Should I run the AC?

Last night’s low was 45 deg F. By the time I’ll be doing the inspection, it should be a high around 65 deg F with plenty of sun. Should I run the AC or not?



This was just discussed in the members only by David.
Log in and you find it.

No, …

There ain’t no taking temp splits in low ambient temperatures!

There ain’t no cold beer feeling in the suction line when there’s low ambient temperatures.

Without adequate head pressure the metering device “starves the evaporator” and it can not operate as intended! Period.

The only thing you can do is turn it on and see if it runs. There is no analysis.
You have no load and you have no head pressure to drive the refrigerant through the system to properly operate to do anything but see if it runs.

For $300 I’ll sell you a device that you can put on the equipment and make it work like it’s 100° outside. You have to have an EPA certification to use it.

Not all air conditioners have crankcase eaters. Only heat pumps (not A/C units) have crankcase heaters or some sort of sump heater unless they (A/C’s) were designed to run in low ambient conditions. So there’s a high probability that there is no heater in the equipment you are inspecting in a residential setting.

If some idiot thinks that they know more about HVAC that you do, then they should have the inspectors hat on. If you can’t convince them that you know what you’re talking about, then maybe you shouldn’t be there either.

I have no idea where the hell anybody came up with a home inspector is responsible for the operation of the HVAC equipment. Well, maybe I do. It’s some moron home inspector trying to outdo the other guy.

There’s a 99% probability that you won’t damage the equipment if you turn it on in the cold ambient temperatures. With your luck you’ll be 1%.

:razz: David A. Andersen & Associates
Clarksville - Nashville Home Inspector Lic#40
ITC Level II Thermographer Cert#1958
Building Science Thermographer Cert#33784
HVAC Certification EPA Cert#2046620

BPI# 5015804

Does anyone really understand why you can’t run the equipment at low ambient temperatures?

It has nothing to do with the temperature at the time you want to turn it on.
Refrigerant has an affinity to mix with and move into the oil in the compressor.
The refrigerant is driven out of the house by the heat in the house and the heat from the furnace being on.

If it is cold enough outside for the heater to run, then you are driving the refrigerant out of the house where it basically condenses into the oil in the compressor.
Inspector Joe comes along when the temperature is 65° (or any outdoor air temperature) and decides it’s okay to turn on the air conditioner.
As the compressor starts to suck, it pulls the refrigerant out of the oil which begins to foam violently (like shaking up a 2 L bottle of Coke and popping the cap) and gets sucked through the compressor and discharged towards the evaporator inside the house.

Two things happen. The oil does not compress in the compression chamber of the compressor. Very tight tolerances exist to ensure compressor efficiency. There is no room for the oil and through hydraulic pressure, something breaks.
Second, the oil is pumped out of the compressor, now the compressor can run without any oil for a while. This oil logs the evaporator coil and prevents absorbing heat from the room (therefore you have insufficient Delta T). Because you cannot absorb heat, refrigerant pressures remain low. Because the pressures are low, you have no velocity. No velocity means that the oil cannot get back to the compressor.
Are we starting to get the picture?

Seeing as you cannot see what’s happening with the oil in a residential HVAC unit, you are guessing at what’s going on. There are procedures to monitor if this condition is occurring, but nobody is willing to do them.

It is not about what the temperature is right now. It is about what the temperature was and what it has done to the refrigerant in the system.

Can you come and explain that to the class I just taught in 2010.
I almost lost my marbles trying to explain Heat pumps and air conditioning from Carson Dunlop schooling guidelines.
Thanks for bringing me back. Now my nightmares will start all over again. :roll::roll::roll::roll:

:shock:Sorry Kevin!:shock:

Sweet dreams… :twisted:

Great stuff but maybe now you guys could answer the question better by explaining under what conditions he can test it.

24 hours above------=OK to test
48 hours above-----=OK to test ???

Out here we have hot days and cold nights, so not to test the air conditioner just because it was cold last night and hot right now would have all of us home inspectors out of business in this part of the world.

I agree with Russel. Many AC systems now can be run without problems with ambient temps as low as 45 F . Along with that many have scroll compressors.

How many Inspectors know how to tell if it is scroll compressor? I know BTW but i am sure is a few that do not.
I was taught 60 For 24 hours unless there is a crank case heater.
Works for me.

Here is a picture of a


I was also taught that a scroll compressor is taller, and narrower,
which certainly matches that picture.
The problem for most inspectors is that they possibly can’t see inside the condenser,
with its covers and minimal visibility to the interior.
And it wouldn’t make much difference what kind of compressor is used,
unless it does have a crankcase heater, as you mentioned.
But I’m of the opinion that no AC testing, unless its been 60 degrees plus for more than 24 hours…
most understand when I explain why.

A scroll can handle a slug, but can it run without oil?

Some are still not paying attention…

When someone posts the physics and fluid dynamics of refrigerant, and can prove when it’s ok (in their area) to “run cold”, we will say it’s ok for YOU.

Until then, if you don’t understand it (and there is no reason you should) you are subject to the old wives tales that the lawyers will use against you in that “demand letter” you get if by some chance (and I said 1%) you happen to run it and it breaks for any possible reason.

Just do what you will.

My stance is that you are not an HVAC contractor.
You are not EPA certified, so you can’t use the tools you need to run a compressor in the cold and keep it from breaking…

The SOP (regardless of which one) states how and when it will be tested.

I’m just trying for you to understand the “why” so you can answer those that think you should be running it.

Wayne that picture is smaller than the avatars here.
Could you explain what to look for?

A scroll is normally Smaller diameter and taller . As someone mentioned . When running you will notice a difference in sound. a better look

Thanks ,“much better”

Though sound ID means you are already running it.:slight_smile:

LOL yes Bob That is a true statement.