At my last inspection the AC supply temperature (taken at the coldest supply duct at the plenem) fluctuated between 45 and 50 degrees in a rhythmic motion with each cycle lasting about a minute. The temperature would move up slowly to 50 degrees and then back down slowly to 45 degrees. I’m here in Texas and the outside temperature was 63 degrees. We normally check the AC here when the outside temperature is above 60 degrees. I use an SPK 2 thermometer at the coldest supply register and if I don’t get the state required 15-22 differential I check directly at the plenum and look for at least an 18-degree differential. I swapped out thermometers in case the one I was using was faulty and got the same reading. Any ideas on what may be causing the temperature to fluctuate? I’ve had this issue once before and I have inspected 1,000 homes. Thanks.
Ok, I’ll discuss this again…
You CAN NOT get a Delta T when the outdoor temperature is below design conditions and there is no indoor load! Remember reading, “Don’t run below 65F”? Well I don’t believe that either and I just got done arguing this point elsewhere here.
I know your idiot TREC rules want you to take them, so just take them and call it a day.
Try; Compressor Failure - #10 by dandersen
EVERYONE PAY ATTENTION: Good observation Gene!
This is what you find when you run the A/C when the outdoor air temp is below the thermostat set-point.
There is no load on the equipment (indoor air temp/moisture).
There is insufficient head pressure because of the OA temperature.
The unit was not modified to run in low ambient conditions.
There is a good chance of damaging the compressor running the A/C under these conditions for a long time, trying to get a Delta T that you will never get.
What you see here is called “Hunting”.
It is like your truck trying to go 70 MPH (80 MPH in Texas) with a bad fuel pump. No pressure at the Carb/Fuel Injectors and it will run just like this A/C.
There is a design refrigerant Delta P that is required for the system to operate properly. R-22 maxes out around 250 PSIG high side, 70 PSIG low side (180 Delta P). When it’s 60F OA, 101 PSIG high side, 40 PSIG low side (61 Delta P). Pressure drops 119 #.
When you starve the evaporator coil the pressure drops as does the refrigerant temperature. However, this temperature is not consistent across the coil so the coil gets warmer overall, Increasing temp and increases refrigerant pressure. On the outdoor condenser coil, as the load from the evaporator coil decreases, the temp of the condenser which is designed to run 30 degrees above OA Ambient Temp now runs colder (0 Delta P) lessens pressure to the metering device. Unless you increase the load indoors, or reduce the heat transfer of the condenser, it will hunt.
Understand? Your Sate doesn’t! They should if they make laws, no?
You can make this go away by wrapping the outdoor coil with something, or turn off the outdoor fan. This will raise the head pressure to design. You can run the heat up in the house to create a 95F load. Just don’t want to keep this up when the head pressure goes above 250 PSIG (R-22). It will trip off on a high pressure control.
It’s my guess you won’t want to try this, but that is what you need to do to satisfy the State Mandate.
I appreciate your knowledge on this topic Mr. Andersen, but how you would explain the fact that the other 2 AC systems at this house operated fine? I’ve also been doing inspections for 6 years now and have only seen this issue a few times now.
All 3 AC units were 2012 Rheem, R-410 refrigerant.
What? Texas requires in their SOP you to measure the temperature differential? I always do, but only as information for the client.
Don’t see an HVAC license in your Qualifications.
No need to be so critical. Having a license doesn’t make one knowledgeable. Likewise gaining knowledge in a specific area doesn’t require a license. Some of the smartest people I’ve ever known never graduated high school. And some of the dumbest souls I’ve encountered have multiple degrees and letters after their name. We have to not accept everything stated as truth. But those statements should challenge us to investigate and discover for ourselves what is truth and what is merely opinion. That’s the great thing about this forum.
There could be several reasons for what would cause the issue. Having stated that 2 other systems seem to be ‘steady’ and consistent in the TD ( I’m assuming this was what was inferred), the ‘hunting’ usually, but not always indicates air or non-condensable gases occupying part of the circuit. Usually a dead giveaway is the change in sound of the refrigerant as it enters the coil TXV, it will flash off irregularly. You can ask the owner if they’ve had any repairs to that system recently involving opening up the sealed system or any brazing work done. Almost every time it is poor service practices such as running a nitrogen purge while brazing to prevent oxidation, not correctly using a vacuum pump when complete, or not changing the filter dryer as part of the procedure. Anyways, the only way to know is to have it followed up on by is by a licensed and competent HVAC company that don’t take short-cuts. It is possible this system has been running this way for some time as a lot of time it goes un-noticed by the homeowner, as it will still seem to function, but meantime, damage is already occurring internally to the components.
No two systems are exactly the same.
Refrigerant charge, length of the line set or lack there of.
I can only tell you if you want to pay me to come out there and figure it out. I have seen this hundreds of times…
As I said, they have a name for that, “HUNTING”.
Yea, and I just made all that shit up! 
HVAC Certification EPA Cert#2046620
This is the only thing I need to touch anything with refrigerant in it…
If it makes you feel better;
I went to a Tech School for HVAC after graduation from high school.
I have a degree from Northeastern University in HVAC System Design.
I owned my own HVAC company.
This is only one of four careers I participated in over 70 years…
And No, NACHI does not have a sticker for me to use to satisfy your needs. Maybe you should talk to Nick about that…
Good post Michael, but heads will pop like zits from all that information!
I think you may agree that there is no reason to even try to analyze an HVAC unit running outside of it’s system design.
If a Home Inspector is going to step outside their own Inspection Standards. Operate equipment outside of HVAC design conditions. Then try to make sense out of why it’s not working, come here and argue about it, all because some idiot at TREC makes up a rule they are supposed to follow…
Sheep 
Take the damn Delta T, report it, and don’t worry about the numbers.
Yes, I know. A lot of guys are just curious as to the ‘why’ and look to experts in the field for answers. I’ve was in the field over 30 years, and I have asked others with expertise in other trades because it’s almost always educational. If you or I were on this inspection would we call it out based on what we know ?. Would we at least put it in the report that it seemed to be cycling irregularly but that it could be attributed to the weather and recommend a follow-up when the weather warms up ?
Even in Florida it gets cold in winter and in my employment, we had maintenance contracts where our boss required us to record the pressures and TD. It was always a battle with them trying to tell them these numbers don’t mean squat and we could be damaging the system, but they insisted. On a high-rise with compressors on the roof, it could take 10 minutes to get up there and put your gauges on. I guess they figured it was ‘ job security’ if it was compressor suicide cos the customer would be calling us to fix it. It’s always been a pet peeve of mine that a lot of A/C company owners may know how to run a company yet know little about the tech end of this business. I have to keep reminding myself as a new inspector to stick to the SOP and stay in my lane.
I would not. But that is just me and that in my assessment, it is to be expected.
I’m not saying others should not. I just don’t want argument of my explanation (which is for those that might want to really know why) because I don’t have a NACHI sticker in my signature block…
As did one of my former bosses. I told him in front of the owners son, if you can tell me why you need those numbers (sensible temperature differential) I’ll start doing it. Bosses son asked why he puts up with me. “Because I can’t afford not to have him here”.
It was my job in that company to fix what others could not figure out. I cases just like this one, they never cleaned the coils properly (because it was beneath them). Even when they were asked “Did you clean the coils?” numerous times, I became the highest paid coil cleaner in the company when I arrived. I had no intention of walking around anyone’s house or business covered in crap, even with my paper shoe covers to take temps at supply and return registers (which I have explained numerous times here why it is a waste of time as it is not the proper locations to do diagnostic testing).
As for the comment that other two units that are operating properly, If they are dirty, they will not “hunt”. It is the same thing I said about wrapping the outdoor coil above. Wrap the coil and restrict the air flow and it will quit hunting. You can not diagnose HVAC equipment if they are operated outside of design, and are not in “like new” condition.
You guys do your job as you see fit. My comments are only intended for those have a want/need to know.