Originally Posted By: mtimpani This post was automatically imported from our archived forum.
I get the do not run a/c under 65 degrees. But I am confused on the “or if it has not run in 24 hours”. I don’t get that. If it is 88 degrees outside for four days in a row and the a/c has not been used for two weeks, I can’t start it? Could you guys set me straight? Thank you
Originally Posted By: jpope This post was automatically imported from our archived forum.
prand wrote:
Mark, I think its supposed to be 65 deg. for 24 hours.
I've heard that somewhere as well, and I say phooey 
There are many days here in So. Cal where we run heaters at night (cuz we can't bear temps below 65) and A/C during the day (cuz we can't bear temps above 85) . Indian summers here will fall into the low 60's at night and jump to 90 during the day.
If the outside temp is 66 degrees, I'm checking the unit. I've also been told by several HVAC techs that even if it's only 55 degrees outside, we can run them for a short duration without worry of damage.
Heat pumps, I'm told, are a different story.
-- Jeff Pope
JPI Home Inspection Service
"At JPI, we'll help you look better"
(661) 212-0738
Originally Posted By: jpeck This post was automatically imported from our archived forum.
It’s an “or”.
Most (if not all) compressors have crankcase heaters nowadays (I think).
The problem comes in when the temperature is below 60 degrees (65 just adds another safety factor) and the unit is not 'on' (not that it has been run, but that it is 'on' with power to it), leaving the oil cold and stiff. Cranking up the compressor then could damage the compressor.
If it is 60 or warmer, for that same 24 hour period, that should not be a problem.
If it is colder than 60 degrees but the a/c has been 'on' (again, not necessarily 'running' but have power to it) for 24 hours or more, the oil in the crank case is warm enough to work as intended and not damage the compressor.
Thus, it's an 'or', either makes it 'safe' to operate the compressor.
Now, if the breaker to the condenser (compressor) unit is OFF, you know the crankcase heater is also OFF, and that you do not want to operate it UNLESS it is 60 degrees (65 degrees is safer) or warmer, also for 24 hours. Then the oil is warm enough to operate the compressor.
Anyone out there who can explain it better than me, go for it (or correct me if I'm wrong in my explanation).
Originally Posted By: csessums This post was automatically imported from our archived forum. Anyone out there who can explain it better than me, go for it (or correct me if I’m wrong in my explanation).
Jerry Peck
South Florida
You're kidding right? "correct me if I'm wrong in my explanation" 
I was taught the same @ ITA
Originally Posted By: Joe Healy This post was automatically imported from our archived forum.
As far as I know it, the reason that you don’t want to operate the compressor under 65 degrees has more to do with the refrigerant slugging into the compressor. Remember that a compressor compresses gas and not liquid. If it is too cold out, the freon isn’t taking on enough heat to remain in the gaseous state and therefore would be going into the compressor as a liquid. This would cause a problem for the compressor. I hope this helps a little bit.
Originally Posted By: rsummers This post was automatically imported from our archived forum.
When I finish installing a new Hvac system they are ran and motor amps are checked the charge is weight in according to the manufactures charts. Ive operated Tane, Bryant’s, Carriers, Temp Stars, Yorks and Gibson units and have not lost a compressor as of yet. I will cover the condenser fan to force the head pressure up to create a false load. I run every unit at every home I inspect just long enough to hear it come on. I’m sure to state that the out door temp was to low to determine if system is functioning properly.
Originally Posted By: syared This post was automatically imported from our archived forum.
I’ve been told that typically commercial units will have crankcase heaters because they might cool in summer and winter, home units usually do not have a heater.
Originally Posted By: kluce This post was automatically imported from our archived forum.
Joe is completely right on his statement.
I think we are told this so there is no way we can get sued if something would happen when we start the unit. Everyone and there brother will agree that we cannot harm an air conditioner is we start the unit after 24 hours of 65 degree weather.
For myself, I'm not going to worry about it. If it's 65 degrees that day, I'm running it.
Originally Posted By: rwills This post was automatically imported from our archived forum.
I won’t turn on a compressor either when the temperature has been below 60 deg for over 24 hours. I even have a built-in checkbox/statement in my report informing of this.
Here's a question for you and I've only had a few in the past few years ask it: " If you can't check the A/C how do I know it works before I buy the house?"
Originally Posted By: kluce This post was automatically imported from our archived forum.
The answer to the question is…
You don't know.
All I can do as a home inspector is look at the condition of the unit (which varies on the inspector). Going by what I can see as a home inspector and the discloser given by the home owner is what the buyer has to go on.
I tell a lot of my clients that between the discloser, the home inspection and the buyer looking at things, especially at final walk through, the average buyer has a lot better understanding of the house then they would have 20 years ago.
Originally Posted By: kluce This post was automatically imported from our archived forum.
Look at the states before responding. In Indiana or Wisconsin where I use to live, this is an issue.
I was informed that the government is going to raise the size of A/C units again. I was told that 13 seer units will be the smallest that a homeowner can purchase. Does anyone know when this is going to happen?
In this area and the a lot of upper states, this will be a big waste of money to buy a 13 SEER unit due to the payback is so slow because of the cold months and cooler summers.
Originally Posted By: tallen This post was automatically imported from our archived forum.
Mark:
I ran an A/C Friday at 1 pm. 69 degrees. I have asked many techs in this area about this and without exception they all have said it is OK. For what ever reason they don't seem to agree with the 24 hr. "rule"
However, if it is < 65 degrees I don't run it
I always recommend having a tech service the system before close of escrow anyway.
The 2 year old system I checked out Friday was so dirty it looked like it was 10 years old and had never been serviced. The Home owner did put a new filter in for me to see though. 
Originally Posted By: wrobedeau This post was automatically imported from our archived forum.
The reason for not running the unit is the refrigerant will migrate to the coldest part of the system in the form of liquid. This is most likely the compressor mostly due to its mass and the fact that it is generally in the condensing unit outside. Starting the system below 65 would result in slugging of liquid refrigerant to the cylinders and usually damage to or loss of the compressors valves.
Most domestic systems do not have crank case heaters. The purpose of the crankcase heater is to warm the compressor to evaporate any liquid refrigerant and to insure the refrigerant will not migrate to the compressor by keeping it warm.
If there is a crankcase heater and it is on, the system most likely could be started. However checking this is beyond a home inspectors duties as it requires removing a service panel to check. Hence the general rule don't start the unit below 65 degrees. That is about 10 degrees from most inside temperatures.
Originally Posted By: rwashington This post was automatically imported from our archived forum.
Great explanation Bill.
? for you though. Down here in Texas our SOP states to not turn on the A/C below 60 degrees. Is that 5 degrees going to make a difference in doing to damage to the compressor?
Originally Posted By: wrobedeau This post was automatically imported from our archived forum.
There are many variables. Inside temperature, Outside temperature during the day and at night. It is the temperature difference inside to outside that causes the refrigerant to migrate over time. What was the outside temperature the night before compared to the inside temperature? Did it allow more than a ten to fifteen degree differential to exist?
If you are not sure it is best to error on the side of caution and let a technician do it. Remember you could start it and it would run. However if it took out the compressors valves it will not cool again until the compressor or condensing unit is replaced even though it runs. the damaged valves will not allow pressure to be built up.
If you are 100% sure it has a crank case heater and it has been on for a while, (ie. the compressor is warm) then it can be started.
Originally Posted By: Blaine Wiley This post was automatically imported from our archived forum.
Fortunately here in FL there are very few days the temp doesn’t get above 65.
When I was in VA a good friend of mine owned an HVAC company, and did that part of the inspection for me frequently simply so he could put his sticker on the units. (he got quite a bit of business from that) We ran the A/C units in any weather to check and see if the compressor at least started. He said that the unit can be started and run for a couple of minutes without damage. Whether he was wrong or right, it was his money on the line if something broke as we agreed on, but no compressor ever broke in 7 years of running them in temps even in the teens.
When started in cold temps, I did put a disclaimer in the report something like: The A/C unit did start and run during the inspection, however the outdoor temperature was too cold to accurately assess the freon charge or system performance. Blah, blah, blah, check it again in the spring.