the 65 degree question

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???
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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.

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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.

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 :smiley:

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…”