is so important!
Another time this must be considered, is when air infiltration occurs in the winter. Outdoor air at 30 degrees 90% rh enters the house and becomes 68 degrees 20% rh. Thus the use of humidifiers.
If your talking about me, Test# 2 was a very conservative but realistic set of numbers. My point about using 0% rh was to point out what is being measured by those that only use dry bulb thermometers.
Just used those DB and WB points to illustrate that a test on a system cannot merely go by DB temperature drop and rule of thumb tests.
Let me clarify. 100% RH “return air”, which is what the chart is based on, just isn’t realistic for an operating system.
While I agree there are limitations to taking dry bulb temperature splits, I still think it’s a useful indicator for a HI if done correctly, with a clear understanding of the limitations, and as a supplement to other observations (like feeling the suction/liquid lines at the condenser to get relative temperatures). I think the real issue is it’s rarely done correctly, or too much faith is placed on just the temperature split/drop.
It should be done with a probe type thermometer in the stream of airflow directly before and after the equipment (not with an IR of the grilles), after the system has been running for a while. And with a clear understanding of under what conditions (e.g. very muggy/humid) those readings will be off from the typical ranges HI’s look for. And with an understanding that just because you get a good split/drop under average operating conditions does not mean the system will effectly cool a house.
An unusually low/high split under average local conditions, after the system has been running for a while, simply means it may need further evaluation/servicing by a specialist. Thats it.
The point here is not to change anyones mind about the usefulness of taking (dry bulb) temperature splits (Dave and I would just end up agreeing to disagree again anyway … 
), but to give those taking those readings a better understanding of the right way to do it and the limitations to keep in mind.
JMO
As complicated as some of you guys want to make air conditioning, you would think that homeowners could never figure out that their equipment is working or not.
Bk what did I say at the beginning of this thread.
BK you would be surprised at the number of times hvac companies get called out on a/c’s that won’t keep up when the outdoor temps hit the 90’s. But they worked quite well in the low 80’s. And the system is less than a pound short of refrigerant. So unless the system has huge refrigerant loss or a mechanical problem the house will usually stay quite comfortable for a day or more pulling basement air through all the return leaks. All bets off for attic installs.
It’s quite simple. I tell the homeowner if his house feels warm, go look at the suction line, if it’s not wet go get a beer!
If the suction line doesnt feel like the beer, drink the beer and wait on the HVAC guy! 
The point: if you know what the refrigerant lines should feel like, it is a lot more accurate than air temps. And it takes no equipment, training, or certification.
If the suction line is warm, the system may be overloaded (if so the liquid line will be warm).
If the suction line is warm and the liquid line is cool, it’s under charged.
If the suction line is cool and the liquid line is hot, the condenser coil is dirty (even if it looks clean).
If you see ice anywhere, there is an air restriction and/or the system is undercharged (nothing can be determined until you get rid of the ice).
If the suction line is cool and you can not feel heat in the liquid line (it’s 98 degrees like your hand), your OK.
If you get it wrong, it doesnt matter because you are not required to make this assessment anyway. Seeing you only “laid hands” on the unit, how far outside the scope can you be accused of?
I try to make this as simple as possible, but no one wants simple!
Actually the correct method is easier and faster than going thru all the motions of determining an arbitrary split in temps that (1) isn’t correct in its basic assumptions (2) isn’t found or recognized in any HVAC technical manuals or publications that I have ever seen or heard of. When I first started HVAC training I asked my instructor about “temp splits” and he looked at me like I had three heads with horns. He is a working HVAC business owner and also teaches the EPA course at the local Jr. College so his credentials are impeccable. He was kind and didn’t tell me I was full of crap but explained as best he could due to my newbieness that the method I was describing would tell me little if nothing due to the huge number of other variables in play. He is also the one who pointed out I would getting the temps off the metal grills and registers and not the air.
The very first day and first lesson my instructor did was hold up his hands and make the statement, “These are going to be one of the most important tools in your arsenal”. “Learn to rely on your sense of touch and feel as much as your eyes, ears and instruments.”
You are right, it is not hard, but wrong is wrong. The short list above that David posted is actually more than a HI needs to know but is a valuable tool to nail down those times when he/she is in doubt about the piece of equipment. I believe if you use those guidelines you will find it easier and may avoid paying for an unnecessary service call. I almost always have to call for service calls due simply to the fact that most people neglect the system so much it is overdue and filthy. I would say roughly 8 out of 10 have never been service since installation. Evap coils and circulating fans are severely clogged up, drain pans and lines are clogged or restricted, or someone didn’t know beans from wild owl droppings about how to build a good duct system or wire the system components. HVAC equipment is one of the most expensive pieces of equipment in the home and yet the most neglected (cheap filters and never serviced). Most people just let them run until they quit leading to such brillient statements like. “It was running real good till it quit!”
Who runs the ac in the low 80’s? 
lol, BK, I think he is referring to the outside air temp being in the 80s, but you would be surprised at what some people will do to save a quarter. Had a lady ask me if I was going to pay for the electricity and water I used during a home inspection once. She had her water heater on a light switch (which I wrote up).
Were getting more technical than a H/I ought to be. Most H/I’s don’t have the background that some of you, and in part I do, and I’ve been out of the HVAC business since the late 90’s.
A rule of thumb is 16 - 20 TD. If we try to calculate all the what if’s, rule of thumb goes out the window.
Rarely do I check or disclose the TD of a system. I would defer to an HVAC expert to inspect, calculate and verify the operation of the system. The expert will also be familiar with his states “Energy Code” to help him determine design vs. operating conditions.
We keep trying to step outside our background/expertise, were gonna get bit.
David,
Where can I get one, CHEAP hygrometer for $35, I just had another Tramex w/dewpoint crap out on me and tire of coughing up about $150 S&H everytime this happens?
Luckily PE is standing behind it only 3 months old but a backup would be nice at that price.
Thanks