the 65 degree question

[Charley L. Bottger]

Originally Posted by rray

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.

RR I know where Vince got his 24 hour thing from was just trying to drag it out of him?????
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[David A. Andersen]

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.

[Matt Berman, CMI]

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 Matt.

[Charley L. Bottger]

Originally Posted by dandersen

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.

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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. Nice job well explained.


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