The manufacture specs rule. The 65 deg is a guideline when you do not have better information or the appropriate equipment.
Tadd I wonder why you would pull up a 6 year old post to make your first post on NACHI.
I am sure this has been disused more then once in the last 6 years … Thanks … Roy
Love to see you help but more up to date would be great.
Here is to hoping that it actually gets above 65 degrees before summer so we can test some A/C systems 
Here in Florida, almost every home has a heat pump. The compressor runs whether it is heating or cooling. The only difference is the reversing valve sends the hot freon to the condenser to cool and to the evaporator to heat.
Do you test the defrost control then?
If the compressor can break at temps under 65, then why in my car does the ac safely run each time I turn on the defrost even at very low temps?
I use the 65 degree rule.
This past week it has been unusually warm for April and in the low 70s.
I found 2 non functioning AC units this past week.
My clients were grateful.
For 5-6 months of the year there is not much I can do to assure them the unit is functioning.
Mike, this isn’t targeted at you, but I want to use your post as a basis for my question…
Let’s turn this thread around for a minute to get the perspective of another viewpoint…
Q: A typical homeowner that knows nothing about home ownership, (and frankly doesn’t care to know), walks up to the thermostat and turns on the AC. It is currently 58 degrees outside, and 72 degrees inside, but she is hot from doing chores all day and wants to cool down before continuing with the rest of the chores. What say you?
Use a fan in the window☺
I know you’re smarter than that.
My point being, most homeowners don’t have a clue about “65 degrees”! In fact, more than most don’t even give a damn. I have never seen a Warning on a thermostat to check the outdoor temperature before operating the AC, and most homes do not even have an outdoor thermometer to do so. So, do you mean to tell me that every single home with an AC unit is at risk of the homeowner/spouse/CHILDREN/visitor turning on the AC when it is “too cold” outside and damaging the unit?
I would appreciate one of our MB “experts” to explain this scenario, and not just another “cut & paste” reply they got off the internet.
It’s not the same system design now is it?
There is a crap load of a/c equipment out there that has to run 365.
It’s designed to do so.
If you have a wife with hot flashes and want to run your a/c (instead of opening a window or fan) then call your a/c guy and convert it to run in low ambient conditions.
That simple…
Aside from all the off point comments and beliefs, the only time you have to concern yourself with starting the unit under 65F is when the equipment has been sitting off for a long long time and your the one starting it up for the first time, for no good reason…
As Charley pointed out, Refrigerant has an affinity for oil and colder components. Refrigerant and oil combine and will stratify in the bottom of the compressor. When you start up the compressor and there is a significant change of pressure, there is a violent reaction and the refrigerant and oil try to discharge out of the compressor discharge (like mentos in a bottle of coke). Oil can’t compress and the hydrolic pressure will break something with ease. Now, to throw a curve into it, many (if not all new) equipment can handle this if someone turns it on anyway! Scroll compressors have no problem. Many reciprocal compressors (commercial) have spring loaded heads that lift away from the piston and prevent breakage. The primary issue is that the oil leaves the compressor. Under low load conditions, the oil can not get back to the compressor, so it runs without oil for a long time. Short time is not an issue as all the parts sit in the oil.
Whoever said you can’t run a Heat Pump under 60F, explain how it lasts when it runs in a/c every 45 min in the heating mode without issue?
Heat pumps have heaters, accumulators, and if are really old, have compressors more strongly built compared to A/C compressors of the same age.
Like your car A/C. It’s designed to run in the winter. How do you think your window defroster works?
To reiterate: If it is spring or fall and the a/c was running “yesterday” but it’s below 65F the am of your inspection, just because it is 65F does not mean you will damage anything.
Most equipment will handle a one time abuse. But if you run it all the time in the cold, without a low ambient modification, it will fail prematurely. And this is not what we are concerned about in HI.
Well in most cases the home inspector is the first time runner in the non AC seasons so using 65 as the cut off for not testing is a reasonable way to ensure you don’t break something especially older units that do not have scroll type compressors.
Yup. There is nothing wrong with the 65 rule.
But the OP must understand WHY he is not doing it so he can support his decision to the client.
Basing a decision just upon a thermometer reading doesn’t cut it.
This goes for anything Home Inspection…
“A/C unit was not tested because…”
I agree. The why of such things including codes is valuable. Learn it.
Who here is worried that after a long hard winter and then a string of 80 degree days you go into a house and start up the A/C not realizing that another inspector, two months earlier, when it was 40 degrees out, ran the unit and damaged it without anybody noticing what happened?
There would be no way to prove that the previous inspector, not you, damaged the compressor.
So what? You followed protocol and found a bad unit.
Stuff happens.
One of my clients this week said no AC problems were disclosed.
I said it could have been working last summer and died sometime during the Winter and he would not necessarily have known.
But I would rather find the problem than not and get a call 3 months later.
With no disrespect, but we can get away with what is factual and is not the subject of discussion. You have a responsibility to the homeowner, the real estate agent, the buyer and the insurance agency that may be involved.
Just because we can get away with something, does not mean that is a good standard of practice. It is my opinion that no home inspector should ever touch anything beyond the thermostat!.
I am more than happy to identify the very minor things that can be used to identify adequate operation (beyond the idiotic assumption that temperature differential has any bearing). I will go to my grave trying to teach idiots that a thermometer cannot evaluate HVAC system performance!
Pay attention!: Your basic home inspection training indicates that if you simply touch the suction and refrigeration lines of the HVAC system you can determine without violating any potential Montréal protocol, by simply determining what it “feels like” when you touch the two refrigeration lines!
If the suction (the big one) is not like a cold beer from the fridge, and the unit has been running for two hours, there is probably something going on…
If the liquid line (the little guy) has any heat (that’s above 97°) means the equipment is not clean and rejecting heat. There is absolutely nothing more than you can determine until the system is adequately cleaned to its original factory specification!
OUTSIDE YOUR HI STANDARDS!
For a home inspector, that means it’s not going to get cleaned while you’re there. Someone else has got to come back behind you. Therefore! At this point it is out of your hands. It is the responsibility of another company that is licensed (just like some of you are) to perform within the specifications that unfortunately home inspectors are not qualified to determine!
One more thing to consider…
I know I am no expert, but I have always found even if you do test below 65 deg., you don’t get much of a result to begin with.
I would test usually down to 55-50 deg.,but I was mainly checking for basic function. Most equipment doesn’t seem to do much in these temps. Some did better than others. At least I knew it would start up and run.
Now living in FL I don’ think this will ever become a real issue for me.