I have to check all registers to check on air flow because in our area there are many broken ducts in crawlspaces or seperated ducts in attics. If Temp DIff is low at a few regiseters but ok at the rest, I now know where to check in entering attic or crawlspace.
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I appreciate your points! This is why I asked my questions.
We’re talking about a home inspection rather than an HVAC inspection by a certified HVAC technician. I am attempting to give my clients my judgement about whether or not the system is functioning as expected considering the many environmental factors present at the time of inspection. That way they can determine if they need to negotiate better terms with the seller.
I described how I attempt to do that. But if there’s a more appropriate way to ascertain proper functionality, I’d like to know. I’m certainly not the expert here and want to improve my processes.
Thanks so much for!
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First things first, follow the SoP. Which is surprisingly sparse if following the NACHI SoP. So now you want to give your clients a little more, fine. But you are now on your own. There is no sure fire way to evaluate without being technically exhaustive.
So, back to the basics. Does it operate? Is each room conditioned? What are my senses telling me? Does it feel dank, humid or warm? Is the condenser short cycling? Is the t-stat demand being met? What does my IR read? What is the visible apparent level of care?
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I do the same thing. And I’m confident that it gives me a really good idea if the central air conditioner is working or not, producing cold air.
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The it’s beer can cold approach is more accurate.
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I do that too at the refrigerant lines. All I know is I’ve had at least a half a dozen insurance companies except my reports when buyer’s had home warranties and the AC went out on them within the first year.
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I used the air handler blower example since I am intimately familiar with a motor failure on my own system. A brand new system where the motor failed after one year but the failure was not that it stopped rotating the cage. Instead it went bat shit crazy and appeared to be rotating at over 100% design speed (ECM controlled motor). The only reason I knew it was a problem is I am in tune with my house and noticed it happening. The system still cooled acceptably but not as well as before (used to turn the house into a refrigerator) and the audible volume of air flow at the supply registers increased noticeably.
Brian’s post below yours is about the closest to evaluating a system that an Inspector can get. And that is, as Brian alludes, an opinion based on what he can feel and determine not on actual design and original installation abilities of the system. We have no idea how the system performed when installed and have no comparison from that standpoint.
Trying to label a system as “satisfactory” or “acceptable” is a subjective call that can bite an Inspector in the rear. Unfortunately Inspectors are between a rock and a hard spot on this subject. Texas Inspectors are in a worse place because of our requirements.
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I use 14-22 spread
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It’s not about being wrong. It’s more about how you present the results. Measuring Delta T accurately is not using IR. BUT, IR can give you generalized information that can indicate a problem. My two cents, unless you are in Texas, avoid reporting temps with your IR unless you see something that makes you suspicious of a problem. Then disclose that you did not measure Delta T because that is far more involved than what home inspectors do, but you did see some indications that the AC system is not performing as it should and you recommend evaluation by a HVAC tech.
Good news for you and all of us, is that few HVAC techs bother to accurately measure Delta T or understand it either. Most HVAC techs are mechanics who know how to disassemble and reassemble a system and some basic diagnostics. Few understand the science…not that we have to understand it either, but we are generalists.
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Interestingly, I have read many, many service invoices for HVAC repair and the common data on most of them is RLA motor readng, Delta T (or the return/supply temps indivudually listed), and refrigerant pressures.
They don’t dislose how they obtain the data, many use probe termometers, others use the sensor that attaches to the manifold gauge console.
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I’ve seen that and also seen HVAC techs using a $20 laser thermometer. Few are calculating air flow, understanding blower speed and condition, doing wet bulb measurements, and evaluating other performance factors. And most of the time, simple is good enough.
I have devolved to keep it simple. If it feels like cold air coming out, I move on.
Another thing is understanding some basics about the split range. For instance, I have seen when the relative temperature inside the house is high, like upper 80s, the AC split is 20F, but 4 hours later, the split is now 16F and the inside temperature is around 70 degrees.
Also, I’ve been in a closed-up house where the AC has been running for the last month. The temperature in the house is about 68 degrees. I drop the thermostat to its lowest setting around 55F. When I check the split an hour later, the air temp in the house is still 68F and the split is 10 degrees. While in ultra-low humidity conditions, you might get an AC system to blow under 55F air, with normal humidity and people coming and going from a house, I rarely have seen air cooler than 60F coming out. So, my example with 10F split is actually performing well.
Something that is obvious, but lots of people don’t realize until they think about it, is that an AC system is not designed to turn a house into a meat locker. When you are at the bottom end of the capacity of a home AC system to cool, you can see very low Delta T in a performing system.
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Taking the temperature differential is attempting to measure the Btu capacity of the unit.
You can not measure this with a sensible heat thermometer. Period.
When is this temperature being taken, heat of cooling. They will not be the same.
In heating, you are adding sensible heat to raise the the air temp. In cooling you are removing latent heat which subsequently lowers the sensible heat temperature.
Depending where you are standing on the face of the planet, this temperature delta will be different. It basically has to do with water vapor in the air being treated.
I am not going to go through all this (again) because it is complicated with many variables and is not what this test is after.
If you come to this question in your test, do not answer it until you finish the test. In most cases they will come back to this in another question where they will tell you the answer they are looking for.
It’s not your job to do Thermodynamic calculations.
This has come about in the HVAC field where Delta-T is used by installers to run test the install. If the unit is installed correctly, where the air duct system has been balanced properly for air flow, you will get close to these temps. If your off the chart, there is something wrong.
I have also had to deal with idiot code inspectors inspecting new installation. One 102F August day I had to go out for the company I was working for because the Code guy was wanting to measure this Delta, and the unit had to run for his predetermined time and the furnace kept shutting off. Well when it’s 106F inside the house, how hot is that going to get? Well above the heat exchanger safety switch rating. He would take none of this, so I changed the temperature safety switch to the highest rating sensor I had and let it rip. He assumed I was fixing the alleged problem. When he left, I took it out.
This is an example of what goes on if as an HI you call these Delta measurements out. Also, as mentioned here, using an IR Thermometer is wrong. Even the accurate thermistor is wrong if you don’t follow the industry measurement locations. You need to make your measurements four duct withs downstream of the plenum. This requires you to put a hole in the duct and professionally repair it when your done. Does this sound like the HI’s job? Will you find any questions on this, No.
Stick with the Beer Can procedure if you truly want to inspect the operation of an AC Unit. But don’t don’t do this when it is above 95F as this is the design operation of most systems.
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That is not part of our job! We simply report what we find, and after that, we do not need to worry about the other parties involved and what they will do with our report… We remain impartial to all parties involved in the transaction; we are like Switzerland!
As for measuring the “Delta T” of a system. We stopped doing that a decade or more ago in our reports. We may use that data for our own information and retain it if asked, but we do not include it in our reports. We simply report that X is cooling or heating as designed. Or we report that X is not cooling or heating, and service is needed. We also add a sentence if the units are aged, say over ten years of age, that they have had a good life and could fail at any time. We then recommend a service check-up due to their age.
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I was with you until this sentence. HVAC units reliably last 25-30 years.
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Along with splits I also look for the lowest temp in cool mode. If the lowest temp of the unit is in the 70s the split temps are usually no good. 95% of the units get into the 55 to 60 degree range. If they are working fine but are 65 - 70 range - it usually means they are low on refrigerant. If they blow warmer, than of course it could be many things wrong. My threshold is 60. When it hits 60 , 59, 58, 57, 56, things are good.
The 60 range threshold for me is more important then the split. I was at an inspection with an hvac company at the same time years ago. The house had been vacant and very hot inside. He announced to the buyer, " 10 degree split, everything’s good!" The supply temps after running for a half hour were only 80 degrees. Obviously the unit was not cooling properly. But because the return was 90, he said everything was great. In my opinion you shouldn’t just go by the split temps.
Around here, it’s usually 15 (+/- a few).
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AC units have a shorter life, in part due to changes in available refrigerants.
Not in the South!
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