Thread: Measuring temp differential

What is the best, or at least a good, way to measure the plenum temperature. I tried an infrared but if I aim directly at the sheet metal the reading is incorrect. I get a more believable result if I stick a piece of tape on the plenum and aim at that - but I don't know whether that is reasonably accurate or not.

Thanks,

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Originally Posted by Peter Wigle

What is the best, ..., way to measure the plenum temperature.

A good old fashioned dial or digital thermometer. One that reads air temperature.

Like these ( pocket thermometers, dial pocket, digital pocket, glass pocket, probe pocket thermometer ) and available almost anywhere.

Originally Posted by Jerry Peck

A good old fashioned dial or digital thermometer. One that reads air temperature.

Like these ( pocket thermometers, dial pocket, digital pocket, glass pocket, probe pocket thermometer ) and available almost anywhere.

But, I think if you do some searching on the board and archive, you'll find some discouraging words about using temp differential as an indicator that all is well, or not. Too many variables affect TD, is the main argument, I believe.

Originally Posted by John Arnold

But, I think if you do some searching on the board and archive, you'll find some discouraging words about using temp differential as an indicator that all is well, or not. Too many variables affect TD, is the main argument, I believe.

John,

That is absolutely correct.

But IF you are going to waste your time doing it ...

... at least do it with something which measures the "air temperature", not with something which measures "surface temperature".

Originally Posted by Jerry Peck

But IF you are going to waste your time doing it ...

... at least do it with something which measures the "air temperature", not with something which measures "surface temperature".

If you don't correct for emissivity of that surface you don't know what you are measuring.

By all means shoot black electrical tape rather than the sheet metal because that will give you a much more accurate measurement with an infrared device.

Peter, what are you trying to determine and why? So many variables, esp. for someone not trained as an HVAC tech. Where to sample, what equipment, when in the heating or cooling cycle, etc, etc. Are you looking for the delta T or just plenum temp.?

Originally Posted by Bruce Breedlove

If you don't correct for emissivity of that surface you don't know what you are measuring.

Does not matter. See below (and my post above) for why.

By all means shoot black electrical tape rather than the sheet metal because that will give you a much more accurate measurement with an infrared device.

It does not matter because, when measuring temperature drop *OF THE AIR* ... one should measure the temperature of "the air", not of a surface being heated or cooled by that air.

Thus, there is no need to bother with corrections for emissivity, reflected background temperature, etc., on "just needs to use the correct tool for the correct job", in this case use a thermometer which measures "air temperature" instead of one which measured "surface temperature".

Originally Posted by Jerry Peck

It does not matter because, when measuring temperature drop *OF THE AIR* ... one should measure the temperature of "the air", not of a surface being heated or cooled by that air.

Thus, there is no need to bother with corrections for emissivity, reflected background temperature, etc., on "just needs to use the correct tool for the correct job", in this case use a thermometer which measures "air temperature" instead of one which measured "surface temperature".

Excellent post Jerry, you nailed it.

If you want to provide your clients with a little more value take wet bulb temperature drops at the equipment in addition to just dry bulb readings.
Dry bulb readings alone are useless in a cooling application.

Originally Posted by John Arnold

But, I think if you do some searching on the board and archive, you'll find some discouraging words about using temp differential as an indicator that all is well, or not. Too many variables affect TD, is the main argument, I believe.

We, here in TX, are required to measure the differential. I have 2 digital thermometers with magnets. They stick so well that sometimes I leave them at the house....aargh.

I do understand that the measurements are not a great indicator for the HVAC and explain that to my clients.

I have used my IR thermometer and digital simultaneously to see what kind of difference there is. The outcome: the readings were close.

Bruce

OK, I am not HVAC trained, so when the outside temp is 95 deg. and I need to know if the a/c is cooling properly, what do I do?

What I have been doing, as suggested by local HVAC professionals, is checking for a TD of between 15 and 20 deg. using a manual thermometer.

I drop it in the register and get a reading, and then stick it in the return for a reading. I leave it in each for about 10 min. with the a/c running the entire time. The a/c will have been running for at least 30 min. before I start.

I just got an email from a client about this very thing.
The HVAC guy (a different one) said everything was fine.
I got a 10 deg. TD when I checked it.

What else can I do to make sure the a/c is working properly?

Originally Posted by wes owens

OK, I am not HVAC trained, so when the outside temp is 95 deg. and I need to know if the a/c is cooling properly, what do I do?

What I have been doing, as suggested by local HVAC professionals, is checking for a TD of between 15 and 20 deg. using a manual thermometer.

I drop it in the register and get a reading, and then stick it in the return for a reading. I leave it in each for about 10 min. with the a/c running the entire time. The a/c will have been running for at least 30 min. before I start.

I just got an email from a client about this very thing.
The HVAC guy (a different one) said everything was fine.
I got a 10 deg. TD when I checked it.

What else can I do to make sure the a/c is working properly?

Well, IMHO taking TD does not tell you much.

I stopped taking TD's many years back. What I want to feel is cool air shooting out of the registers. I like to see temps in the mid 50's blowing out of the registers. I also like to see temps at the coil in the 40's.

I know, this is not what you were taught in the schools! But, you know what? It is logical and it works!

On a 95f degree day with RH in the high 70's and a room temp around 74f, I would be happy as a tick on lazy dog if I could get temps out of the register in the low 60's. And if I can get around 55f, that would be great. But if I see temps down in the 40's that is too cool or up around 65f-70f then we have a problem.

Bottom line is if the air does not feel cool on your hand then the unit has a problem!

Experience is the best teacher you can get

I agree with you about feeling the air, but my concern is what if the client moves in and says it's not cooling right and wants to know how you checked it to determine it was ok?

I figured with the TD, I would at least have something to defend myself with.

Having said that, checking TD's could (per the email I received) cause a disagreement between me an the HVAC guys.

This was the situation:
Average temperature at registers was 58º F.
Temperature at return was 68º F.
Temperature difference was 10º F.

The outside temp was 90F

The HVAC guy said he checked the system out and everything was fine.

I said it was not cooling properly.

Now we have a problem.

You have to remember that TD is relative. When you got the 10 degree differential, was there high humidity? That affects the "normal" range. Was it a "high efficiency unit?
No offense, but 10 degrees td may be normal under some circumstances, only knowledge and experience will tell.

Peter, the bottom line is the AC getting cold? Regardless of how hot the return air is. If you walk in a house that is 85 degrees and turn down the AC to 60, you can assume that 85 degree air is going through the return air, if the thermostat is set at 72 degrees when you walk in then you can assume that 72 degrees is going through the return. If the AC runs for 30 minutes its not going to get much colder at the A-coil than it is after 30 minutes. Just because you set the thermostat to 60 does not mean the AC is going to get colder, its just going to run longer to try to meet that temp. If the supply air coming out of the ducts is under 65 degrees then it is a good assumption that the AC unit can cool the house to 65 degrees if kept running. Any thing over 65 degrees may not cool a house in 100 degree weather. Many other factors may play apart such as insulation and windows but if the AC can't produce at least 65 degrees then it most likely needs service.KISS Keep It Simple Simon

My opinion. Use your IR Thermometer at the return and supply initially. If the temp differential is lower than the norm then there is something going on with the system.

!0 to 12 degrees is pretty much the norm on the 13 seer and up units. Anywhere from the low end of 15 up to 20 degrees (never gets there) on the upper end.

Almost all will say it is not telling you anything. I say, (uh oh, gonna get it now) that is bull, it will give you an excellent starting point to look for further concerns. If you have a 12 degree differential in the newer units your system is most likely in pretty good order. If you have at least 15 degrees or better with the older systems they are most likely in pretty good shape as well..

Jim nailed it perfectly with the high winds and or high humidity you will generally get lower TD.

If you have low reading then (you will anyway I hope) dig deeper to find air leaks in the HVAC unit, bad duct work, no attic ventilation or very poor, dirty coils inside or out, low charge in the condenser etc.

It is an excellent way to start. To put a thermometer up every orifice in the home, return and supply, is ludicrous and a waste of time. You IR thermometer will work just fine. Point it at he grill if you wish but point it at the grill of every vent. You will get an excellent tell from that point. Temp is temp. Just take it all like you did the first one.

Oh yheah. Just my opinion, but it works and have done it for years.

If you have crappy old leaky poorly insulated metal duct work the differential willm be very low on the AC end of things.

I stopped taking TD readings a few years ago. A waste of time for the most part. There are too many variables to consider. Even if you get a good TD reading that you have confidence in, all it may mean is that the system may be working.
The real point is (and I know I will get feedback on this) whether it is working effectively or not.
When I get to the house I turn the AC/Heat on almost immediately. My concern is whether or not the space is coming up or down to temperature while I am doing my inspection.
As an example, I did a house last winter, 3.5 hour insp. 3-4 degree temp rise in the house with the unit running the entire time. Mr dumbass flipper put in a new furnace and some ductwork without having a clue as to what he/she was doing. My client bought the house after getting credit to properly install a whole new HVAC system.
Does the system bring the space up or down to set temp or not in a reasonable timeframe? That is what I look for. TD for me is not the primary concern.

You kinda back youself into a corner when you start putting numbers on items that are inspected. I have found it is best to stick to general terms.

OK, I'm trying to get a handle on this.

If the outside temp is 90 - 95 deg., do you run the heat?

If so, it will surly get up to the temp setting at the thermostat with no problem. And fast.

But, the a/c probably won't hit the 60 deg. mark because (as I always been told), you can't expect more than a 20 deg. difference between the inside and the outside temps on a hot day.

So lets try and simplify.

First, you walk into a house on a 90 -95 deg. day with the owners home, or the buyers present and you run the heat? If so, do you take a temp. reading or do you wait for it to hit the mark on the thermostat?

Then, you run the a/c with the thermostat set where? Or do you take a temp reading or feel the air blowing out of the registers?

Lastly, you sum it all up with results based on what as far as the client is concerned, providing they ask?

Believe me, I'm not trying to be difficult.

I'm just trying to find the simplest way to check a system that will also satisfy my client, as well as cover me the first time the a/c isn't cooling on a dog day after my client moves in.

Originally Posted by wes owens

OK, I'm trying to get a handle on this.

If the outside temp is 90 - 95 deg., do you run the heat?

It depends. If it is a heat pump NO. Conventional furnace, YES.

If so, it will surly get up to the temp setting at the thermostat with no problem. And fast.

But, the a/c probably won't hit the 60 deg. mark because (as I always been told), you can't expect more than a 20 deg. difference between the inside and the outside temps on a hot day.

So lets try and simplify.

First, you walk into a house on a 90 -95 deg. day with the owners home, or the buyers present and you run the heat? If so, do you take a temp. reading or do you wait for it to hit the mark on the thermostat?

Why do you want to take a temperature reading? You are looking at the burners and how they are burning. If all of the burners are working and I don't see anything that looks screwy, IMO the system is working.

Then, you run the a/c with the thermostat set where? Or do you take a temp reading or feel the air blowing out of the registers?

Lastly, you sum it all up with results based on what as far as the client is concerned, providing they ask?

Believe me, I'm not trying to be difficult.

I'm just trying to find the simplest way to check a system that will also satisfy my client, as well as cover me the first time the a/c isn't cooling on a dog day after my client moves in.

OK if it is 95f outside and the system is cooling the house down to 68f, like you said the return air was. The blasted system is working fine. Even if you have only a 10f TD. It's not going to get any better. If a system can cool down a home 25 degrees less than the outside temperature then it is doing good.

In my report, I simply report that the system was cooling or it was heating. I stay away from specific numbers, they can get you into trouble. If you report that it was cooling or if you report a TD of X number of degrees, what is the difference if the system craps out the day after your client moves in? NO, difference at all. They are still going to call you and their agent. One good reason to always recommend a home warranty. You can always fall back on Didn't you take out the home warranty like I told you to do?

When you take temp readings you are not by any means giving any definitive answers to your clients from those readings. Those readings are for you, no matter how you take them. It just gives you a round about, hmm ok, This is where I am at so lets see what is going on. Taking the temp of a supply vent dead center on the metal or up into the duct, take it the same place on every vent. Sticking a thermometer in every vent is not going to get you anywhere any faster or any better. It is an average. If the master bedroom vent reads 65 and the bedroom next to it reads 72 then you know you have other concerns to check out as why the big differential between the supply vents. If all of them are within a few degrees you are starting off in pretty good shape as far as even temp throughout the home. If there is a 5 degree difference between the return and suppl then you know you have more serious concerns going on.

It is not rocket science. It is a generalised starting point. You do not need a $3,000.00 millionth degree accuracy.

That of course is only one of a multitude of things to take into mind. The temp is not a big hang up but it is tremendously useful on giving you a better idea of things that may be going on.

I don't care if it is 88 degrees in the home. You turn the heat on. Yes it is going to get hot. If the air coming out of the supply is only 95 degrees you know you have a concern.

It is just part of the overall picture.

Home inspection is like a painting. You do not get the whole picture until you finish your entire inspection.

Today I did an inspection on a new home. The inside temp started at 83 and never went below 81 in over three hours. The two unit looked great, good install everything appeared to be in place and functioning. Of course being a new home the temp TD at the return and supply would be lower than an older home (nothing exact boys, calm down) The TD was only 8 degrees. A decent running system on the hundreds and hundreds (who knows how many) of new systems I have inspected is about 12 degrees. 8 degrees doesn't cut it.

The suction line connection at the outside of the AC condenser wasn't even breaking a sweat. Its temp was about 86 degrees. Doesn't cut it. I wrote the AC condenser up for repair. New install. Probably not enough juice or a pin leak somewhere. Outside air temp low 90's.

Not brain surgery, just the big picture. Heat worked like a champ. All duct connections tight. Unit sealed up well. No found air leaks etc. etc.

I am not finished with my painting until the entire canvas is covered.

Scot'\t

"what do you want to take a temperature reading for"



Well if you have 68 or there abouts in all the supply vents and 75 at one of the bedrooms. It will tell you of a concern to look for in the attic such as a bad duct connection or pinched off duct or who knows what. To not take temps at every return duct is a mistake.

I inspected a home where one of the duct had to go down then back up because of the framing config. That duct was also the lowest coming out of the plenum. It was filled with water. Practically no air coming out. If I did not take a temp reading I would not have found it. When the AC repaired the system he emptied 3 gallons of water out of that duct before replacing it.

That is only one of the reasons everyone should take temp readings at all vents. Return or supply.

I need someone to educate me here. It seems to me that what is most important is the temp at the supply. If the temp is 55-60 then its working properly. What does it mean when the temp at the return is 15-20-25 higher than the supply? Why would it be that much higher? Does this tell you there is an insulation problem? Also I've been told that you do not turn on the ac after having the heat on. You should wait at least an hour so you don't cause damage to the ac. Thanks, I'll hang up and listen.

If it is summer and the house is already cool when I get there, I open the doors to warm it up while I get situated. Once it warms up into the 80's I will turn the AC on while I am doing the insp. I have multiple temp meters, one stays on the floor by the stat (just to see if they read similar). Stat gets set to 70. Later I bring it down to 67 to see if it can get there. If it isn't at/near 70 within 20 minutes then I start looking for problems. I do check temp in the various rooms as I insp them. I may stick the probe in the register from time to time just to see.
If the house isn't cool when I get there, well then I just turn it on and see what happens.
Later when I look at the furnace I will run it on the heat side for a few to see if it works, etc.
In the winter of course I don't turn on the AC, just look it over to see if all checks out. The furnace gets run during the insp. Stat gets set to 74, then 80 for a bit to see if it can get there.
If the system can't get to set temp then it's probably the system; unit, ductwork layout etc.
If the system can't get to set temp in a reasonable time, then it could be other factors such as drafty windows/doors, poor/no insulation, open FP damper, etc.
I used to have a temp diff line on my reports and took it out. Customers don't know what it means. It isn't a (usually) relevant number. Let the HVAC tech do that when he's trying to figure out why the system is X?.
The important issue is 'customer comfort'. When customer's aren't happy, they may choose to make bitchy phone calls. Therefore, run the system, check room temps, ask yourself if the place is comfortable during the insp. That way you can point out rooms that aren't heating/cooling and it won't be you getting the phone call.

Scott, why is temps in the 40s a problem?

Originally Posted by David O'Keefe

I need someone to educate me here. It seems to me that what is most important is the temp at the supply. If the temp is 55-60 then its working properly.

David,

Let's start with a simple example to show the answer to that question.

Assumed example: air coming out the supply is 55-60 degrees (from your question)

Q: Without knowing what the return temperature is, how do you know that is working?

A: Because it is blowing 55-60 degree cold air out.

Q: Now let us presume that the inside of the house is 60 degrees. You have supply air which is 55-60 degrees. Is the system cooling? No, it is only blowing air through it.

I know that was very simplistic, but if you do not know what the return is, how can you say it is cooling just because you are measuring cold air at the supplies? You can't.

What does it mean when the temp at the return is 15-20-25 higher than the supply?

All that means is that the air entering the AHU is being cooled down 15-20-25 degrees, which means 'it is working to some extent' - simply because 'it is' cooling the air.

However, you can have a dirty coil, restricted air flow for any reason, and the TD (temperature differential) between the return and the supply will give a false high reading than it would with a clean coil and no air restriction. Thus, just because it has a high TD (temperature differential) *does not mean it is working properly*.

In fact, you could have a new unit with the fan speed set too high, blowing too much air too fast over the coil. The TD would now be reading lower than it should. I had that happen in a house of ours in South Florida. The HVAC contractor brought in the factory representative to convince me that was not the problem and to find out what the problem was - the first thing the factory representative said when he stepped in the front door was "The fan speed is too high." But that was not the only problem.

Now take the case of a properly set fan speed, the unit is working correctly, then the supply duct comes off in the attic, there is now too much air moving too fast over the coil, yet the fan speed is not the problem. The low TD told you almost nothing.

Why would it be that much higher?

Under the right conditions (relative humidity, fan speed, air flow, etc.) the typical ac unit will reduce the temperature of the air "about" 20 degrees "across the coil". The further from the unit you measure the return and the supply, the more skewed your readings will be, and being as they start out telling you almost nothing, they now tell you even less.

Also I've been told that you do not turn on the ac after having the heat on. You should wait at least an hour so you don't cause damage to the ac.

Never heard that one before.

Now, for what Ted is recommending, and for the reason Ted is recommending it (and only for that reason), Ted's recommendation is a good one. I used to do it.

The ONLY reason for doing it is to 'compare' room-to-room-to-room temperatures, nothing else. If all the rooms are close in temperature, 'mostly likely' there are no duct work issues. However, if there are one or more rooms 'not like the others', that is an indication of 'something is not right' - but it does not necessarily indicate a duct problem, it could just be an 'amount of air' problem. Talking about measuring 'room air temperature' here.

If you want to start shooting the supplies with an IR thermometer, and I did that too, you should be able to measure the temperature drop as the air travels further from the AHU. Each further supply will not be as cold as the next closer one was (providing, again, that there are no air supply problems). Now, if you get a supply reading which is way off the others, that *is* an indication to look for things, including ductwork, but it may not be ductwork.

All you are doing is 'adding to what you do in order to try to determine *what seems odd* when going through the house'. Those things *which seem odd* are usually indications of problems.

As far as measuring TD goes ... I stopped bothering to do that years before I retired, why do something which as little to no meaning? You can use the TD to say 'something is not right', but never use it to say that 'it is operating within normal parameters'.

Like a cancer test, it can give you a "false negative", but if it says you have cancer, you have cancer. Use the TD the same way if you are going to bother to take a TD: a low or high TD "indicates *something* needs to be checked out", a TD within the typical 15-22 degree range "indicates *nothing* of consequence".

Jerry, Thank you for the education. I'm going back to the source that told me to wait before turning on the ac,I most likely misunderstood him.

David

Jerry,

If you would be so kind as to briefly run through a scenario as if you were performing the inspection.


You arrive at the residence, which is a 1 1/2 or 2 story home.

The downstairs has an electric packaged heat pump, or a gas pack.

The upstairs has an electric split system unit with the air handler in the attic.

The outside temp is 90 deg. with 50% RH.

GO:

Originally Posted by wes owens

GO:

I operate both units and look at both units.

'Looking at them' tells me more than 'operating them' does.

Originally Posted by Tony Mount

Scott, why is temps in the 40s a problem?

You start to get into the "supper cooling" area when you hit the 40's with the air at the registers.

Originally Posted by Scott Patterson

You start to get into the "supper cooling" area when you hit the 40's with the air at the registers.

Tony,

If the supply air temperature is 40 degrees, you can rest assured that the coil temperature is less than 35 degrees, and as the coil temperature approaches 32 degrees (between 35 degrees and 32 degrees, which is not much difference) the coil begins to frost up. Once it begins to frost up, the frost restricts the air flow, exposing less and less coil to the air flow, causing the coil to frost up into a block of ice. That's not a good thing.

Most AHU are designed (coil size, refrigerant charge, air flow rate, etc.) to produce a TD around 20 degrees, plus or minus. Ac techs measure the "superheat" at the refrigerant lines, not an air TD across the coil.

Originally Posted by Jerry Peck

Most AHU are designed (coil size, refrigerant charge, air flow rate, etc.) to produce a TD around 20 degrees, plus or minus. Ac techs measure the "superheat" at the refrigerant lines, not an air TD across the coil.

At what location?

Originally Posted by Charles Sessums

At what location?

At what location for what? "Super heat" measurements?

Typically this is done at the refrigerant lines where they enter/exit the AHU enclosure, just outside the coil where the coil refrigerant lines were stubbed out and the 'site installed' refrigerant lines are brazed to them.

David, in addition to Jerry's detailed response.
Think about the temp variance at the return depending on where that return is located and how long the system has been running. You will probably get a 20-25 degree higher reading if it's a high return in a 2nd floor. A floor return on the 1st floor will likely have a lower spread. Another example of why to forget about temp diff for a standard HI.
Wes, your post was not directed at me, nonetheless I find it's wording a little odd. What's your point? Are you trying to bait Jerry or are you just a newer HI. Just wondering.
Poor Peter, he got his thread hijacked and haven't seen him since.

Originally Posted by Jerry Peck

At what location for what? "Super heat" measurements?

Typically this is done at the refrigerant lines where they enter/exit the AHU enclosure, just outside the coil where the coil refrigerant lines were stubbed out and the 'site installed' refrigerant lines are brazed to them.

Yes and thank you

Originally Posted by Scott Patterson

You start to get into the "supper cooling" area when you hit the 40's with the air at the registers.

And for those of us who like to eat our supper while it's hot, that's not a good thing.

"Wes, your post was not directed at me, nonetheless I find it's wording a little odd. What's your point? Are you trying to bait Jerry or are you just a newer HI."

Neither.

First of all, it's not odd. If you read it carefully, I was staging a normal inspection for me and wanted Jerry's input.

A step by step of what Jerry would do.

I have had, and continue to have HVAC's telling me and others different ways to check this very thing we're discussing.

I was just asking about a quick, simple way to determine if the a/c is working properly.

Even the more experienced guys on here have different ways of doing that.

That leads me to my point.
Jerry probably has more experience on this board than anyone, and I value his opinion. Thats why I was asking.

Besides, Jerry's to smart to be baited.

I must be missing something here ...
- Turn stat to AC and set temp
- Is condensing unit running
- Is air handling unit running
- Is cold air coming through duct and out vents
- Does space come to set temp after a reasonable time (20 minutes in my book, varies depending on factors)
If yes, system works
If not, look deeper so you can provide client with relevant info so they don't get screwed by some BS service tech.
Want to spend time checking for superheat/supercool, temp diff, go ahead. Waste of time but knock yourself out.
Want to provide a useful service for the client? Check duct leakage where possible, return placement, are there any high returns; are the ducts at the perimeter walls actually supplies and not returns, lift some floor registers (is the boot/floor connection sealed or is air going into the joist cavity instead of the room) etc.
Now you are inspecting things that matter to the client. Of course you also have to look at the equipment.
If it doesn't bring the space to set temp it doesn't work. Refer to hvac tech

I agree with that, but your comment

"Does space come to set temp after a reasonable time"

leads to earlier question.

On a 95 deg. day with mid to high RH, what would you set the stat. at?

No matter how well the system is working, it's not going to cool but so much on a blistering day.

So if the stat. is set too low, it will never reach it.

Like I said before, I'm not trying to be difficult.

I'm just asking how others check it because if there is an easier or better way than what I'm doing, I want to know so I can change how I do it.

From what I've read on this post, several guys have several different ways of doing it. Even the seasoned ones.

"Does space come to set temp after a reasonable time"

Well, on a Summer day it can run for hours and not drop 2 degrees in a vacant house. That test will only work in certain areas, or in a dwelling that has a operating or functioning system, that is in use.

On a 95 deg. day with mid to high RH, what would you set the stat. at?

1 or 2 degrees less than the currently set temperature for starters. See what happens while you go about your business, and then decide what steps to take next. There are many possible outcomes.

Dom.

For all the people saying it is a waste of time to take temperature differentials, the bottom line is that a temperature differential is taken by everyone who evaluates the function of an A/C system in one form or another.
You may not take a reading at the supply and return and come up with a specific diagnostic differential reading, but everyone uses something to tell you whether there is cold air coming out of the registers, be it waving your hand in front of the registers, a IR gun, camera or the thermostat of the unit.
If you don't, then you don't know that the unit is producing cool air since all the motors, compressors can be running smoothly without producing cool air and temperature cannot be determined with the naked eye.

My bottom line is this, I use the TD method as A tool, but not THE tool. There really is no one "simple" test to pass or fail a A/C unit.
You really need to look at the entire unit and check the function by whatever method you choose. But don't assume that the unit is functioning properly just because you get a "good" TD nor should you condemn a unit because you get a TD slightly outside the normal range.
JMO

Originally Posted by Jim Luttrall

But don't assume that the unit is functioning properly just because you get a "good" TD

That could indeed be a "bad thing" to do, and if you do that, *someday* you *will* pay the price for doing so.

nor should you condemn a unit because you get a TD slightly outside the normal range.

That is not a bad thing to do. At worst, you recommend the HVAC system be cleaned and serviced when it is 'not that bad' - however - short of a brand spanking new install, has anyone ever seen 'perfectly clean system which could not use servicing and cleaning'? Nope, didn't think so. Thus, that is 'not a bad thing'.

You may even have the HVAC technician find something major which needed addressing, and that would be a 'good thing'.

Word!

Jim you are correct that we all measure TD when we measure room temps. However the thread didn't start out that way, it was more specific and then it continued to wander.
My comments are based on an occupied house, vacant is another story. If it's 95 outside why is the AC completely off. There shouldn't be a condition in place that requires the unit to drop temp 25 degrees. Unless maybe the seller is that stingy on the E bill or there is a known problem.
Set temp is 70. Much above that and most clients are going to complain. You can go up from that depending. What do you set your AC at?
It is not just an issue of whether the AC unit works or not. Literally half of the calls I used to get about ' I need a new AC, ours isn't big enough to cool the house' we were able to resolve by redoing ductwork, specifically high returns.
It doesn't matter how much cold air you pump in, if you aren't sucking some of the hot air out, the space will never get cool and comfortable. All you will end up with are layers of stratified air.
It doesn't matter if it is 95 outside. If the system is working properly, it will cool the space to a comfortable level. On a hot day like that it will likely take more than 20 minutes so ... how long is your home insp.
Average house for me is 3 hours. If the house isn't comfortable within that time, it never will be on a hot day.
That doesn't mean the AC unit isn't working. It could be many other factors.
When you go to check the air handler or condenser. Check the freon line, is it cold?
Often times if the equipment is running and sized adequately, yet the space is not comfortable (hot or cold), the problem will be bad ductwork or no returns.
How often have you seen a wild return, no high return, old massive gravity feed ducts being used with a new furnace?
Those items are the real efficiency factors.
Time to go to one of the best mex joints in town for dinner, later.

Originally Posted by Markus Keller

Set temp is 70. Much above that and most clients are going to complain. You can go up from that depending. What do you set your AC at?

Our thermostat is set at 78 degrees, and when it feels cold we raise it to 79, if it feels too warm (like when we have many people over, we lower it to 77, maybe 76. Some people say it is too cold then. We have one of those programmable thermostats so you can adjust it 1 degree at a time.

Has a lot to do with the relative humidity.

In South Florida, we had on of those regular thermostats where you were guessing at what was 75, give or take, maybe ... we had to keep that down closer to 74-75-76 as the relative humidity was higher down there.

We had some friends down there who kept their house at 70 degrees, it was like a meat locker in there - just too dang cold.

Jerry that's pretty amazing. Obviously geographic location brings differences in how people deal with climate.
My temp settings are based on what clients and friends around here like. From my discussions many people set temp at 70-72, some at 74. I can 't remember anyone stating setting it much higher than 75. At that point, what's the point?
I also use 70 as a set point, since as you mentioned, homeowners will set temp lower when a lot of friends are over due to the heat gain.
I can't imagine setting the stat at 80 and then putting in the report that the AC works. From my experience that would lead to complaints.
I don't have AC. I don't like AC. It tends to make me feel sick. I live on ceiling fans. The heat generally doesn't bother me below 100.

Jerry,

70 sounds very comfortable around here.

When I come home after being in the heat all day, I crank it down to where it feels like a Dairy Queen.

The wife is running around with a sweater, but I'm happy as a chilled pig in mud.

August is when I always expect to have the highest utility bill of the year and I received it in the mail today and I'm happy to pay it.

rick

The building where I live is pretty tight. I keep the thermostat on mid 70's, 76 or so. The last place I lived I had to keep it around 72 or 73. Obviously more outside air being sucked in and keeping the home to humid. The drier the easier to cool and a higher temp feels OK.

Most HVAC guys are not going to measure the delta T at the grilles they do it at the equipment.
If they measured it at the grilles they would have to acknowledge the radiant gains that are occurring in the ducts they have installed.
This more than likely describes the reason for the conflict with the HVAC guy.

If you want to take temperature differentials that's great but quit focusing on dry bulb differentials only. Too many variables are involved to make a blanket statement concerning their usefulness.
You need to add the wet bulb differential in addition to converting it over to an enthalpy value to get any real meaning from the readings.
Time to pull out the psychrometric chart guys.

Just how does one take a wet bulb. Just where do you stick it. I've been told a wet rag over the probe is used to obtain the reading.

Bacharach 12-7012 Sling Psychrometer Quantity in Basket: None
Code: 12-7012
Price: $72.99
Shipping Weight: 3.00 pounds
Quantity: +25°F +120°F Red Spirit Filled The uniquely compact Bacharach Sling Psychrometer accurately determines percent relative humidity without the necessity of consulting complex tables. There is no need to wet the wick each time a reading is taken; the Sling Psychrometer contains a slide rule calculator which correlates wet and dry bulb thermometer indications for direct reading of relative humidity. When not in use, the thermometer case telescopes into the handle for protection.
Features & Benefits:

• Designed for portability (7.5" long x 1" diameter)
• Slide rule construction quickly converts temperature to relative humidity
• Built-in water reservoir holds sufficient water for several hours of testing
• Thin bulb design gives fast thermal response
• Thermometers constructed of shock-resistant glass; stems have deep-etched numerals and 1° scale divisions for easy reading
• Accurate to within ±5% R.H.

Applications:

• Measurement of comfort conditions in air conditioned environments, hospital operating rooms, storage areas and laboratories
• Tests in industrial atmospheres where high or low humidity may be critical in manufacturing processes
• School athletics – to determine safe conditions for practice

From Wikipedia:
Hygrometers are instruments used for measuring humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature. Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer, which measures dry-bulb temperature. When the air temperature is below freezing, however, the wet bulb is covered with a thin coating of ice and yet may be warmer than the dry bulb. Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on a psychrometric chart. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.

They do a better job of describing than I could.

Originally Posted by Charles Sessums

Just how does one take a wet bulb. Just where do you stick it. I've been told a wet rag over the probe is used to obtain the reading.

The easiest way to get an approximate wet bulb reading is to place a wet wick over your dry bulb thermometer just like the wick used in a sling psychrometer.

The only problem with a sling psychrometer is you're going to have to make a pretty large hole in the ducts to get that thing in there.

You can take the differential readings at the equipment like most HVAC guys do and you can also take an average of the grilles to see what type of an effect the duct system is having.

Wet bulb readings alone are handy but their real value is when they are plugged into a psychrometric chart.

Easiest just to plug them into an enthalpy chart.

Ahhhhhh


I think we left home inspecting a long time ago

Originally Posted by Ted Menelly

Ahhhhhh


I think we left home inspecting a long time ago

Yep, but Charles asked...

Originally Posted by Ted Menelly

Ahhhhhh


I think we left home inspecting a long time ago

Was hoping I made my point.

Ahhhhhh


I think we left home inspecting a long time ago

It never hurts to know the "why" behind the "what" we do.
And while I understand the principles behind wet bulb readings, enthalpy charts and the like, I will categorically state there is no way I ever take wet bulb readings nor do I remember how to read enthalpy charts.
For home inspections, it is enough to remember we are generalists and realize that there are more things to consider beside a "normal" temperature differential.

Heck, I'll bet DavidR doesn't take wet bulb readings unless he has a really unusual case. Right David?

I have a hard enough time remembering to take myself to the next inspection.


I do appreciate the good info about wet bulbs though.

Take a 24 hour Phase inspection course and see how much of all that code stuff about inspection y'all forgot. My head was swimming for three days. Looking at the books and the instructors with my head cocked half the time saying, Oh yeaaaaaaaaaaaahhhhhhhh.


Here is one for Jerry

They must have used the SU PANEL words about 20 times thru the electrical portion on Saturday. Always with both words sub or auxiliary. Jerry would have been ripping his hair out.

Originally Posted by Jim Luttrall

Heck, I'll bet DavidR doesn't take wet bulb readings unless he has a really unusual case. Right David?

Lol...........I'm in a tad bit of a different position, I live and die by wet bulb readings.
To not take them for me is bad business.

Originally Posted by DavidR

Lol...........I'm in a tad bit of a different position, I live and die by wet bulb readings.
To not take them for me is bad business.

You must be in a very variable climate, I would bet there are not 1 in 10 a/c technicians here that use a wet bulb reading.
Summers are long, hot and pretty dry here. Spring and fall are a different story, but we don't size or tune units for humidity control, it is just the by product of air conditioning and not many people are willing to pay extra for a variable speed unit.

Originally Posted by Ted Menelly

Here is one for Jerry

They must have used the SU PANEL words about 20 times thru the electrical portion on Saturday. Always with both words sub or auxiliary. Jerry would have been ripping his hair out.

Ted,

Nope, not ripping my hair out, just taking that on one territory at a time.

There are other converts out there, so 'that side' (the dark side) is slowly being reduced in numbers and those persons are being replaced with ones who have the force with them.