the 65 degree question

can someone supply some good solid literature on why you should’nt run the AC when temps are too low?

looking to have ammo when i’m confronted.

opinions and/or personal practices also welcome. :slight_smile:

I believe it is HeatPumps that shouldn’t be tested in cooling mode under 65
AC’s are not tested under 60 (some HI make a practice not to test if the temp has dropped below 60 (for an extended period of time) in the last 24 hours or if the unit has not been energized for the past 24 hours).

The best argument would be your SOP
and the fact that most Inspection (Orgs and State) SOP’s recognize the possibility of damage to a AC unit if operated when conditions are not conducive to testing.

I.E.= Industry standard argument

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Under 65 degrees (an arbitrary number but with a good margin of safety), the refrigerant may not vaporize completely and you then have liquid refrigerant being sent to the compressor.

The compressor is designed for gas,which is more easily compressed than fluid. If you attempt to pump liquid through the system, you risk damaging the compressor. This can result in you writing a big check.

Since I HATE, paying my clients, (Much prefer they write checks to me!) I explain this to them when weather warrants and I report the AC was not inspected due to weather conditions.

Don’t have any literature but have a head full experience.

The proper term is called (liquid migration) which occurs during the off cycle of the unit. Liquid freon will seek the coldest area of an enclosed system between the furnace A-coil and the outside condensing unit. When outside ambients are below 60 degrees the MFG recommends not to operate these units because the coldest area of the system will be the outside unit and if liquid freon is lying in the compressor you can can damage the valves in the compressor when you start it up.

Freon is simply stated, all about pressure temperature relationship if the home or the area of the furnace A-coil is in a warmer area than the condensing unit the freon will migrate toward the coldest area in a shut down mode. Once the compressor is actually started and operated in a normal mode no damage will occur to the compressore the damage will occur on the initial starting of the unit.

If the outside unit is equipped with a crankcase heater the possibility of liquid migration is almost nil as the heat will drive the liquid freon from the crankcase

If this sounds like Greek I can try another approach.


Learn something new everyday! :cool:

It may only be semantics but a liquid cannot be compressed.


Per the US Dept. of Energy:

Question - Why does air compress and why does liquid not compress?

Gases, like air, consist of molecules separated from each other, bouncing
around. In a liquid, the molecules are all touching. Compressing a gas
simply forces the molecules to come closer to each other. If a gas is
compressed enough, or cooled enough, the molecules will pack right next to
each other, condensing into a liquid or a solid. It is hard to compress the
condensed matter further.

Liquids actually can be compressed somewhat. It is just a lot harder than
compressing gases, that is, it takes a lot more energy to reduce the volume
of a liquid than to reduce the volume of a gas by the same amount. Solids
also can be compressed, but since they are condensed phases too, it takes
much energy.

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois

doesnt sound greek to me.

you mention the MFG states no running the unit under 60 degrees.
All or most MFG’s?

crankcase heater…how would i know if a unit has one?
newer technology or older one?

now the big question…what’s everyone’s threshold on not running ac units due to outside temps?

I will not start any ac unit if the ambient (outside) air temperature is below 65 degrees and if it has not been started at least once by the seller or sellers agent. Experience tells me it could break down on an initial start up and i dont want to buy an ac unit for someone else.Just my policy.Matt

I think you will find that all MFG recommend not to operate at below 60 degrees this can vary within different SOP’s per your State. My SOP states 60 degrees. Generally speaking if the outside ambient is below 60 degrees you will be getting squirrelly temps on the unit anyway won’t tell you much of any thing other than it starts.

Crankcase heater have been around since Mobey Dick was a Minnow

Two kinds the stainless steel band around the bottom of the compressor with two wires going back to the top side of the contactor as this heater requires 220-volts when the unit is shut down.

The second kind is embedded within the crankcase of the compressor and all you will see are two wires that appear to disappear into the metal housing of the compressor at the bottom. Both kinds of heaters will be activated in the off cycle of the unit.

I beg to disagree what do you suppose a water pump does when it increases the pressure from say 25 PSIG to 150 PSIG it compresses the liquid???

If the temperature is as you stated I would think the unit breaking down at startup would fall under failed under testing.

If a garage door comes off the track when you open it are you responsible for fixing it?

I never check the units if the outside temperature has been under 60% within a 24 hour period.

Matt that is just fine you made the decision to choose 65 degrees just stick with it. be consistent.

I personally have operated in my other life units that were serving commercial areas with no windows that had high sources of heat generated from within the building and A/C units were operated 365 starting and stopping with an outside ambient at zero some with crankcase heaters some without. There is no rule that says Puff the magic dragon the valves will blow if you start this unit in temps below 60 or 65 as an HI pick a temp and just stay with it.

Vince thats fine if you made this decision but can you explain why you choose those numbers.

What’s with the “24 hour period”? That sure would preclude everyone in the desert mountains between, say, here and El Paso from using their cooling systems since it regularly gets down to 40° at night and up to 90° during the day. If I had those inspection protocols, I would never be able to test the cooling systems here. Hmmmmmm. Certainly would make the inspection go faster. :margarit:

RR I know where Vince got his 24 hour thing from was just trying to drag it out of him???

The literature that you are looking for would be from the equipment manufacturer.

The reasoning of the 65° in home inspection SOP/state laws is that at this point, if you don’t understand HVAC principles there is a “potential” to damage the equipment. Home inspection practices never jeopardize a potential to damage the property we are inspecting. Therefore, they are not supposed to the operated.

A heat pump can be operated in any temperature at any time. It is designed differently than an air conditioner and the system operates normally in both heating and cooling below 65°. This occurs almost every 45 minutes during a normal run cycle.

Crankcase heaters have two extra wires that go into the bottom of the compressor or have a metal band wrapped around the compressor with two wires coming from them. Heat pumps have these devices. Air conditioners normally will not (this would be an aftermarket modification). There are compressor designs that have internal heaters as well. There is a small resistance circuit within the compressor that takes 120 V away from the 240 V supply current and feeds this current through the start winding of the motor that keeps the compressor warm during the off cycle. This is the reason why you have single poll magnetic contactors on some compressors. One leg of power remains on in the compressor providing a heat source to prevent the migration of refrigerant. These compressors are frequently found on air conditioners and there is no way of visually determining their existence.

Concerning the issues of compressor damage from liquid slugging, it is very unlikely but possible to damage a reciprocal compressor. Seeing as it is “possible” for damage, it is outside the scope of home inspection SOP.

Under normal low ambient temperature operation, liquid refrigerant enters the compressor at the top and is dumped on top of the mechanical and electric motor within the compressor. These components are become warm during operation and will vaporize the liquid to a vapor in most cases.

Scroll compressors (versus reciprocal compressors) can handle liquid slugging due to their inherent design. There are no pistons or valves to be damaged.

The major concern is what Charlie is describing as “liquid migration”. Even without the lower temperature, refrigerant will migrate into the refrigerant oil in the compressor. When the compressor starts, the refrigerant will violently boil and take the oil with it as it is being pumped out of the compressor, draining the oil out of the compressor. If it does not return a reasonable period time, the compressor will be damaged due to the lack of oil. Another concern is that the liquid refrigerant dumping into a compressor will also wash out the oil if this is excessive. Liquid refrigerant causes the crankcase oil to boil and reduces its lubrication qualities. This condition generally occurs from long-term operation when the evaporator coil or air flow is restricted, it is not generally a startup occurrence when the temperature is below 65.

The issue on liquid compression (whether it can be compressed or not) is a matter of semantics when you study the reality of it all. It is not whether the liquid can be compressed or not, it has to do with the extremely small clearance between the piston and the head of a compressor. These tolerances are extremely small to maintain compressor pump efficiency and any object that is not in gaseous form can damage the working components. When you pump a liquid you use a water pump, when you pump air use an air compressor. They have different design and you wouldn’t pump water with an air compressor?! And to broaden things a little further, there are non-condensable gasses which can do just as much damage to the compressor if the refrigerant gas is contaminated.

The 24 hour issue is based upon first time startup of the equipment after a seasonal change. If the equipment has never experienced a 90° day in the past three months and/or electric power has been shut off to a compressor that has a crankcase heater, the compressor should not be operated for 24 hours after these heaters have been energized or the weather temperature has exceeded the 65° (or any other arbitrary number) for an extended period. This practice is for HVAC contractors, not for home inspectors. So, Russel Ray and others that live in these temperate climates can safely operate their equipment as they normally would.

I expounded on these issues not to condone or reprimand testing practices below 65°, rather for the purpose of expanding your understanding of the operation and for your reading enjoyment. Air conditioners can be operated in any environment if designed to do so.

To reiterate, we are not required to operate equipment below 65°. If it has been below freezing for an extended period of time prior to you doing an inspection and the outdoor air temperature has just reached an all time high for the week of 67°, I would not recommend starting that equipment. We get wrapped up in trying to do more than the other guy, and adding these liabilities only increases your risks of doing business. If you are a risk taker, go for it. The point is, it’s unecessary.

Very well stated David,this is exactly why i as a home inspector do not start an ac unit under 65 degrees and i wont be the first one to start it for the season.My 43 years in the hvac business and 16 years as a home inspector tells me i know better :wink: Matt.

Shame On you David you just pi??ed in my strawberry’s I was going to have some fun with the above statement that the other Gentleman posted about the 24 hour thing.:slight_smile: :slight_smile: Nice job well explained.


Below is a link to a carrier air owner’s manual which states not to run the cooling cycle below 55 degrees and the heating cycle above 66 degrees. Yes, its for a particular model, and probably without the case heater. But, here is some documentation.